NCERT Textbook Class 9

Activity _____________ 10.6 (a) • Take a beaker filled with water. (b) • Take a piece of cork and an iron nail of Fig. 10.6: (a) Observe the elongation of the rubber equal mass. string due to the weight of a piece of stone • Place them on the surface of water. suspended from it in air. (b) The elongation • Observe what happens. decreases as the stone is immersed in water. The cork floats while the nail sinks. This happens because of the difference in their • Observe what happens to elongation densities. The density of a substance is of the string or the reading on the defined as the mass per unit volume. The balance. density of cork is less than the density of water. This means that the upthrust of water You will find that the elongation of the string on the cork is greater than the weight of the or the reading of the balance decreases as the cork. So it floats (Fig. 10.5). stone is gradually lowered in the water. However, no further change is observed once The density of an iron nail is more than the stone gets fully immersed in the water. the density of water. This means that the What do you infer from the decrease in the upthrust of water on the iron nail is less than extension of the string or the reading of the the weight of the nail. So it sinks. spring balance? Therefore objects of density less than that We know that the elongation produced in of a liquid float on the liquid. The objects of the string or the spring balance is due to the density greater than that of a liquid sink in weight of the stone. Since the extension the liquid. decreases once the stone is lowered in water, it means that some force acts on the stone in Q uestions upward direction. As a result, the net force 1. Why is it difficult to hold a school on the string decreases and hence the bag having a strap made of a thin elongation also decreases. As discussed and strong string? earlier, this upward force exerted by water is 2. What do you mean by buoyancy? known as the force of buoyancy. 3. Why does an object float or sink when placed on the surface of What is the magnitude of the buoyant water? force experienced by a body? Is it the same in all fluids for a given body? Do all bodies 10.6 Archimedes’ Principle in a given fluid experience the same buoyant force? The answer to these questions is Activity _____________ 10.7 • Take a piece of stone and tie it to one end of a rubber string or a spring balance. • Suspend the stone by holding the balance or the string as shown in Fig. 10.6 (a). • Note the elongation of the string or the reading on the spring balance due to the weight of the stone. • Now, slowly dip the stone in the water in a container as shown in Fig. 10.6 (b). GRAVITATION 141

contained in Archimedes’ principle, stated as 10.7 Relative Density follows: As you know, the density of a substance is When a body is immersed fully or partially defined as mass of a unit volume. The unit of in a fluid, it experiences an upward force that density is kilogram per metre cube (kg m–3). is equal to the weight of the fluid displaced The density of a given substance, under by it. specified conditions, remains the same. Therefore the density of a substance is one Now, can you explain why a further of its characteristic properties. It is different decrease in the elongation of the string was for different substances. For example, the not observed in activity 10.7, as the stone was density of gold is 19300 kg m-3 while that of fully immersed in water? water is 1000 kg m-3. The density of a given sample of a substance can help us to Archimedes was a Greek determine its purity. scientist. He discovered the It is often convenient to express density of a substance in comparison with that of principle, subsequently water. The relative density of a substance is the ratio of its density to that of water: named after him, after Relative density = Density of a substance noticing that the water in a Density of water bathtub overflowed when he Since the relative density is a ratio of similar quantities, it has no unit. stepped into it. He ran Example 10.7 Relative density of silver is Archimedes through the streets shouting 10.8. The density of water is 103 kg m–3. “Eureka!”, which means “I What is the density of silver in SI unit? have got it”. This knowledge helped him to Solution: determine the purity of the gold in the crown Relative density of silver = 10.8 made for the king. Relative density His work in the field of Geometry and Density of silver = Density of water Mechanics made him famous. His Density of silver understanding of levers, pulleys, wheels- = Relative density of silver and-axle helped the Greek army in its war × density of water with Roman army. = 10.8 × 103 kg m–3. Archimedes’ principle has many applications. It is used in designing ships and submarines. Lactometers, which are used to determine the purity of a sample of milk and hydrometers used for determining density of liquids, are based on this principle. Q uestions 1. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg? 2. You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why? 142 SCIENCE

GRAVITATION What you have learnt • The law of gravitation states that the force of attraction between any two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them. The law applies to objects anywhere in the universe. Such a law is said to be universal. • Gravitation is a weak force unless large masses are involved. • The force of gravity decreases with altitude. It also varies on the surface of the earth, decreasing from poles to the equator. • The weight of a body is the force with which the earth attracts it. • The weight is equal to the product of mass and acceleration due to gravity. • The weight may vary from place to place but the mass stays constant. • All objects experience a force of buoyancy when they are immersed in a fluid. • Objects having density less than that of the liquid in which they are immersed, float on the surface of the liquid. If the density of the object is more than the density of the liquid in which it is immersed then it sinks in the liquid. Exercises 1. How does the force of gravitation between two objects change when the distance between them is reduced to half ? 2. Gravitational force acts on all objects in proportion to their masses. Why then, a heavy object does not fall faster than a light object? 3. What is the magnitude of the gravitational force between the earth and a 1 kg object on its surface? (Mass of the earth is 6 × 1024 kg and radius of the earth is 6.4 × 106 m.) 4. The earth and the moon are attracted to each other by gravitational force. Does the earth attract the moon with a force that is greater or smaller or the same as the force with which the moon attracts the earth? Why? 5. If the moon attracts the earth, why does the earth not move towards the moon? 143

6. What happens to the force between two objects, if (i) the mass of one object is doubled? (ii) the distance between the objects is doubled and tripled? (iii) the masses of both objects are doubled? 7. What is the importance of universal law of gravitation? 8. What is the acceleration of free fall? 9. What do we call the gravitational force between the earth and an object? 10. Amit buys few grams of gold at the poles as per the instruction of one of his friends. He hands over the same when he meets him at the equator. Will the friend agree with the weight of gold bought? If not, why? [Hint: The value of g is greater at the poles than at the equator.] 11. Why will a sheet of paper fall slower than one that is crumpled into a ball? 1 12. Gravitational force on the surface of the moon is only 6 as strong as gravitational force on the earth. What is the weight in newtons of a 10 kg object on the moon and on the earth? 13. A ball is thrown vertically upwards with a velocity of 49 m/s. Calculate (i) the maximum height to which it rises, (ii) the total time it takes to return to the surface of the earth. 14. A stone is released from the top of a tower of height 19.6 m. Calculate its final velocity just before touching the ground. 15. A stone is thrown vertically upward with an initial velocity of 40 m/s. Taking g = 10 m/s2, find the maximum height reached by the stone. What is the net displacement and the total distance covered by the stone? 16. Calculate the force of gravitation between the earth and the Sun, given that the mass of the earth = 6 × 1024 kg and of the Sun = 2 × 1030 kg. The average distance between the two is 1.5 × 1011 m. 17. A stone is allowed to fall from the top of a tower 100 m high and at the same time another stone is projected vertically upwards from the ground with a velocity of 25 m/s. Calculate when and where the two stones will meet. 18. A ball thrown up vertically returns to the thrower after 6 s. Find (a) the velocity with which it was thrown up, (b) the maximum height it reaches, and (c) its position after 4 s. 144 SCIENCE

19. In what direction does the buoyant force on an object immersed in a liquid act? 20. Why does a block of plastic released under water come up to the surface of water? 21. The volume of 50 g of a substance is 20 cm3. If the density of water is 1 g cm–3, will the substance float or sink? 22. The volume of a 500 g sealed packet is 350 cm3. Will the packet float or sink in water if the density of water is 1 g cm–3? What will be the mass of the water displaced by this packet? GRAVITATION 145

C 11hapter WORK AND ENERGY In the previous few chapters we have talked draws diagrams, organises her thoughts, about ways of describing the motion of collects question papers, attends classes, objects, the cause of motion and gravitation. discusses problems with her friends, and Another concept that helps us understand and performs experiments. She expends a lot of interpret many natural phenomena is ‘work’. energy on these activities. In common Closely related to work are energy and power. parlance, she is ‘working hard’. All this ‘hard In this chapter we shall study these concepts. work’ may involve very little ‘work’ if we go by the scientific definition of work. All living beings need food. Living beings have to perform several basic activities to You are working hard to push a huge rock. survive. We call such activities ‘life processes’. Let us say the rock does not move despite all The energy for these processes comes from the effort. You get completely exhausted. food. We need energy for other activities like However, you have not done any work on the playing, singing, reading, writing, thinking, rock as there is no displacement of the rock. jumping, cycling and running. Activities that are strenuous require more energy. You stand still for a few minutes with a heavy load on your head. You get tired. You Animals too get engaged in activities. For have exerted yourself and have spent quite a example, they may jump and run. They have bit of your energy. Are you doing work on the to fight, move away from enemies, find food load? The way we understand the term ‘work’ or find a safe place to live. Also, we engage in science, work is not done. some animals to lift weights, carry loads, pull carts or plough fields. All such activities You climb up the steps of a staircase and require energy. reach the second floor of a building just to see the landscape from there. You may even Think of machines. List the machines that climb up a tall tree. If we apply the scientific you have come across. What do they need for definition, these activities involve a lot of work. their working? Why do some engines require fuel like petrol and diesel? Why do living In day-to-day life, we consider any useful beings and machines need energy? physical or mental labour as work. Activities like playing in a field, talking with friends, 11.1 Work humming a tune, watching a movie, attending a function are sometimes not considered to What is work? There is a difference in the be work. What constitutes ‘work’ depends way we use the term ‘work’ in day-to-day life on the way we define it. We use and define and the way we use it in science. To make the term work differently in science. To this point clear let us consider a few examples. understand this let us do the following activities: 11.1.1 NOT MUCH ‘WORK’ IN SPITE OF WORKING HARD! Activity _____________ 11.1 Kamali is preparing for examinations. She • We have discussed in the above spends lot of time in studies. She reads books, paragraphs a number of activities which we normally consider to be work 2018-19

in day-to-day life. For each of these Activity _____________ 11.3 activities, ask the following questions and answer them: • Think of situations when the object is (i) What is the work being done on? not displaced in spite of a force acting (ii) What is happening to the object? on it. (iii) Who (what) is doing the work? • Also think of situations when an object 11.1.2 SCIENTIFIC CONCEPTION OF WORK gets displaced in the absence of a force acting on it. To understand the way we view work and define work from the point of view of science, • List all the situations that you can let us consider some situations: think of for each. Push a pebble lying on a surface. The • Discuss with your friends whether pebble moves through a distance. You exerted work is done in these situations. a force on the pebble and the pebble got displaced. In this situation work is done. 11.1.3 WORK DONE BY A CONSTANT FORCE A girl pulls a trolley and the trolley moves How is work defined in science? To through a distance. The girl has exerted a force on the trolley and it is displaced. understand this, we shall first consider the Therefore, work is done. case when the force is acting in the direction Lift a book through a height. To do this you must apply a force. The book rises up. of displacement. There is a force applied on the book and the book has moved. Hence, work is done. Let a constant force, F act on an object. A closer look at the above situations Let the object be displaced through a reveals that two conditions need to be satisfied for work to be done: (i) a force should distance, s in the direction of the force (Fig. act on an object, and (ii) the object must be displaced. 11.1). Let W be the work done. We define work If any one of the above conditions does to be equal to the product of the force and not exist, work is not done. This is the way we view work in science. displacement. A bullock is pulling a cart. The cart Work done = force × displacement moves. There is a force on the cart and the cart has moved. Do you think that work is W = Fs (11.1) done in this situation? Fig. 11.1 Activity _____________ 11.2 Thus, work done by a force acting on an • Think of some situations from your object is equal to the magnitude of the force daily life involving work. multiplied by the distance moved in the direction of the force. Work has only • List them. magnitude and no direction. • Discuss with your friends whether In Eq. (11.1), if F = 1 N and s = 1 m then work is being done in each situation. the work done by the force will be 1 N m. • Try to reason out your response. Here the unit of work is newton metre (N m) • If work is done, which is the force acting or joule (J). Thus 1 J is the amount of work on the object? • What is the object on which the work is done? • What happens to the object on which work is done? WORK AND ENERGY 147

done on an object when a force of 1 N Fig. 11.4 displaces it by 1 m along the line of action of the force. Consider a situation in which an object is moving with a uniform velocity along a Look at Eq. (11.1) carefully. What is the particular direction. Now a retarding force, F, work done when the force on the object is is applied in the opposite direction. That is, zero? What would be the work done when the angle between the two directions is 180º. the displacement of the object is zero? Refer Let the object stop after a displacement s. In to the conditions that are to be satisfied to such a situation, the work done by the force, say that work is done. F is taken as negative and denoted by the minus sign. The work done by the force is Example 11.1 A force of 5 N is acting on an F × (–s) or (–F × s). object. The object is displaced through 2 m in the direction of the force (Fig. It is clear from the above discussion that 11.2). If the force acts on the object all the work done by a force can be either positive through the displacement, then work or negative. To understand this, let us do the done is 5 N × 2 m =10 N m or 10 J. following activity: Fig. 11.2 Activity _____________ 11.4 Q uestion1. A force of 7 N acts on an object. • Lift an object up. Work is done by the The displacement is, say 8 m, in force exerted by you on the object. The the direction of the force object moves upwards. The force you (Fig. 11.3). Let us take it that the exerted is in the direction of force acts on the object through displacement. However, there is the the displacement. What is the force of gravity acting on the object. work done in this case? • Which one of these forces is doing Fig. 11.3 positive work? Consider another situation in which the • Which one is doing negative work? force and the displacement are in the same • Give reasons. direction: a baby pulling a toy car parallel to the ground, as shown in Fig. 11.4. The baby Work done is negative when the force acts has exerted a force in the direction of opposite to the direction of displacement. displacement of the car. In this situation, the Work done is positive when the force is in the work done will be equal to the product of the direction of displacement. force and displacement. In such situations, the work done by the force is taken as positive. Example 11.2 A porter lifts a luggage of 15 kg from the ground and puts it on his head 1.5 m above the ground. Calculate the work done by him on the luggage. Solution: Mass of luggage, m = 15 kg and displacement, s = 1.5 m. 148 SCIENCE

Work done, W = F × s = mg × s raised hammer falls on a nail placed on a = 15 kg × 10 m s-2 × 1.5 m piece of wood, it drives the nail into the wood. = 225 kg m s-2 m We have also observed children winding a = 225 N m = 225 J toy (such as a toy car) and when the toy is placed on the floor, it starts moving. When a Work done is 225 J. balloon is filled with air and we press it we notice a change in its shape. As long as we Q uestions press it gently, it can come back to its original 1. When do we say that work is shape when the force is withdrawn. However, done? if we press the balloon hard, it can even 2. Write an expression for the work explode producing a blasting sound. In all done when a force is acting on these examples, the objects acquire, through an object in the direction of its different means, the capability of doing work. displacement. An object having a capability to do work is 3. Define 1 J of work. said to possess energy. The object which does 4. A pair of bullocks exerts a force the work loses energy and the object on which of 140 N on a plough. The field the work is done gains energy. being ploughed is 15 m long. How much work is done in How does an object with energy do work? ploughing the length of the field? An object that possesses energy can exert a force on another object. When this happens, 11.2 Energy energy is transferred from the former to the latter. The second object may move as it Life is impossible without energy. The demand receives energy and therefore do some work. for energy is ever increasing. Where do we Thus, the first object had a capacity to do get energy from? The Sun is the biggest work. This implies that any object that natural source of energy to us. Many of our possesses energy can do work. energy sources are derived from the Sun. We can also get energy from the nuclei of atoms, The energy possessed by an object is thus the interior of the earth, and the tides. Can measured in terms of its capacity of doing you think of other sources of energy? work. The unit of energy is, therefore, the same as that of work, that is, joule (J). 1 J is the Activity _____________ 11.5 energy required to do 1 joule of work. Sometimes a larger unit of energy called kilo • A few sources of energy are listed above. joule (kJ) is used. 1 kJ equals 1000 J. There are many other sources of energy. List them. 11.2.1 FORMS OF ENERGY • Discuss in small groups how certain Luckily the world we live in provides energy sources of energy are due to the Sun. in many different forms. The various forms include mechanical energy (potential energy • Are there sources of energy which are + kinetic energy), heat energy, chemical not due to the Sun? energy, electrical energy and light energy. The word energy is very often used in our Think it over ! daily life, but in science we give it a definite and precise meaning. Let us consider the How do you know that some entity is a following examples: when a fast moving form of energy? Discuss with your friends cricket ball hits a stationary wicket, the wicket and teachers. is thrown away. Similarly, an object when raised to a certain height gets the capability to do work. You must have seen that when a WORK AND ENERGY 149

James Prescott Joule was an outstanding British physicist. He is best known for his research in electricity and thermodynamics. Amongst other things, he Fig. 11.5 formulated a law • The trolley moves forward and hits the wooden block. James Prescott Joule for the heating (1818 – 1889) effect of electric • Fix a stop on the table in such a current. He also manner that the trolley stops after hitting the block. The block gets verified experimentally the law of displaced. conservation of energy and discovered • Note down the displacement of the block. This means work is done on the the value of the mechanical equivalent block by the trolley as the block has gained energy. of heat. The unit of energy and work • From where does this energy come? called joule, is named after him. • Repeat this activity by increasing the 11.2.2 KINETIC ENERGY mass on the pan. In which case is the displacement more? Activity _____________ 11.6 • In which case is the work done more? • In this activity, the moving trolley does • Take a heavy ball. Drop it on a thick work and hence it possesses energy. bed of sand. A wet bed of sand would be better. Drop the ball on the sand A moving object can do work. An object bed from height of about 25 cm. The moving faster can do more work than an ball creates a depression. identical object moving relatively slow. A moving bullet, blowing wind, a rotating wheel, • Repeat this activity from heights of a speeding stone can do work. How does a 50 cm, 1m and 1.5 m. bullet pierce the target? How does the wind move the blades of a windmill? Objects in • Ensure that all the depressions are motion possess energy. We call this energy distinctly visible. kinetic energy. • Mark the depressions to indicate the A falling coconut, a speeding car, a rolling height from which the ball was stone, a flying aircraft, flowing water, blowing dropped. wind, a running athlete etc. possess kinetic energy. In short, kinetic energy is the energy • Compare their depths. possessed by an object due to its motion. The • Which one of them is deepest? kinetic energy of an object increases with its • Which one is shallowest? Why? speed. • What has caused the ball to make a How much energy is possessed by a deeper dent? moving body by virtue of its motion? By • Discuss and analyse. definition, we say that the kinetic energy of a body moving with a certain velocity is equal to Activity _____________ 11.7 the work done on it to make it acquire that velocity. • Set up the apparatus as shown in Fig. 11.5. SCIENCE • Place a wooden block of known mass in front of the trolley at a convenient fixed distance. • Place a known mass on the pan so that the trolley starts moving. 150

Let us now express the kinetic energy of Solution: an object in the form of an equation. Consider Mass of the object, m = 15 kg, velocity of the object, v = 4 m s–1. an object of mass, m moving with a uniform From Eq. (11.5), velocity, u. Let it now be displaced through a distance s when a constant force, F acts on it in the direction of its displacement. From Ek = 1m v2 2 Eq. (11.1), the work done, W is F s. The work done on the object will cause a change in its 1 = 2 × 15 kg × 4 m s–1 × 4 m s–1 velocity. Let its velocity change from u to v. = 120 J Let a be the acceleration produced. The kinetic energy of the object is 120 J. In section 8.5, we studied three equations of motion. The relation connecting the initial velocity (u) and final velocity (v) of an object moving with a uniform acceleration a, and the displacement, s is Example 11.4 What is the work to be done to increase the velocity of a car from v2 – u2 = 2a s (8.7) 30 km h–1 to 60 km h–1 if the mass of This gives the car is 1500 kg? v2 – u2 (11.2) Solution: s= 2a From section 9.4, we know F = m a. Thus, Mass of the car, m =1500 kg, initial velocity of car, u = 30 km h–1 using (Eq. 11.2) in Eq. (11.1), we can write the work done by the force, F as v2 - u2 30 × 1000 m W =ma × = 60 × 60s = 25/3 m s–1. 2a or Similarly, the final velocity of the car, ( )W = 1 m v2 – u 2 (11.3) v = 60 km h–1 2 = 50/3 m s–1. If the object is starting from its stationary Therefore, the initial kinetic energy of the car, position, that is, u = 0, then W = 1m v2 (11.4) E = 1m u2 2 ki 2 It is clear that the work done is equal to the change in the kinetic energy of an object. 1 = 2 × 1500 kg × (25/3 m s–1)2 If u = 0, the work done will be 1 m v2 . = 156250/3 J. 2 Thus, the kinetic energy possessed by an The final kinetic energy of the car, object of mass, m and moving with a uniform velocity, v is 1 Ekf = 2 × 1500 kg × (50/3 m s–1)2 Ek = 1 m v2 (11.5) 2 = 625000/3 J. Thus, the work done = Change in kinetic energy Example 11.3 An object of mass 15 kg is = Ekf – Eki moving with a uniform velocity of 4 = 156250 J. m s–1. What is the kinetic energy possessed by the object? WORK AND ENERGY 151

Q uestions Activity ___________ 11.11 1. What is the kinetic energy of an object? • Lift an object through a certain 2. Write an expression for the kinetic height. The object can now do work. energy of an object. It begins to fall when released. 3. The kinetic energy of an object of mass, m moving with a velocity • This implies that it has acquired some of 5 m s–1 is 25 J. What will be its energy. If raised to a greater height it kinetic energy when its velocity can do more work and hence possesses is doubled? What will be its more energy. kinetic energy when its velocity is increased three times? • From where did it get the energy? Think and discuss. 11.2.3 POTENTIAL ENERGY In the above situations, the energy gets Activity _____________ 11.8 stored due to the work done on the object. The energy transferred to an object is stored • Take a rubber band. as potential energy if it is not used to cause a • Hold it at one end and pull from the change in the velocity or speed of the object. other. The band stretches. You transfer energy when you stretch a • Release the band at one of the ends. rubber band. The energy transferred to the • What happens? band is its potential energy. You do work while • The band will tend to regain its original winding the key of a toy car. The energy transferred to the spring inside is stored as length. Obviously the band had potential energy. The potential energy acquired energy in its stretched possessed by the object is the energy present position. in it by virtue of its position or configuration. • How did it acquire energy when stretched? Activity ____________11.12 Activity _____________ 11.9 • Take a bamboo stick and make a bow as shown in Fig. 11.6. • Take a slinky as shown below. • Ask a friend to hold one of its ends. • Place an arrow made of a light stick on it with one end supported by the You hold the other end and move away stretched string. from your friend. Now you release the slinky. • Now stretch the string and release the arrow. • What happened? • How did the slinky acquire energy when • Notice the arrow flying off the bow. Notice the change in the shape of the stretched? bow. • Would the slinky acquire energy when • The potential energy stored in the bow it is compressed? due to the change of shape is thus used in the form of kinetic energy in Activity ____________11.10 throwing off the arrow. • Take a toy car. Wind it using its key. Fig.11.6: An arrow and the stretched string • Place the car on the ground. on the bow. • Did it move? • From where did it acquire energy? SCIENCE • Does the energy acquired depend on the number of windings? • How can you test this? 152

11.2.4 POTENTIAL ENERGY OF AN OBJECT The potential energy of an object atMore to know a height depends on the ground level AT A HEIGHT or the zero level you choose. An object in a given position can have a An object increases its energy when raised certain potential energy with respect through a height. This is because work is to one level and a different value of done on it against gravity while it is being potential energy with respect to raised. The energy present in such an object another level. is the gravitational potential energy. It is useful to note that the work done by The gravitational potential energy of an gravity depends on the difference in vertical object at a point above the ground is defined heights of the initial and final positions of as the work done in raising it from the ground the object and not on the path along which to that point against gravity. the object is moved. Fig. 11.8 shows a case where a block is raised from position A to B It is easy to arrive at an expression for by taking two different paths. Let the height the gravitational potential energy of an object AB = h. In both the situations the work done at a height. on the object is mgh. Fig. 11.7 Consider an object of mass, m. Let it be Fig. 11.8 raised through a height, h from the ground. Example 11.5 Find the energy possessed by an object of mass 10 kg when it is at A force is required to do this. The minimum a height of 6 m above the ground. Given, g = 9.8 m s–2. force required to raise the object is equal to Solution: the weight of the object, mg. The object gains Mass of the object, m = 10 kg, displacement (height), h = 6 m, and energy equal to the work done on it. Let the acceleration due to gravity, g = 9.8 m s–2. From Eq. (11.6), work done on the object against gravity be Potential energy = mgh = 10 kg × 9.8 m s–2 × 6 m W. That is, = 588 J. The potential energy is 588 J. work done, W = force × displacement 153 = mg × h = mgh Since work done on the object is equal to mgh, an energy equal to mgh units is gained by the object. This is the potential energy (EP) of the object. Ep = mgh (11.7) WORK AND ENERGY

Example 11.6 An object of mass 12 kg is 11.2.6 LAW OF CONSERVATION OF ENERGY at a certain height above the ground. If the potential energy of the object is In activities 11.13 and 11.14, we learnt that 480 J, find the height at which the object is with respect to the ground. the form of energy can be changed from one Given, g = 10 m s–2. form to another. What happens to the total Solution: energy of a system during or after the process? Mass of the object, m = 12 kg, potential energy, Ep = 480 J. Whenever energy gets transformed, the total Ep = mgh energy remains unchanged. This is the law of 480 J = 12 kg × 10 m s–2 × h conservation of energy. According to this law, 480 J h = 120 kg m s–2 = 4 m. energy can only be converted from one form The object is at the height of 4 m. to another; it can neither be created or 11.2.5 ARE VARIOUS ENERGY FORMS destroyed. The total energy before and after INTERCONVERTIBLE? the transformation remains the same. The law Can we convert energy from one form to of conservation of energy is valid in another? We find in nature a number of instances of conversion of energy from one all situations and for all kinds of form to another. transformations. Activity ____________11.13 Consider a simple example. Let an object • Sit in small groups. • Discuss the various ways of energy of mass, m be made to fall freely from a height, conversion in nature. h. At the start, the potential energy is mgh and • Discuss following questions in your kinetic energy is zero. Why is the kinetic group: (a) How do green plants produce food? energy zero? It is zero because its velocity is (b) Where do they get their energy from? (c) Why does the air move from place zero. The total energy of the object is thus mgh. to place? As it falls, its potential energy will change into (d) How are fuels, such as coal and kinetic energy. If v is the velocity of the object petroleum formed? (e) What kinds of energy conversions at a given instant, the kinetic energy would be sustain the water cycle? ½mv2. As the fall of the object continues, the Activity ___________ 11.14 potential energy would decrease while the • Many of the human activities and the kinetic energy would increase. When the object gadgets we use involve conversion of energy from one form to another. is about to reach the ground, h = 0 and v will • Make a list of such activities and be the highest. Therefore, the kinetic energy gadgets. would be the largest and potential energy the • Identify in each activity/gadget the kind of energy conversion that takes least. However, the sum of the potential energy place. and kinetic energy of the object would be the same at all points. That is, potential energy + kinetic energy = constant or mgh + 1 mv2= constant. (11.7) 2 The sum of kinetic energy and potential energy of an object is its total mechanical energy. We find that during the free fall of the object, the decrease in potential energy, at any point in its path, appears as an equal amount of increase in kinetic energy. (Here the effect of air resistance on the motion of the object has been ignored.) There is thus a continual transformation of gravitational potential energy into kinetic energy. 154 SCIENCE

Activity ___________ 11.15 A stronger person may do certain work in relatively less time. A more powerful vehicle • An object of mass 20 kg is dropped would complete a journey in a shorter time from a height of 4 m. Fill in the blanks than a less powerful one. We talk of the power in the following table by computing of machines like motorbikes and motorcars. the potential energy and kinetic The speed with which these vehicles change energy in each case. energy or do work is a basis for their classification. Power measures the speed of Height at Potential Kinetic Ep + Ek work done, that is, how fast or slow work is which object energy energy done. Power is defined as the rate of doing work or the rate of transfer of energy. If an is located (Ep= mgh) (Ek = mv2/2) agent does a work W in time t, then power is given by: m J JJ 4 Power = work/time 3 2 or P=W (11.8) 1 t Just above the ground The unit of power is watt [in honour of • For simplifying the calculations, take James Watt (1736 – 1819)] having the symbol the value of g as 10 m s–2. W. 1 watt is the power of an agent, which Think it over ! does work at the rate of 1 joule per second. What would have happened if nature had We can also say that power is 1 W when the not allowed the transformation of energy? rate of consumption of energy is 1 J s–1. There is a view that life could not have been possible without transformation of 1 watt = 1 joule/second or 1 W = 1 J s–1. energy. Do you agree with this? We express larger rates of energy transfer in kilowatts (kW). 1 kilowatt = 1000 watts 1 kW = 1000 W 1 kW = 1000 J s–1. 11.3 Rate of Doing Work The power of an agent may vary with time. Do all of us work at the same rate? Do This means that the agent may be doing work machines consume or transfer energy at the same rate? Agents that transfer energy do at different rates at different intervals of time. work at different rates. Let us understand this Therefore the concept of average power is from the following activity: useful. We obtain average power by dividing Activity ___________ 11.16 the total energy consumed by the total time • Consider two children, say A and B. taken. Let us say they weigh the same. Both start climbing up a rope separately. Example 11.7 Two girls, each of weight 400 Both reach a height of 8 m. Let us say N climb up a rope through a height of 8 A takes 15 s while B takes 20 s to m. We name one of the girls A and the accomplish the task. other B. Girl A takes 20 s while B takes 50 s to accomplish this task. What is the • What is the work done by each? power expended by each girl? • The work done is the same. However, Solution: A has taken less time than B to do the work. (i) Power expended by girl A: • Who has done more work in a given Weight of the girl, mg = 400 N time, say in 1 s? Displacement (height), h = 8 m WORK AND ENERGY 155

Time taken, t = 20 s Q uestions From Eq. (11.8), 1. What is power? 2. Define 1 watt of power. Power, P = Work done/time taken 3. A lamp consumes 1000 J of electrical energy in 10 s. What is mgh its power? =t 4. Define average power. 400 N × 8 m 11.3.1 COMMERCIAL UNIT OF ENERGY = 20s = 160 W. The unit joule is too small and hence is inconvenient to express large quantities of (ii) Power expended by girl B: energy. We use a bigger unit of energy called Weight of the girl, mg = 400 N kilowatt hour (kW h). Displacement (height), h = 8 m Time taken, t = 50 s What is 1 kW h? Let us say we have a machine that uses 1000 J of energy every mgh second. If this machine is used continuously Power, P = t for one hour, it will consume 1 kW h of energy. Thus, 1 kW h is the energy used in one hour 400 N × 8 m at the rate of 1000 J s–1 (or 1 kW). = 50 s 1 kW h = 1 kW ×1 h = 64 W. = 1000 W × 3600 s = 3600000 J Power expended by girl A is 160 W. Power expended by girl B is 64 W. 1 kW h = 3.6 × 106 J. The energy used in households, industries Example 11.8 A boy of mass 50 kg runs and commercial establishments are usually up a staircase of 45 steps in 9 s. If the expressed in kilowatt hour. For example, height of each step is 15 cm, find his electrical energy used during a month is power. Take g = 10 m s–2. expressed in terms of ‘units’. Here, 1 ‘unit’ means 1 kilowatt hour. Solution: Example 11.9 An electric bulb of 60 W is Weight of the boy, used for 6 h per day. Calculate the ‘units’ mg = 50 kg × 10 m s–2 = 500 N of energy consumed in one day by the bulb. Height of the staircase, h = 45 × 15/100 m = 6.75 m Solution: Time taken to climb, t = 9 s Power of electric bulb = 60 W From Eq. (11.8), = 0.06 kW. power, P = Work done/time taken Time used, t = 6 h mgh Energy = power × time taken =t = 0.06 kW × 6 h 500 N × 6.75 m = 0.36 kW h = 9s = 0.36 ‘units’. = 375 W. The energy consumed by the bulb is 0.36 ‘units’. Power is 375 W. 156 SCIENCE

Activity ___________ 11.17 • How many ‘units’ are used during night? • Tabulate your observations. • Take a close look at the electric meter • Draw inferences from the data. installed in your house. Observe its • Compare your observations with features closely. the details given in the monthly • Take the readings of the meter each electricity bill (One can also estimate day at 6.30 am and 6.30 pm. the electricity to be consumed by specific appliances by tabulating their • Do this activity for about a week. known wattages and hours of • How many ‘units’ are consumed operation). during day time? What you have learnt • Work done on an object is defined as the magnitude of the force multiplied by the distance moved by the object in the direction of the applied force. The unit of work is joule: 1 joule = 1 newton × 1 metre. • Work done on an object by a force would be zero if the displacement of the object is zero. • An object having capability to do work is said to possess energy. Energy has the same unit as that of work. • An object in motion possesses what is known as the kinetic energy of the object. An object of mass, m moving with velocity v has a kinetic energy of 1 mv2 . 2 • The energy possessed by a body due to its change in position or shape is called the potential energy. The gravitational potential energy of an object of mass, m raised through a height, h from the earth’s surface is given by m g h. • According to the law of conservation of energy, energy can only be transformed from one form to another; it can neither be created nor destroyed. The total energy before and after the transformation always remains constant. • Energy exists in nature in several forms such as kinetic energy, potential energy, heat energy, chemical energy etc. The sum of the kinetic and potential energies of an object is called its mechanical energy. • Power is defined as the rate of doing work. The SI unit of power is watt. 1 W = 1 J/s. • The energy used in one hour at the rate of 1kW is called 1 kW h. WORK AND ENERGY 157

Exercises 1. Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’. • Suma is swimming in a pond. • A donkey is carrying a load on its back. • A wind-mill is lifting water from a well. • A green plant is carrying out photosynthesis. • An engine is pulling a train. • Food grains are getting dried in the sun. • A sailboat is moving due to wind energy. 2. An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object? 3. A battery lights a bulb. Describe the energy changes involved in the process. 4. Certain force acting on a 20 kg mass changes its velocity from 5 m s–1 to 2 m s–1. Calculate the work done by the force. 5. A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer. 6. The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why? 7. What are the various energy transformations that occur when you are riding a bicycle? 8. Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going? 9. A certain household has consumed 250 units of energy during a month. How much energy is this in joules? 10. An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down. 11. What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer. 12. Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher. 158 SCIENCE

13. A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer. 14. An electric heater is rated 1500 W. How much energy does it use in 10 hours? 15. Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy? 16. An object of mass, m is moving with a constant velocity, v. How much work should be done on the object in order to bring the object to rest? 17. Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h? 18. In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero. 19. Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why? 20. Find the energy in kW h consumed in 10 hours by four devices of power 500 W each. 21. A freely falling object eventually stops on reaching the ground. What happenes to its kinetic energy? WORK AND ENERGY 159

C 12hapter SOUND Everyday we hear sounds from various Fig. 12.1: Vibrating tuning fork just touching the sources like humans, birds, bells, machines, suspended table tennis ball. vehicles, televisions, radios etc. Sound is a form of energy which produces a sensation Activity _____________ 12.2 of hearing in our ears. There are also other forms of energy like mechanical energy, light • Fill water in a beaker or a glass up to energy etc. We have talked about mechanical the brim. Gently touch the water surface energy in the previous chapters. You have with one of the prongs of the vibrating been taught about conservation of energy, tuning fork, as shown in Fig. 12.2. which states that we can neither create nor destroy energy. We can just change it from • Next dip the prongs of the vibrating one form to another. When you clap, a sound tuning fork in water, as shown in Fig. is produced. Can you produce sound without 12.3. utilising your energy? Which form of energy did you use to produce sound? In this • Observe what happens in both the chapter we are going to learn how sound is cases. produced and how it is transmitted through a medium and received by our ears. • Discuss with your friends why this happens. 12.1 Production of Sound Fig. 12.2: One of the prongs of the vibrating tuning Activity _____________ 12.1 fork touching the water surface. • Take a tuning fork and set it vibrating by striking its prong on a rubber pad. Bring it near your ear. • Do you hear any sound? • Touch one of the prongs of the vibrating tuning fork with your finger and share your experience with your friends. • Now, suspend a table tennis ball or a small plastic ball by a thread from a support [Take a big needle and a thread, put a knot at one end of the thread, and then with the help of the needle pass the thread through the ball]. Touch the ball gently with the prong of a vibrating tuning fork (Fig. 12.1). • Observe what happens and discuss with your friends. 2018-19

Fig. 12.3: Both the prongs of the vibrating tuning fork plucked vibrates and produces sound. If you have never done this, then do it and observe dipped in water. the vibration of the stretched rubber band. From the above activities what do you Activity _____________ 12.3 conclude? Can you produce sound without a vibrating object? • Make a list of different types of musical instruments and discuss with your In the above activities we have produced friends which part of the instrument sound by striking the tuning fork. We can vibrates to produce sound. also produce sound by plucking, scratching, rubbing, blowing or shaking different objects. 12.2 Propagation of Sound As per the above activities what do we do to the objects? We set the objects vibrating and Sound is produced by vibrating objects. The produce sound. Vibration means a kind of matter or substance through which sound rapid to and fro motion of an object. The is transmitted is called a medium. It can be sound of the human voice is produced due solid, liquid or gas. Sound moves through a to vibrations in the vocal cords. When a bird medium from the point of generation to the flaps its wings, do you hear any sound? Think listener. When an object vibrates, it sets the how the buzzing sound accompanying a bee particles of the medium around it vibrating. is produced. A stretched rubber band when The particles do not travel all the way from the vibrating object to the ear. A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle. As a result of which the adjacent particle gets displaced from its position of rest. After displacing the adjacent particle the first particle comes back to its original position. This process continues in the medium till the sound reaches your ear. The disturbance created by a source of sound in Can sound make a light spot dance? Take a tin-can. Remove both ends to make it a hollow cylinder. Take a balloon and stretch it over the can, then wrap a rubber band around the balloon. Take a small piece of mirror. Use a drop of glue to stick the piece of mirror to the balloon. Allow the light through a slit to fall on the mirror. After reflection the light spot is seen on the wall, as shown in Fig. 12.4. Talk or shout directly into the open end of the can and observe the dancing light spot on the wall. Discuss with your friends what makes the light spot dance. Fig. 12.4: A beam of light from a light source is made to fall on a mirror. The reflected light is falling on the wall. SOUND 161

the medium travels through the medium and Q uestion not the particles of the medium. 1. How does the sound produced by a vibrating object in a medium A wave is a disturbance that moves reach your ear? through a medium when the particles of the medium set neighbouring particles into 12.2.1 SOUND NEEDS A MEDIUM TO TRAVEL motion. They in turn produce similar motion in others. The particles of the medium do not Sound is a mechanical wave and needs a move forward themselves, but the material medium like air, water, steel etc. for disturbance is carried forward. This is what its propagation. It cannot travel through happens during propagation of sound in a vacuum, which can be demonstrated by the medium, hence sound can be visualised as a following experiment. wave. Sound waves are characterised by the motion of particles in the medium and are Take an electric bell and an airtight glass called mechanical waves. bell jar. The electric bell is suspended inside the airtight bell jar. The bell jar is connected Air is the most common medium through to a vacuum pump, as shown in Fig. 12.6. If which sound travels. When a vibrating object you press the switch you will be able to hear moves forward, it pushes and compresses the the bell. Now start the vacuum pump. When air in front of it creating a region of high the air in the jar is pumped out gradually, pressure. This region is called a compression the sound becomes fainter, although the (C), as shown in Fig. 12.5. This compression same current is passing through the bell. starts to move away from the vibrating object. After some time when less air is left inside When the vibrating object moves backwards, the bell jar you will hear a very feeble sound. it creates a region of low pressure called What will happen if the air is removed rarefaction (R), as shown in Fig. 12.5. As the completely? Will you still be able to hear the object moves back and forth rapidly, a series sound of the bell? of compressions and rarefactions is created in the air. These make the sound wave that propagates through the medium. Compression is the region of high pressure and rarefaction is the region of low pressure. Pressure is related to the number of particles of a medium in a given volume. More density of the particles in the medium gives more pressure and vice versa. Thus, propagation of sound can be visualised as propagation of density variations or pressure variations in the medium. Fig. 12.5: A vibrating object creating a series of Fig. 12.6: Bell jar experiment showing sound cannot compressions (C) and rarefactions (R) in travel in vacuum. the medium. 162 SCIENCE

Q uestions waves. In these waves the individual particles 1. Explain how sound is produced of the medium move in a direction parallel to by your school bell. the direction of propagation of the 2. Why are sound waves called disturbance. The particles do not move from mechanical waves? one place to another but they simply oscillate 3. Suppose you and your friend are back and forth about their position of rest. on the moon. Will you be able to This is exactly how a sound wave propagates, hear any sound produced by hence sound waves are longitudinal waves. your friend? There is also another type of wave, called 12.2.2 SOUND WAVES ARE LONGITUDINAL a transverse wave. In a transverse wave particles do not oscillate along the direction WAVES of wave propagation but oscillate up and down about their mean position as the wave travels. Activity _____________ 12.4 Thus, a transverse wave is the one in which the individual particles of the medium move • Take a slinky. Ask your friend to hold about their mean positions in a direction one end. You hold the other end. perpendicular to the direction of wave Now stretch the slinky as shown in propagation. When we drop a pebble in a Fig. 12.7 (a). Then give it a sharp push pond, the waves you see on the water surface towards your friend. is an example of transverse wave. Light is a transverse wave but for light, the oscillations • What do you notice? If you move your are not of the medium particles or their hand pushing and pulling the slinky pressure or density – it is not a mechanical alternatively, what will you observe? wave. You will come to know more about transverse waves in higher classes. • If you mark a dot on the slinky, you will observe that the dot on the slinky 12.2.3 CHARACTERISTICS OF A SOUND will move back and forth parallel to the direction of the propagation of the WAVE disturbance. We can describe a sound wave by its (a) • frequency • amplitude and (b) • speed. Fig. 12.7: Longitudinal wave in a slinky. A sound wave in graphic form is shown in Fig. 12.8(c), which represents how density The regions where the coils become closer and pressure change when the sound wave are called compressions (C) and the regions moves in the medium. The density as well as where the coils are further apart are called the pressure of the medium at a given time rarefactions (R). As we already know, sound varies with distance, above and below the propagates in the medium as a series of average value of density and pressure. compressions and rarefactions. Now, we can Fig. 12.8(a) and Fig. 12.8(b) represent the compare the propagation of disturbance in a density and pressure variations, respectively, slinky with the sound propagation in the as a sound wave propagates in the medium. medium. These waves are called longitudinal Compressions are the regions where particles are crowded together and represented by the upper portion of the curve in Fig. 12.8(c). The peak represents the region of maximum compression. Thus, compressions are regions where density as SOUND 163

Fig. 12.8: Sound propagates as density or pressure variations as shown in (a) and (b), (c) represents graphically the density and pressure variations. well as pressure is high. Rarefactions are the photoelectric effect which was later regions of low pressure where particles are explained by Albert Einstein. The SI unit of spread apart and are represented by the frequency was named as hertz in his honour. valley, that is, the lower portion of the curve in Fig. 12.8(c). A peak is called the crest and a Frequency tells us how frequently an event valley is called the trough of a wave. occurs. Suppose you are beating a drum. How many times you are beating the drum in unit The distance between two consecutive time is called the frequency of your beating compressions (C) or two consecutive the drum. We know that when sound is rarefactions (R) is called the wavelength, as propagated through a medium, the density shown in Fig. 12.8(c), The wavelength is of the medium oscillates between a maximum usually represented by λ (Greek letter value and a minimum value. The change in lambda). Its SI unit is metre (m). density from the maximum value to the minimum value, then again to the maximum Heinrich Rudolph Hertz value, makes one complete oscillation. The was born on 22 February number of such oscillations per unit time is 1857 in Hamburg, the frequency of the sound wave. If we can Germany and educated at count the number of the compressions or the University of Berlin. He rarefactions that cross us per unit time, we confirmed J.C. Maxwell’s will get the frequency of the sound wave. It is electromagnetic theory by usually represented by ν (Greek letter, nu). Its his experiments. He laid the SI unit is hertz (symbol, Hz). H. R. Hertz foundation for future development of radio, telephone, telegraph The time taken by two consecutive and even television. He also discovered the compressions or rarefactions to cross a fixed point is called the time period of the wave. In 164 SCIENCE

other words, we can say that the time taken The magnitude of the maximum for one complete oscillation is called the time disturbance in the medium on either side of period of the sound wave. It is represented by the mean value is called the amplitude of the the symbol T. Its SI unit is second (s). wave. It is usually represented by the letter A, Frequency and time period are related as as shown in Fig. 12.8(c). For sound its unit follows: will be that of density or pressure. ν= 1 The loudness or softness of a sound is . determined basically by its amplitude. The amplitude of the sound wave depends upon T the force with which an object is made to A violin and a flute may both be played vibrate. If we strike a table lightly, we hear a at the same time in an orchestra. Both soft sound because we produce a sound wave sounds travel through the same medium, of less energy (amplitude). If we hit the table that is, air and arrive at our ear at the same hard we hear a louder sound. Can you tell time. Both sounds travel at the same speed why? A sound wave spreads out from its irrespective of the source. But the sounds source. As it moves away from the source its we receive are different. This is due to the amplitude as well as its loudness decreases. different characteristics associated with the Louder sound can travel a larger distance as sound. Pitch is one of the characteristics. it is associated with higher energy. Fig. 12.10 How the brain interprets the frequency of shows the wave shapes of a loud and a soft an emitted sound is called its pitch. The faster sound of the same frequency. the vibration of the source, the higher is the frequency and the higher is the pitch, as shown in Fig. 12.9. Thus, a high pitch sound corresponds to more number of compressions and rarefactions passing a fixed point per unit time. Objects of different sizes and conditions vibrate at different frequencies to produce sounds of different pitch. Fig. 12.10: Soft sound has small amplitude and louder sound has large amplitude. Fig. 12.9: Low pitch sound has low frequency and The quality or timber of sound is that high pitch of sound has high frequency. characteristic which enables us to distinguish one sound from another having the same pitch and loudness. The sound which is more SOUND 165

pleasant is said to be of a rich quality. A sound The time taken by the wave to travel a of single frequency is called a tone. The sound distance, d of 1.5 km is which is produced due to a mixture of several frequencies is called a note and is pleasant to Thus sound will take 2.1 s to travel a listen to. Noise is unpleasant to the ear! Music distance of 1.5 km. is pleasant to hear and is of rich quality. Q uestions Q uestions 1. What are wavelength, frequency, 1. Which wave property determines time period and amplitude of a (a) loudness, (b) pitch? sound wave? 2. Guess which sound has a higher 2. How are the wavelength and pitch: guitar or car horn? frequency of a sound wave related to its speed? The speed of sound is defined as the 3. Calculate the wavelength of a distance which a point on a wave, such as a sound wave whose frequency is compression or a rarefaction, travels per unit 220 Hz and speed is 440 m/s in time. a given medium. We know, 4. A person is listening to a tone of 500 Hz sitting at a distance of speed, v = distance / time 450 m from the source of the sound. What is the time interval λ between successive compressions =T from the source? Here λ is the wavelength of the sound wave. It is the distance travelled by the sound wave The amount of sound energy passing each in one time period (T) of the wave. Thus, second through unit area is called the intensity of sound. We sometimes use the v=λν terms “loudness” and “intensity” interchangeably, but they are not the same. or v = λ ν Loudness is a measure of the response of the ear to the sound. Even when two sounds are That is, speed = wavelength × frequency. of equal intensity, we may hear one as louder than the other simply because our ear detects The speed of sound remains almost the it better. same for all frequencies in a given medium under the same physical conditions. Q uestion 1. Distinguish between loudness Example 12.1 A sound wave has a and intensity of sound. frequency of 2 kHz and wave length 35 cm. How long will it take to travel 12.2.4 SPEED OF SOUND IN DIFFERENT 1.5 km? MEDIA Solution: Sound propagates through a medium at a Given, finite speed. The sound of a thunder is heard Frequency, ν = 2 kHz = 2000 Hz a little later than the flash of light is seen. Wavelength, λ = 35 cm = 0.35 m We know that speed, v of the wave = wavelength × frequency v =λν = 0.35 m × 2000 Hz = 700 m/s 166 SCIENCE

So, we can make out that sound travels with Sonic boom: When the speed of any object a speed which is much less than the speed of exceeds the speed of sound it is said to be light. The speed of sound depends on the travelling at supersonic speed. Bullets, jet properties of the medium through which it aircrafts etc. often travel at supersonic travels. You will learn about this dependence speeds. When a sound, producing source in higher classes. The speed of sound in a moves with a speed higher than that of medium depends on temperature of the sound, it produces shock waves in air. medium. The speed of sound decreases when These shock waves carry a large amount we go from solid to gaseous state. In any of energy. The air pressure variation medium as we increase the temperature, the associated with this type of shock waves speed of sound increases. For example, the produces a very sharp and loud sound speed of sound in air is 331 m s–1 at 0 ºC and called the “sonic boom”. The shock waves 344 m s–1 at 22 ºC. The speeds of sound at a produced by a supersonic aircraft have particular temperature in various media are enough energy to shatter window glass and listed in Table 12.1. You need not memorise even damage buildings. the values. 12.3 Reflection of Sound Table 12.1: Speed of sound in different media at 25 ºC Sound bounces off a solid or a liquid like a rubber ball bounces off a wall. Like light, sound State Substance Speed in m/s gets reflected at the surface of a solid or liquid Solids and follows the same laws of reflection as you Aluminium 6420 have studied in earlier classes. The directions Nickel 6040 in which the sound is incident and is reflected Steel 5960 make equal angles with the normal to the Iron 5950 reflecting surface at the point of incidence, and Brass 4700 the three are in the same plane. An obstacle of Glass (Flint) 3980 large size which may be polished or rough is needed for the reflection of sound waves. Liquids Water (Sea) 1531 Water (distilled) 1498 Activity _____________ 12.5 Ethanol 1207 Methanol 1103 • Take two identical pipes, as shown in Fig. 12.11. You can make the pipes Gases Hydrogen 1284 using chart paper. The length of the Helium 965 pipes should be sufficiently long Air 346 as shown. Oxygen 316 Sulphur dioxide 213 • Arrange them on a table near a wall. • Keep a clock near the open end of one Q uestion 1. In which of the three media, air, of the pipes and try to hear the sound water or iron, does sound travel of the clock through the other pipe. the fastest at a particular • Adjust the position of the pipes so temperature? that you can best hear the sound of the clock. • Now, measure the angles of incidence and reflection and see the relationship between the angles. • Lift the pipe on the right vertically to a small height and observe what happens. (In place of a clock, a mobile phone on vibrating mode may also be used.) SOUND 167

excessive reverberation is highly undesirable. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fibreboard, rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties. Fig. 12.11: Reflection of sound Example 12.2 A person clapped his hands near a cliff and heard the echo after 2 s. 12.3.1 ECHO What is the distance of the cliff from the person if the speed of the sound, v is If we shout or clap near a suitable reflecting taken as 346 m s–1? object such as a tall building or a mountain, we will hear the same sound again a little Solution: later. This sound which we hear is called an echo. The sensation of sound persists in our Given, brain for about 0.1 s. To hear a distinct echo Speed of sound, v = 346 m s–1 the time interval between the original sound Time taken for hearing the echo, and the reflected one must be at least 0.1s. t=2s If we take the speed of sound to be 344 m/s Distance travelled by the sound at a given temperature, say at 22 ºC in air, the sound must go to the obstacle and reach = v × t = 346 m s–1 × 2 s = 692 m back the ear of the listener on reflection after In 2 s sound has to travel twice the 0.1s. Hence, the total distance covered by the distance between the cliff and the sound from the point of generation to the person. Hence, the distance between reflecting surface and back should be at least the cliff and the person (344 m/s) × 0.1 s = 34.4 m. Thus, for hearing distinct echoes, the minimum distance of the = 692 m/2 = 346 m. obstacle from the source of sound must be half of this distance, that is, 17.2 m. This Q uestion distance will change with the temperature of 1. An echo is heard in 3 s. What is air. Echoes may be heard more than once the distance of the reflecting due to successive or multiple reflections. The surface from the source, given rolling of thunder is due to the successive that the speed of sound is reflections of the sound from a number of 342 m s–1? reflecting surfaces, such as the clouds and the land. 12.3.3 USES OF MULTIPLE REFLECTION 12.3.2 REVERBERATION OF SOUND A sound created in a big hall will persist by 1. Megaphones or loudhailers, horns, repeated reflection from the walls until it is musical instruments such as trumpets reduced to a value where it is no longer and shehanais, are all designed to audible. The repeated reflection that results send sound in a particular direction in this persistence of sound is called without spreading it in all directions, reverberation. In an auditorium or big hall as shown in Fig 12.12. 168 SCIENCE

soundboard may be placed behind the stage so that the sound, after reflecting from the sound board, spreads evenly across the width of the hall (Fig 12.15). Megaphone Horn Fig. 12.14: Curved ceiling of a conference hall. Fig 12.12: A megaphone and a horn. In these instruments, a tube followed by a conical opening reflects sound successively to guide most of the sound waves from the source in the forward direction towards the audience. 2. Stethoscope is a medical instrument used for listening to sounds produced within the body, mainly in the heart or lungs. In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound, as shown in Fig.12.13. Fig. 12.15: Sound board used in a big hall. Fig.12.13: Stethoscope Question 1. Why are the ceilings of concert halls curved? 3. Generally the ceilings of concert halls, 12.4 Range of Hearing conference halls and cinema halls are curved so that sound after reflection The audible range of sound for human beings reaches all corners of the hall, as shown extends from about 20 Hz to 20000 Hz (one in Fig 12.14. Sometimes a curved Hz = one cycle/s). Children under the age of SOUND 169

five and some animals, such as dogs can hear 12.5 Applications of Ultrasound up to 25 kHz (1 kHz = 1000 Hz). As people grow older their ears become less sensitive to Ultrasounds are high frequency waves. higher frequencies. Sounds of frequencies Ultrasounds are able to travel along well- below 20 Hz are called infrasonic sound or defined paths even in the presence of infrasound. If we could hear infrasound we obstacles. Ultrasounds are used extensively would hear the vibrations of a pendulum just in industries and for medical purposes. as we hear the vibrations of the wings of a bee. Rhinoceroses communicate using • Ultrasound is generally used to clean infrasound of frequency as low as 5 Hz. parts located in hard-to-reach places, Whales and elephants produce sound in the for example, spiral tube, odd shaped infrasound range. It is observed that some parts, electronic components etc. animals get disturbed before earthquakes. Objects to be cleaned are placed in a Earthquakes produce low-frequency cleaning solution and ultrasonic infrasound before the main shock waves waves are sent into the solution. Due begin which possibly alert the animals. to the high frequency, the particles of Frequencies higher than 20 kHz are called dust, grease and dirt get detached and ultrasonic sound or ultrasound. Ultrasound drop out. The objects thus get is produced by animals such as dolphins, thoroughly cleaned. bats and porpoises. Moths of certain families have very sensitive hearing equipment. These • Ultrasounds can be used to detect moths can hear the high frequency squeaks cracks and flaws in metal blocks. of the bat and know when a bat is flying Metallic components are generally nearby, and are able to escape capture. Rats used in construction of big structures also play games by producing ultrasound. like buildings, bridges, machines and also scientific equipment. The cracks Hearing Aid: People with hearing loss may or holes inside the metal blocks, which need a hearing aid. A hearing aid is an are invisible from outside reduces the electronic, battery operated device. The strength of the structure. Ultrasonic hearing aid receives sound through a waves are allowed to pass through the microphone. The microphone converts the metal block and detectors are used to sound waves to electrical signals. These detect the transmitted waves. If there electrical signals are amplified by an is even a small defect, the ultrasound amplifier. The amplified electrical signals gets reflected back indicating the are given to a speaker of the hearing aid. presence of the flaw or defect, as shown The speaker converts the amplified in Fig. 12.16. electrical signal to sound and sends to the ear for clear hearing. Q uestions Fig 12.16: Ultrasound is reflected back from the 1. What is the audible range of the defective locations inside a metal block. average human ear? 2. What is the range of frequencies SCIENCE associated with (a) Infrasound? (b) Ultrasound? 170

Ordinary sound of longer wavelengths Fig.12.17: Ultrasound sent by the transmitter and cannot be used for such purpose as it will bend around the corners of the defective received by the detector. location and enter the detector. The transmitter produces and transmits • Ultrasonic waves are made to reflect ultrasonic waves. These waves travel through from various parts of the heart and water and after striking the object on the form the image of the heart. This tech- seabed, get reflected back and are sensed nique is called ‘echocardiography’. by the detector. The detector converts the ultrasonic waves into electrical signals which • Ultrasound scanner is an instrument are appropriately interpreted. The distance which uses ultrasonic waves for of the object that reflected the sound wave getting images of internal organs of can be calculated by knowing the speed of the human body. A doctor may image sound in water and the time interval between the patient’s organs such as the liver, transmission and reception of the gall bladder, uterus, kidney, etc. It ultrasound. Let the time interval between helps the doctor to detect transmission and reception of ultrasound abnormalities, such as stones in the signal be t and the speed of sound through gall bladder and kidney or tumours seawater be v. The total distance, 2d travelled in different organs. In this technique by the ultrasound is then, 2d = v × t. the ultrasonic waves travel through the tissues of the body and get The above method is called echo-ranging. reflected from a region where there is The sonar technique is used to determine the a change of tissue density. These depth of the sea and to locate underwater waves are then converted into hills, valleys, submarine, icebergs, sunken electrical signals that are used to ship etc. generate images of the organ. These images are then displayed on a Example 12.3 A ship sends out ultrasound monitor or printed on a film. This that returns from the seabed and is technique is called ‘ultrasonography’. detected after 3.42 s. If the speed of Ultrasonography is also used for ultrasound through seawater is examination of the foetus during 1531 m/s, what is the distance of the pregnancy to detect congenial defects seabed from the ship? and growth abnormalities. Solution: • Ultrasound may be employed to break small ‘stones’ formed in the kidneys Given, into fine grains. These grains later get Time between transmission and flushed out with urine. detection, t = 3.42 s. 12.5.1 SONAR The acronym SONAR stands for SOund Navigation And Ranging. Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects. How does the sonar work? Sonar consists of a transmitter and a detector and is installed in a boat or a ship, as shown in Fig. 12.17. SOUND 171

Speed of ultrasound in sea water, 12.6 Structure of Human Ear v = 1531 m/s How do we hear? We are able to hear with Distance travelled by the ultrasound the help of an extremely sensitive device = 2 × depth of the sea = 2d called the ear. It allows us to convert pressure variations in air with audible frequencies into where d is the depth of the sea. electric signals that travel to the brain via the 2d = speed of sound × time auditory nerve. The auditory aspect of human = 1531 m/s × 3.42 s = 5236 m ear is discussed below. d = 5236 m/2 = 2618 m. Thus, the distance of the seabed from the ship is 2618 m or 2.62 km. Q uestion 1. A submarine emits a sonar pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m/s, how far away is the cliff? As mentioned earlier, bats search out prey Fig. 12.19: Auditory parts of human ear. and fly in dark night by emitting and detecting reflections of ultrasonic waves. The The outer ear is called ‘pinna’. It collects high-pitched ultrasonic squeaks of the bat the sound from the surroundings. The are reflected from the obstacles or prey and collected sound passes through the auditory returned to bat’s ear, as shown in Fig. 12.18. canal. At the end of the auditory canal there The nature of reflections tells the bat where is a thin membrane called the ear drum or the obstacle or prey is and what it is like. tympanic membrane. When a compression Porpoises also use ultrasound for navigation of the medium reaches the eardrum the and location of food in the dark. pressure on the outside of the membrane increases and forces the eardrum inward. Fig. 12.18: Ultrasound is emitted by a bat and it is Similarly, the eardrum moves outward when reflected back by the prey or an obstacle. a rarefaction reaches it. In this way the eardrum vibrates. The vibrations are amplified several times by three bones (the hammer, anvil and stirrup) in the middle ear. The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear. In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound. 172 SCIENCE

What you have learnt • Sound is produced due to vibration of different objects. • Sound travels as a longitudinal wave through a material medium. • Sound travels as successive compressions and rarefactions in the medium. • In sound propagation, it is the energy of the sound that travels and not the particles of the medium. • Sound cannot travel in vacuum. • The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation. • The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength, λ. • The time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period, T. • The number of complete oscillations per unit time is called the frequency (ν), ν = 1 . T • The speed v, frequency ν, and wavelength λ, of sound are related by the equation, v = λν. • The speed of sound depends primarily on the nature and the temperature of the transmitting medium. • The law of reflection of sound states that the directions in which the sound is incident and reflected make equal angles with the normal to the reflecting surface at the point of incidence and the three lie in the same plane. • For hearing a distinct sound, the time interval between the original sound and the reflected one must be at least 0.1 s. • The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation. • Sound properties such as pitch, loudness and quality are determined by the corresponding wave properties. • Loudness is a physiological response of the ear to the intensity of sound. • The amount of sound energy passing each second through unit area is called the intensity of sound. • The audible range of hearing for average human beings is in the frequency range of 20 Hz – 20 kHz. SOUND 173

• Sound waves with frequencies below the audible range are termed “infrasonic” and those above the audible range are termed “ultrasonic”. • Ultrasound has many medical and industrial applications. • The SONAR technique is used to determine the depth of the sea and to locate under water hills, valleys, submarines, icebergs, sunken ships etc. Exercises 1. What is sound and how is it produced? 2. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound. 3. Cite an experiment to show that sound needs a material medium for its propagation. 4. Why is sound wave called a longitudinal wave? 5. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room? 6. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why? 7. A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 m s–1. 8. Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child. 9. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute? 10. Does sound follow the same laws of reflection as light does? Explain. 11. When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day? 12. Give two practical applications of reflection of sound waves. 13. A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g = 10 m s–2 and speed of sound = 340 m s–1. 14. A sound wave travels at a speed of 339 m s–1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible? 174 SCIENCE

15. What is reverberation? How can it be reduced? 16. What is loudness of sound? What factors does it depend on? 17. Explain how bats use ultrasound to catch a prey. 18. How is ultrasound used for cleaning? 19. Explain the working and application of a sonar. 20. A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m. 21. Explain how defects in a metal block can be detected using ultrasound. 22. Explain how the human ear works. SOUND 175

C 13hapter WHY DO WE FALL ILL? Activity _____________ 13.1 poisonous substances will accumulate. Under such conditions, the brain will not be able to • We have all heard of the earthquakes in think properly. For all these interconnected Latur, Bhuj, Kashmir etc. or the cyclones activities, energy and raw material are needed. that lashed the coastal regions. Think Food is a necessity for cell and tissue functions. of as many different ways as possible in Anything that prevents proper functioning of which people’s health would be affected cells and tissues will lead to a lack of proper by such a disaster if it took place in our activity of the body. neighbourhood. It is in this context that we will look at the • How many of these ways we can think of notions of health and disease. are events that would occur when the disaster is actually happening? 13.1 Health and its Failure • How many of these health-related events 13.1.1 THE SIGNIFICANCE OF ‘HEALTH’ would happen long after the actual disaster, but would still be because of the We have heard the word ‘health’ used quite disaster? frequently. We use it ourselves as well, when we say things like ‘my grandmother’s health • Why would one effect on health fall into is not good’. Our teachers use it when they the first group, and why would another scold us saying ‘this is not a healthy attitude’. fall into the second group? What does the word ‘health’ mean? When we do this exercise, we realise that If we think about it, we realise that it health and disease in human communities always implies the idea of ‘being well’. We can are very complex issues, with many think of this well-being as effective interconnected causes. We also realise that functioning. For our grandmothers, being able the ideas of what ‘health’ and ‘disease’ mean to go out to the market or to visit neighbours are themselves very complicated. When we is ‘being well’, and not being able to do such ask what causes diseases and how we prevent things is ‘poor health’. Being interested in them, we have to begin by asking what these following the teaching in the classroom so that notions mean. we can understand the world is called a ‘healthy attitude’; while not being interested We have seen that cells are the basic units is called the opposite. ‘Health’ is therefore a of organisms. Cells are made of a variety of state of being well enough to function well chemical substances – proteins, carbo- physically, mentally and socially. hydrates, fats or lipids, and so on. Cell is a dynamic place. Something or the other is 13.1.2 PERSONAL AND COMMUNITY ISSUES always happening inside them. Complex reactions and repair goes on in cells. New cells BOTH MATTER FOR HEALTH are being made. In our organs or tissues, there are various specialised activities going on – the If health means a state of physical, mental and heart is beating, the lungs are breathing, the social well-being, it cannot be something that kidney is filtering urine, the brain is thinking. All these activities are interconnected. For example, if the kidneys are not filtering urine, 2018-19

each one of us can achieve entirely on our own. healthy. Social equality and harmony are The health of all organisms will depend on therefore necessary for individual health. We their surroundings or their environment. The can think of many other such examples of environment includes the physical connections between community issues and environment. So, for example, health is at risk individual health. in a cyclone in many ways. The Five ‘F’s — What is to be done? Human beings live in societies. Our social environment, therefore, is an important factor Protect the water source (H) in our individual health. We live in villages, Treat and store water towns or cities. In such places, even our safely (S) physical environment is decided by our social environment. Wash hands before preparing and taking food (H) Consider what would happen if no agency Wash hands after defecation (S) is ensuring that garbage is collected and disposed. What would happen if no one takes Faecal matter Cover the food (H) Healthy person responsibility for clearing the drains and Control flies (S) ensuring that water does not collect in the streets or open spaces? Clean vegetables and fruits before use (H) So, if there is a great deal of garbage Avoid open defecation (S) thrown in our streets, or if there is open drain- water lying stagnant around where we live, Proper drainage system (H) Hygiene the possibility of poor health increases. Treatment of water (S) Sanitation Therefore, public cleanliness is important for individual health. Prevention of Transmission of Diseases by Maintaining Sanitation and Hygiene Activity _____________ 13.2 13.1.3 DISTINCTIONS BETWEEN ‘HEALTHY’ • Find out what provisions are made by your local authority (panchayat/ AND ‘DISEASE-FREE’ municipal corporation) for the supply of clean drinking water. If this is what we mean by ‘health’, what do we mean by ‘disease’? The word is actually self- • Are all the people in your locality able explanatory – we can think of it as ‘disease’ – to access this? disturbed ease. Disease, in other words, literally means being uncomfortable. However, Activity _____________ 13.3 the word is used in a more limited meaning. We talk of disease when we can find a specific • Find out how your local authority and particular cause for discomfort. This does manages the solid waste generated in not mean that we have to know the absolute your neighbourhood. final cause; we can say that someone is suffering from diarrhoea without knowing • Are these measures adequate? exactly what has caused the loose motions. • If not, what improvements would you We can now easily see that it is possible to suggest? be in poor health without actually suffering • What could your family do to reduce from a particular disease. Simply not being diseased is not the same as being healthy. the amount of solid waste generated ‘Good health’ for a dancer may mean being during a day/week? able to stretch his body into difficult but graceful positions. On the other hand, good We need food to be healthy, and this food health for a musician may mean having enough will have to be earned by doing work. For this, breathing capacity in his/her lungs to control the opportunity to do work has to be available. the notes from his/her flute. To have the opportunity to realise the unique potential in We need to be happy in order to be truly all of us is also necessary for real health. healthy, and if we mistreat each other and are afraid of each other, we cannot be happy or WHY DO WE FALL ILL? 177

So, we can be in poor health without there meningitis, or any one of a dozen different being a simple cause in the form of an diseases. identifiable disease. This is the reason why, when we think about health, we think about Signs of disease are what physicians will societies and communities. On the other look for on the basis of the symptoms. Signs hand, when we think about disease, we think will give a little more definite indication of the about individual sufferers. presence of a particular disease. Physicians will also get laboratory tests done to pinpoint the Q uestions disease further. 1. State any two conditions essential for good health. 13.2.2 ACUTE AND CHRONIC DISEASES 2. State any two conditions essential for being free of disease. The manifestations of disease will be different 3. Are the answers to the above depending on a number of factors. Some questions necessarily the same or diseases last for only very short periods of time, different? Why? and these are called acute diseases. We all know from experience that the common cold lasts 13.2 Disease and Its Causes only a few days. Other ailments can last for a long time, even as much as a lifetime, and are 13.2.1 WHAT DOES DISEASE LOOK LIKE? called chronic diseases. An example is the infection causing elephantiasis, which is very Let us now think a little more about diseases. common in some parts of India. Another In the first place, how do we know that there example is asthma. is a disease? In other words, how do we know that there is something wrong with the body? Activity _____________ 13.4 There are many tissues in the body, as we have seen in Chapter 6. These tissues make • Survey your neighbourhood to find out: up physiological systems or organ systems (1) how many people suffered from that carry out body functions. Each of the acute diseases during the last organ systems has specific organs as its parts, three months, and it has particular functions. So, the (2) how many people developed digestive system has the stomach and chronic diseases during this same intestines, and it helps to digest food taken period, in from outside the body. The musculoskeletal (3) and finally, the total number of system, which is made up of bones and people suffering from chronic muscles, holds the body parts together and diseases in your neighbourhood. helps the body move. • Are the answers to questions (1) and When there is a disease, either the (2) different? functioning of one or more systems of the body will change for the worse. These changes give • Are the answers to questions (2) and rise to symptoms and signs of disease. (3) different? Symptoms of disease are the things we feel as being ‘wrong’. So we have a headache, we have • What do you think could be the cough, we have loose motions, we have a reason for these differences? What do wound with pus; these are all symptoms. you think would be the effect of these These indicate that there may be a disease, but differences on the general health of they don’t indicate what the disease is. For the population? example, a headache may mean just examination stress or, very rarely, it may mean 13.2.3 CHRONIC DISEASES AND POOR HEALTH Acute and chronic diseases have different effects on our health. Any disease that causes poor functioning of some part of the body will affect our health. This is because all functions of the body are necessary for being healthy. 178 SCIENCE

But an acute disease, which is over very soon, difference or the poor nourishment alone will not have time to cause major effects on would not lead to loose motions. But they do general health, while a chronic disease will do become contributory causes of the disease. so. Why was there no clean drinking water for As an example, think about a cough and the baby? Perhaps because the public services cold, which all of us have from time to time. are poor where the baby’s family lives. So, Most of us get better and become well within a poverty or lack of public services become third week or so. And there are no lasting effects on cause of the baby’s disease. our health. But if we get infected with a chronic disease such as tuberculosis of the lungs, then It will now be obvious that all diseases will being ill over the years does make us lose have immediate causes and contributory weight and feel tired all the time. causes. Also, most diseases will have many causes, rather than one single cause. We may not go to school for a few days if we have an acute disease. But a chronic 13.2.5 INFECTIOUS AND NON-INFECTIOUS disease will make it difficult for us to follow what is being taught in school and reduce our CAUSES ability to learn. In other words, we are likely to have prolonged general poor health if we As we have seen, it is important to keep public have a chronic disease. Chronic diseases health and community health factors in mind therefore, have very drastic long-term effects when we think about causes of diseases. We on people’s health as compared to acute can take that approach a little further. It is diseases. useful to think of the immediate causes of disease as belonging to two distinct types. One 13.2.4 CAUSES OF DISEASES group of causes is the infectious agents, mostly microbes or micro-organisms. What causes disease? When we think about Diseases where microbes are the immediate causes of diseases, we must remember that causes are called infectious diseases. This is there are many levels of such causes. Let us because the microbes can spread in the look at an example. If there is a baby suffering community, and the diseases they cause will from loose motions, we can say that the cause spread with them. of the loose motions is probably an infection. Things to ponder But the next question is – where did the infection come from? Suppose we find that the 1. Do all diseases spread to people infection came through unclean drinking coming in contact with a sick person? water. But many babies must have had this unclean drinking water. So, why is it that one 2. What are the diseases that are not baby developed loose motions when the other spreading? babies did not? 3. How would a person develop those One reason might be that this baby is not diseases that don’t spread by contact healthy. As a result, it might be more likely to with a sick person? have disease when exposed to risk, whereas healthier babies would not. Why is the baby On the other hand, there are also diseases not healthy? Perhaps because it is not well that are not caused by infectious agents. Their nourished and does not get enough food. So, causes vary, but they are not external causes lack of good nourishment becomes a second like microbes that can spread in the cause of the disease. Further, why is the baby community. Instead, these are mostly internal, not well nourished? Perhaps because it is from non-infectious causes. a household which is poor. For example, some cancers are caused by It is also possible that the baby has some genetic abnormalities. High blood pressure genetic difference that makes it more likely to can be caused by excessive weight and lack suffer from loose motions when exposed to a of exercise. You can think of many other pathogen. Without the pathogen, the genetic diseases where the immediate causes will not be infectious. WHY DO WE FALL ILL? 179

Peptic ulcers and the Nobel prize The ways in which diseases spread, and the ways in which they can be treated and For many years, everybody used to think prevented at the community level would be that peptic ulcers, which cause acidity– different for different diseases. This would related pain and bleeding in the stomach depend a lot on whether the immediate causes and duodenum, were because of lifestyle are infectious or non-infectious. reasons. Everybody thought that a stressful life led to a lot of acid secretion in the Q uestions stomach, and eventually caused peptic 1. List any three reasons why you ulcers. would think that you are sick and ought to see a doctor. If only one Then two Australians made a discovery of these symptoms were present, that a bacterium, Helicobacter pylori, was would you still go to the doctor? responsible for peptic ulcers. Robin Warren Why or why not? (born 1937), a pathologist from Perth, 2. In which of the following case do Australia, saw these small curved bacteria you think the long-term effects on in the lower part of the stomach in many your health are likely to be most patients. He noticed that signs of unpleasant? inflammation were always present around • if you get jaundice, these bacteria. Barry Marshall (born 1951), • if you get lice, a young clinical fellow, became interested • if you get acne. in Warren’s findings and succeeded in Why? cultivating the bacteria from these sources. 13.3 Infectious Diseases In treatment studies, Marshall and Warren showed that patients could be cured 13.3.1 INFECTIOUS AGENTS of peptic ulcer only when the bacteria were killed off from the stomach. Thanks to this We have seen that the entire diversity seen in pioneering discovery by Marshall and the living world can be classified into a few Warren, peptic ulcer disease is no longer a groups. This classification is based on chronic, frequently disabling condition, but common characteristics between different a disease that can be cured by a short organisms. Organisms that can cause disease period of treatment with antibiotics. are found in a wide range of such categories of classification. Some of them are viruses, For this achievement, Marshall and some are bacteria, some are fungi, some are single-celled animals or protozoans (Fig. 13.1). Warren (seen in the picture) received the Some diseases are also caused by Nobel prize for physiology and medicine in multicellular organisms, such as worms of 2005. different kinds. 180 SCIENCE

Fig. 13.1(a): Picture of SARS viruses coming out (see Fig. 13.1(d): Picture of Leishmania, the protozoan arrows for examples) of the surface of organism that causes kala-azar. The an infected cell. The white scale line organisms are oval-shaped, and each represents 500 nanometres, which is has one long whip-like structure. One half a micrometre, which is one- organism (arrow) is dividing, while a cell thousandth of a millimetre. The scale line of the immune system (lower right) has gives us an idea of how small the things gripped on the two whips of the dividing we are looking at are. organism and is sending cell processes Courtesy: Emerging Infectious up to eat up the organism. The immune Deseases, a journal of CDC, U.S. cell is about ten micrometres in diameter. Fig. 13.1(b): Picture of staphylococci, the bacteria which can cause acne. The scale of the image is indicated by the line at top left, which is 5 micrometres long. Fig. 13.1(c): Picture of Trypanosoma, the protozoan Fig. 13.1(e): Picture of an adult roundworm (Ascaris organism responsible for sleeping lumbricoides) from the small intestine. The sickness. The organism is lying next to ruler next to it shows four centimetres to a saucer-shaped red blood cell to give give us an idea of the scale. an idea of the scale. Copyright: Oregon Health and Science University, U.S. WHY DO WE FALL ILL? 181

Common examples of diseases caused by But viruses do not use these pathways at viruses are the common cold, influenza, all, and that is the reason why antibiotics do dengue fever and AIDS. Diseases like typhoid not work against viral infections. If we have a fever, cholera, tuberculosis and anthrax are common cold, taking antibiotics does not caused by bacteria. Many common skin reduce the severity or the duration of the infections are caused by different kinds of disease. However, if we also get a bacterial fungi. Protozoan microbes cause many infection along with the viral cold, taking familiar diseases, such as malaria and kala- antibiotics will help. Even then, the antibiotic azar. All of us have also come across intestinal will work only against the bacterial part of worm infections, as well as diseases like the infection, not the viral infection. elephantiasis caused by diffferent species of worms. Activity _____________ 13.5 Why is it important that we think of these • Find out how many of you in your categories of infectious agents? The answer class had cold/cough/fever recently. is that these categories are important factors in deciding what kind of treatment to use. • How long did the illness last? Members of each one of these groups – • How many of you took antibiotics (ask viruses, bacteria, and so on – have many biological characteristics in common. your parents if you had antibiotics)? • How long were those who took All viruses, for example, live inside host cells, whereas bacteria very rarely do. Viruses, antibiotics ill? bacteria and fungi multiply very quickly, while • How long were those who didn’t take worms multiply very slowly in comparison. Taxonomically, all bacteria are closely related antibiotics ill? to each other than to viruses and vice versa. • Is there a difference between these two This means that many important life processes are similar in the bacteria group groups? but are not shared with the virus group. As a • If yes, why? If not, why not? result, drugs that block one of these life processes in one member of the group is likely 13.3.2 MEANS OF SPREAD to be effective against many other members of the group. But the same drug will not work How do infectious diseases spread? Many against a microbe belonging to a different microbial agents can commonly move from group. an affected person to someone else in a variety of ways. In other words, they can be As an example, let us take antibiotics. They ‘communicated’, and so are also called commonly block biochemical pathways communicable diseases. important for bacteria. Many bacteria, for example, make a cell-wall to protect Such disease-causing microbes can spread themselves. The antibiotic penicillin blocks through the air. This occurs through the little the bacterial processes that build the cell- droplets thrown out by an infected person who wall. As a result, the growing bacteria become sneezes or coughs. Someone standing close by unable to make cell-walls, and die easily. can breathe in these droplets, and the microbes Human cells don’t make a cell-wall anyway, get a chance to start a new infection. Examples so penicillin cannot have such an effect on us. of such diseases spread through the air are Penicillin will have this effect on any bacteria the common cold, pneumonia and that use such processes for making cell-walls. tuberculosis. Similarly, many antibiotics work against many species of bacteria rather than simply working We all have had the experience of sitting against one. near someone suffering from a cold and catching it ourselves. Obviously, the more crowded our living conditions are, the more likely it is that such airborne diseases will spread. 182 SCIENCE

Diseases can also be spread through The commonest vectors we all know are water. This occurs if the excreta from someone mosquitoes. In many species of mosquitoes, suffering from an infectious gut disease, such the females need highly nutritious food in the as cholera, get mixed with the drinking water form of blood in order to be able to lay mature used by people living nearby. The cholera- eggs. Mosquitoes feed on many warm-blooded causing microbes will enter a healthy person animals, including us. In this way, they can through the water they drink and cause transfer diseases from person to person disease in them. Such diseases are much more (Fig. 13.2). likely to spread in the absence of safe supplies of drinking water. 13.3.3 ORGAN-SPECIFIC AND TISSUE- The sexual act is one of the closest physical SPECIFIC MANIFESTATIONS contact two people can have with each other. Not surprisingly, there are microbial infections The disease-causing microbes enter the body such as syphilis or AIDS that are transmitted through these different means. Where do they by sexual contact from one partner to the go then? The body is very large when other. However, such sexually transmitted compared to the microbes. So there are many diseases are not spread by casual physical possible places, organs or tissues, where they contact. Casual physical contacts include could go. Do all microbes go to the same tissue handshakes or hugs or sports, like wrestling, or organ, or do they go to different ones? or by any of the other ways in which we touch each other socially. Other than the sexual Different species of microbes seem to have contact, the virus causing AIDS (HIV) can also evolved to home in on different parts of the spread through blood-to-blood contact with body. In part, this selection is connected to infected people or from an infected mother to their point of entry. If they enter from the air her baby during pregnancy or through breast via the nose, they are likely to go to the lungs. feeding. This is seen in the bacteria causing tuberculosis. If they enter through the mouth, We live in an environment that is full of they can stay in the gut lining like typhoid- many other creatures apart from us. It is causing bacteria. Or they can go to the liver, inevitable that many diseases will be like the viruses that cause jaundice. transmitted by other animals. These animals carry the infecting agents from a sick person But this needn’t always be the case. An to another potential host. These animals are infection like HIV, that comes into the body thus the intermediaries and are called vectors. via the sexual organs, will spread to lymph nodes all over the body. Malaria-causing Fig. 13.2: Common methods of transmission of microbes, entering through a mosquito bite, will go to the liver, and then to the red blood diseases. cells. The virus causing Japanese encephalitis, or brain fever, will similarly enter through a mosquito bite. But it goes on to infect the brain. The signs and symptoms of a disease will thus depend on the tissue or organ which the microbe targets. If the lungs are the targets, then symptoms will be cough and breathlessness. If the liver is targeted, there will be jaundice. If the brain is the target, we will observe headaches, vomiting, fits or unconsciousness. We can imagine what the symptoms and signs of an infection will be if we know what the target tissue or organ is, and the functions that are carried out by this tissue or organ. WHY DO WE FALL ILL? 183

In addition to these tissue-specific effects that we can conserve our energy. This will of infectious disease, there will be other common enable us to have more of it available to focus effects too. Most of these common effects depend on healing. on the fact that the body’s immune system is activated in response to infection. An active But this kind of symptom-directed immune system recruits many cells to the treatment by itself will not make the infecting affected tissue to kill off the disease-causing microbe go away and the disease will not be microbes. This recruitment process is called cured. For that, we need to be able to kill off inflammation. As a part of this process, there the microbes. are local effects such as swelling and pain, and general effects such as fever. How do we kill microbes? One way is to use medicines that kill microbes. We have seen In some cases, the tissue-specificity of the earlier that microbes can be classified into infection leads to very general-seeming effects. different categories. They are viruses, bacteria, For example, in HIV infection, the virus goes fungi or protozoa. Each of these groups of to the immune system and damages its organisms will have some essential function. Thus, many of the effects of HIV-AIDS biochemical life process which is peculiar to are because the body can no longer fight off that group and not shared with the other the many minor infections that we face groups. These processes may be pathways for everyday. Instead, every small cold can become the synthesis of new substances or respiration. pneumonia. Similarly, a minor gut infection can produce major diarrhoea with blood loss. These pathways will not be used by us Ultimately, it is these other infections that kill either. For example, our cells may make new people suffering from HIV-AIDS. substances by a mechanism different from that used by bacteria. We have to find a drug that It is also important to remember that the blocks the bacterial synthesis pathway without severity of disease manifestations depend on affecting our own. This is what is achieved by the number of microbes in the body. If the the antibiotics that we are all familiar with. number of microbes is very small, the disease Similarly, there are drugs that kill protozoa manifestations may be minor or unnoticed. such as the malarial parasite. But if the number is of the same microbe large, the disease can be severe enough to be life- One reason why making anti-viral threatening. The immune system is a major medicines is harder than making anti- factor that determines the number of microbes bacterial medicines is that viruses have few surviving in the body. We shall look into this biochemical mechanisms of their own. They aspect a little later in the chapter. enter our cells and use our machinery for their life processes. This means that there are 13.3.4 PRINCIPLES OF TREATMENT relatively few virus-specific targets to aim at. Despite this limitation, there are now effective What are the steps taken by your family when anti-viral drugs, for example, the drugs that you fall sick? Have you ever thought why you keep HIV infection under control. sometimes feel better if you sleep for some time? When does the treatment involve medicines? 13.3.5 PRINCIPLES OF PREVENTION Based on what we have learnt so far, it All of what we have talked about so far deals would appear that there are two ways to treat with how to get rid of an infection in someone an infectious disease. One would be to reduce who has the disease. But there are three the effects of the disease and the other to kill limitations of this approach to dealing with the cause of the disease. For the first, we can infectious disease. The first is that once provide treatment that will reduce the someone has a disease, their body functions symptoms. The symptoms are usually are damaged and may never recover because of inflammation. For example, we can completely. The second is that treatment will take medicines that bring down fever, reduce take time, which means that someone pain or loose motions. We can take bed rest so suffering from a disease is likely to be 184 SCIENCE

bedridden for some time even if we can give an infectious microbe does not necessarily proper treatment. The third is that the person mean developing noticeable disease. suffering from an infectious disease can serve as the source from where the infection may So, one way of looking at severe infectious spread to other people. This leads to the diseases is that it represents a lack of success multiplication of the above difficulties. It is of the immune system. The functioning of the because of such reasons that prevention of immune system, like any other system in our diseases is better than their cure. body, will not be good if proper and sufficient nourishment and food is not available. How can we prevent diseases? There are Therefore, the second basic principle of two ways, one general and one specific to each prevention of infectious disease is the disease. The general ways of preventing availability of proper and sufficient food for infections mostly relate to preventing everyone. exposure. How can we prevent exposure to infectious microbes? Activity _____________ 13.6 If we look at the means of their spreading, • Conduct a survey in your locality. we can get some easy answers. For airborne Talk to ten families who are well-off microbes, we can prevent exposure by and ten who are very poor (in your providing living conditions that are not estimation). Both sets of families overcrowded. For water-borne microbes, we should have children who are below can prevent exposure by providing safe five years of age. Measure the heights drinking water. This can be done by treating of these children. Draw a graph of the the water to kill any microbial contamination. height of each child against its age For vector-borne infections, we can provide for both sets of families. clean environments. This would not, for example, allow mosquito breeding. In other • Is there a difference between the words, public hygiene is one basic key to the groups? If yes, why? prevention of infectious diseases. • If there is no difference, do you think In addition to these issues that relate to that your findings mean that being the environment, there are some other general well-off or poor does not matter for principles to prevent infectious diseases. To health? appreciate those principles, let us ask a question we have not looked at so far. Normally, These are the general ways of preventing we are faced with infections everyday. If infections. What are the specific ways? They someone is suffering from a cold and cough in relate to a peculiar property of the immune the class, it is likely that the children sitting system that usually fights off microbial around will be exposed to the infection. But infections. Let us cite an example to try and all of them do not actually suffer from the understand this property. disease. Why not? These days, there is no smallpox anywhere This is because the immune system of our in the world. But as recently as a hundred body is normally fighting off microbes. We have years ago, smallpox epidemics were not at all cells that specialise in killing infecting uncommon. In such an epidemic, people used microbes. These cells go into action each time to be very afraid of coming near someone infecting microbes enter the body. If they are suffering from the disease since they were successful, we do not actually come down with afraid of catching the disease. any disease. The immune cells manage to kill off the infection long before it assumes major However, there was one group of people proportions. As we noted earlier, if the number who did not have this fear. These people would of the infecting microbes is controlled, the provide nursing care for the victims of manifestations of disease will be minor. In other smallpox. This was a group of people who had words, becoming exposed to or infected with had smallpox earlier and survived it, although with a lot of scarring. In other words, if you had smallpox once, there was no chance of WHY DO WE FALL ILL? 185

suffering from it again. So, having the disease to the infecting microbe from turning into once was a means of preventing subsequent actual disease. attacks of the same disease. Many such vaccines are now available for This happens because when the immune preventing a whole range of infectious diseases, system first sees an infectious microbe, it and provide a disease-specific means of responds against it and then remembers it prevention. There are vaccines against tetanus, specifically. So the next time that particular diphtheria, whooping cough, measles, polio microbe, or its close relatives enter the body, and many others. These form the public health the immune system responds with even greater programme of childhood immunisation for vigour. This eliminates the infection even more preventing infectious diseases. quickly than the first time around. This is the basis of the principle of immunisation. Of course, such a programme can be useful only if such health measures are Immunisation available to all children. Can you think of reasons why this should be so? Traditional Indian and Chinese medicinal Some hepatitis viruses, which cause systems sometimes deliberately rubbed the jaundice, are transmitted through water. There is a vaccine for one of them, hepatitis A, in the skin crusts from smallpox victims into the market. But the majority of children in many parts of India are already immune to hepatitis skin of healthy people. They thus hoped to A by the time they are five years old. This is because they are exposed to the virus through induce a mild form of smallpox that would water. Under these circumstances, would you take the vaccine? create resistance against the disease. Activity _____________ 13.7 Famously, two centuries ago, an English • Rabies virus is spread by the bite of physician named Edward Jenner, realised infected dogs and other animals. There are anti-rabies vaccines for both humans that milkmaids and animals. Find out the plan of your local authority for the control of rabies who had had in your neighbourhood. Are these measures adequate? If not, what cowpox did not improvements would you suggest? catch smallpox Q uestions 1. Why are we normally advised to even during take bland and nourishing food when we are sick? epidemics. 2. What are the different means by which infectious diseases are Cowpox is a very spread? 3. What precautions can you take in mild disease. your school to reduce the incidence of infectious diseases? Jenner tried 4. What is immunisation? 5. What are the immunisation deliberately giving programmes available at the nearest health centre in your cowpox to people locality? Which of these diseases are the major health problems in (as he can be seen your area? doing in the picture), and found that they were now resistant to smallpox. This was because the smallpox virus is closely related to the cowpox virus. ‘Cow’ is ‘vacca’ in Latin, and cowpox is ‘vaccinia’. From these roots, the word ‘vaccination’ has come into our usage. We can now see that, as a general principle, we can ‘fool’ the immune system into developing a memory for a particular infection by putting something, that mimics the microbe we want to vaccinate against, into the body. This does not actually cause the disease but this would prevent any subsequent exposure 186 SCIENCE

What you have learnt • Health is a state of physical, mental and social well-being. • The health of an individual is dependent on his/her physical surroundings and his/her economic status. • Diseases are classified as acute or chronic, depending on their duration. • Disease may be due to infectious or non-infectious causes. • Infectious agents belong to different categories of organisms and may be unicellular and microscopic or multicellular. • The category to which a disease-causing organism belongs decides the type of treatment. • Infectious agents are spread through air, water, physical contact or vectors. • Prevention of disease is more desirable than its successful treatment. • Infectious diseases can be prevented by public health hygiene measures that reduce exposure to infectious agents. • Infectious diseases can also be prevented by using immunisation. • Effective prevention of infectious diseases in the community requires that everyone should have access to public hygiene and immunisation. Exercises 1. How many times did you fall ill in the last one year? What were the illnesses? (a) Think of one change you could make in your habits in order to avoid any of/most of the above illnesses. (b) Think of one change you would wish for in your surroundings in order to avoid any of/most of the above illnesses. 2. A doctor/nurse/health-worker is exposed to more sick people than others in the community. Find out how she/he avoids getting sick herself/himself. 3. Conduct a survey in your neighbourhood to find out what the three most common diseases are. Suggest three steps that could be taken by your local authorities to bring down the incidence of these diseases. WHY DO WE FALL ILL? 187

4. A baby is not able to tell her/his caretakers that she/he is sick. What would help us to find out (a) that the baby is sick? (b) what is the sickness? 5. Under which of the following conditions is a person most likely to fall sick? (a) when she is recovering from malaria. (b) when she has recovered from malaria and is taking care of someone suffering from chicken-pox. (c) when she is on a four-day fast after recovering from malaria and is taking care of someone suffering from chicken-pox. Why? 6. Under which of the following conditions are you most likely to fall sick? (a) when you are taking examinations. (b) when you have travelled by bus and train for two days. (c) when your friend is suffering from measles. Why? 188 SCIENCE

C 14hapter NATURAL RESOURCES Our planet, Earth is the only one on which dioxide constitutes up to 95-97% of the life, as we know it, exists. Life on Earth is atmosphere on Venus and Mars. dependent on many factors. Most life-forms we know need an ambient temperature, Eukaryotic cells and many prokaryotic water, and food. The resources available on cells, discussed in Chapter 5, need oxygen to the Earth and the energy from the Sun are break down glucose molecules and get energy necessary to meet the basic requirements of for their activities. This results in the all life-forms on the Earth. production of carbon dioxide. Another process which results in the consumption of oxygen What are these resources on the Earth? and the concomitant production of carbon dioxide is combustion. This includes not just These are the land, the water and the air. human activities, which burn fuels to get The outer crust of the Earth is called the energy, but also forest fires. lithosphere. Water covers 75% of the Earth’s surface. It is also found underground. These Despite this, the percentage of carbon comprise the hydrosphere. The air that covers dioxide in our atmosphere is a mere fraction the whole of the Earth like a blanket, is called of a percent because carbon dioxide is ‘fixed’ the atmosphere. Living things are found in two ways: (i) Green plants convert carbon where these three exist. This life-supporting dioxide into glucose in the presence of zone of the Earth where the atmosphere, the Sunlight and (ii) many marine animals use hydrosphere and the lithosphere interact and carbonates dissolved in sea-water to make make life possible, is known as the biosphere. their shells. Living things constitute the biotic 14.1.1 THE ROLE OF THE ATMOSPHERE component of the biosphere. The air, the water and the soil form the non-living or IN CLIMATE CONTROL abiotic component of the biosphere. Let us study these abiotic components in detail in We have talked of the atmosphere covering the order to understand their role in sustaining Earth, like a blanket. We know that air is a life on Earth. bad conductor of heat. The atmosphere keeps the average temperature of the Earth fairly 14.1 The Breath of Life: Air steady during the day and even during the course of the whole year. The atmosphere We have already talked about the composition prevents the sudden increase in temperature of air in the first chapter. It is a mixture of during the daylight hours. And during the many gases like nitrogen, oxygen, carbon night, it slows down the escape of heat into dioxide and water vapour. It is interesting to outer space. Think of the moon, which is note that even the composition of air is the about the same distance from the Sun that result of life on Earth. In planets such as the Earth is. Despite that, on the surface of Venus and Mars, where no life is known to the moon, with no atmosphere, the exist, the major component of the atmosphere temperature ranges from –190° C to 110° C. is found to be carbon dioxide. In fact, carbon 2018-19

Activity _____________ 14.1 the heating of water bodies and the activities of living organisms. The atmosphere can be • Measure the temperature of the heated from below by the radiation that is following : reflected back or re-radiated by the land or Take (i) a beaker full of water, (ii) a water bodies. On being heated, convection beaker full of soil/sand and (iii) a closed currents are set up in the air. In order to gain bottle containing a thermometer. Keep some understanding of the nature of them in bright Sunlight for three hours. convection currents, let us perform the Now measure the temperature of all 3 following activity: vessels. Also, take the temperature reading in shade at the same time. Activity _____________ 14.2 Now answer • Place a candle in a beaker or wide- mouthed bottle and light it. Light an 1. Is the temperature reading more in incense stick and take it to the mouth activity (i) or (ii)? of the above bottle (Figure 14.1). 2. Based on the above finding, which • Which way does the smoke flow when would become hot faster – the land or the incense stick is kept near the edge the sea? of the mouth? 3. Is the thermometer reading of the • Which way does the smoke flow when temperature of air (in shade) the same the incense stick is kept a little above as the temperature of sand or water? the candle? What do you think is the reason for this? And why does the temperature • Which way does the smoke flow when have to be measured in the shade? the incense stick is kept in other regions? 4. Is the temperature of air in the closed glass vessel/bottle the same as the Fig. 14.1: Air currents being caused by the uneven temperature taken in open air? (i) What heating of air. do you think is the reason for this? (ii) Do we ever come across this The patterns revealed by the smoke show phenomenon in daily life? us the directions in which hot and cold air move. In a similar manner, when air is heated As we have seen above, sand and water by radiation from the heated land or water, it do not heat up at the same rate. What do you rises. But since land gets heated faster than think will be their rates of cooling? Can we water, the air over land would also be heated think of an experiment to test the prediction? faster than the air over water bodies. 14.1.2 THE MOVEMENT OF AIR: WINDS So, if we look at the situation in coastal regions during the day, the air above the land We have all felt the relief brought by cool evening breezes after a hot day. And sometimes, we are lucky enough to get rains after some days of really hot weather. What causes the movement of air, and what decides whether this movement will be in the form of a gentle breeze, a strong wind or a terrible storm? What brings us the welcome rains? All these phenomena are the result of changes that take place in our atmosphere due to the heating of air and the formation of water vapour. Water vapour is formed due to 190 SCIENCE