202110208-APEX-STUDENT-WORKBOOK-PHYSICAL_SCIENCE-G09-PART2

Q9. Two bodies of unequal masses are dropped from the top of building. Which of the following is equal for both bodies at any instance? [Refer to TB page 182 Q14] (A) Speed (B) Force of gravity (C) Potential energy (D) Kinetic energy A. (B) Force of gravity 2.6 Information Skills and Projects Q1. Collect pictures showing various situations where potential energy possessed by an object depends on its shape and position. Prepare a scrap book. [Refer to TB page 183 Q5, Try These] A. Students’ Activity. 2.7 Application to Daily Life, Concern to Bio Diversity Q1. When an apple falls down from a tree what happens to its gravitational potential energy just as it reaches the ground? After it strikes the ground. [Refer to TB page 183 Q8, Try These] A. i. When an apple falls down from a tree, its gravitational potential energy is gradually converted into kinetic energy required for its motion. ii. After it strikes the ground, the whole gravitational potential energy is converted into kinetic energy such that the total mechanical energy is conserved. Q2. When you push your bicycle up an incline, the potential energy of the bicycle and your- self increase. Where does this energy come from? [Refer to TB page 182 Q18] A. The energy comes from the height that increases with the distance through which I push the bicycle. It is the gravitational potential energy. SESSION 2. ENERGY 99

SESSION 3 CONVERSION AND CONSERVATION OF ENERGY 3.1 Mind Map 3.2 Terminology i. Conservation of energy – ‘Energy can neither be created nor be destroyed. The total energy in this universe is always conserved’. This is called law of conservation of energy. ii. Gravitational energy – Energy due to the action of gravity. iii. Power – Rate of work done. 3.3 Solved Examples Q1. Example 9: A person performs 420 J of work in 5 minutes. Calculate the power delivered by him. ( Refer to TB page 179 ) A. Work done by the person, W = 420 J Time taken to complete the work, t = 5 min = 5 x 60 s = 300 s Power delivered, P = W = 400 = 1.4 W t 300 Q2. Example 10: A woman does 250 J of work in 10 seconds and a boy does 100 J of work in 4 seconds. Who delivers more power? ( Refer to TB page 179 ) SESSION 3. CONVERSION AND CONSERVATION OF ENERGY 100

A. Power, P = W t Power delivered by woman = 250 = 25 W 10 Power delivered by boy = 100 = 25 W 4 Both woman and boy deliver same power, i.e., rate of doing the work by woman and boy is equal. 3.4 Key Concepts i. ‘Energy can neither be created nor destroyed. The total energy in this universe is always conserved’. This is called law of conservation of energy. ii. Power is the rate at which work is done. iii. Power (P) = w/t, where w is the work done in time t. 3.5 Conceptual Understanding Q1. Identify the wrong statement among the following. Rewrite them by making necessary corrections. [Refer to TB page 181 Q3] a) Work and energy have different units. b) When an aeroplane takes off the work done by its weight is positive. c) The potential energy of spring increases when it is extended and decreases when it is compressed. d) If the work done by external forces on a system is negative then the energy of the system decreases. e) When a body is falling freely from a height its kinetic energy remains constant. f) The unit of power is watt. A. a) Work and energy have different units. (TRUE) SESSION 3. CONVERSION AND CONSERVATION OF ENERGY 101

b) When an aeroplane takes off the work done by its weight is positive. (FALSE) Since the force caused by the weight of the aeroplane and its displacement are in oppo- site directions, the work done by its weight is taken as negative. Correct statement: When an aeroplane takes off the work done by its weight is negative. c) The potential energy of spring increases when it is extended and decreases when it is compressed. [FALSE] Correct statement: The potential energy of spring increases when it is extended or compressed. d) If the work done by external forces on a system is negative then the energy of the system decreases. [TRUE] e) When a body is falling freely from a height its kinetic energy remains constant. [FALSE] As the body is falling down freely, the acceleration increases and so the velocity also increases. Since K.E. the body gains energy. Correct statement: When a body is falling freely from a height its Kinetic energy in- creases. f) The unit of power is watt.[TRUE] Q2. In which of the following cases is the work done positive or zero or negative? [Refer to TB page 181 Q6] i. Work done by the force by porter on a suitcase in lifting it from the platform on to his head. ii. Work done by the force of gravity on suitcase as the suitcase falls from porter’s head. iii. Work done by porter standing on platform with suitcase on his head. iv. Work done by force of gravity on a ball thrown up vertically into the sky. v. Work done by force applied by hands of a man swimming in pond. SESSION 3. CONVERSION AND CONSERVATION OF ENERGY 102

A. i. Positive ii. Positive iii. Zero iv. Negative v. Negative Q3. State the principle of conservation of energy. [Refer to TB page 181 Q5] A. Principle of conservation of energy: Energy can neither be created nor destroyed. It can only be changed from one form to another form. Q4. A man carrying a bag of total mass 25 kg climbs up to a height of 10 m in 50 seconds. Calculate the power delivered by him on the bag. [Refer to TB page 182 Q10] A. The work done by the man, w = F × s Force acting on the bag, F = mg m = mass of bag = 25 kg; g = 9.8 m /s2 F = 25 × 9.8 N S = height = 10 m W = 25 × 9.8 × 10 = 2450 t = time taken by the man to carry the bag = 50 s Power delivered = w/t = 2450/50 =49 W The power delivered by him on the bag = 49 W Q5. Calculate the work done by a person in lifting a load of 20 kg from the ground and placing it 1 m high on a table. [Refer to TB page 182 Q11] A. Mass of load, m = 20 kg SESSION 3. CONVERSION AND CONSERVATION OF ENERGY 103

Displacement s = 1 m Work done, w = F × s = mg × s = 20 kg × 9.8 m/s2 × 1 m = 196 kg m × m/s2 = 196 Nm = 196 J Work done by the person = 196 J 3.6 Communication Through Drawing and Model Making Q1. Draw a diagram to show conservation of mechanical energy in case of free falling body [Refer to TB page 182 Q17] A. 3.7 Appreciation and Aesthetic Sense, Values Q1. How would you assess the role of energy conversion occurring naturally in maintaining ecological balance of nature. [Refer to TB page 183 Q6, Try These] A. The balance of nature is a theory that says that ecological systems are usually in a stable equilibrium (homeostasis), which is to say that a small change in some particular parameter (the size of a particular population, for example) will be corrected by some negative feedback that will bring the parameter back to its original “point of balance” with the rest of the system. It may apply where populations depend on each other, for example in predator/ prey systems, or relationships between herbivores and their food source. It is also sometimes applied to the relationship between the earth’s ecosystem, the composition of the atmosphere, and the world’s weather. SESSION 3. CONVERSION AND CONSERVATION OF ENERGY 104

1) The chemical changes in the sun providing us the heat energy and light energy to the earth. 2) The heat energy of the sun evoporates the water into water vapour. This helps us to get heavy rainfall. 3) The solar energy is taken by the plant to change CO2 , water and chloroplast into starch which is the food for the plants. This process is called photosynthesis. 4) In photosynthesis, plant releases O2 . 5) The leguminous (pea) plants help the bacteria present at their roots. In return they help the plants to fix the nitrogen in them. The nitrogen cycle is maintained. Conclusion: From the above examples we can understand that the energy conversion is helpful inmaintaining ecological balance of nature. 3.8 Application to Daily Life, Concern to Bio Diversity Q1. When you lift a box from the floor and put it on an almirah the potential energy of the box increases but there is no change in its kinetic energy. Is it violation of conservation of energy? Explain. [Refer to TB page 183 Q7, Try These] A. i. No. It is not a violation of conservation of energy. ii. Actually when we put the box at a higher level from ground, the force of friction offered by the plank of the almirah also increases and so the increased P.E. is thus balanced. As such, there is no increase or decrease in the energy of the system and it is still conserved. Q2. How will the increasing energy needs and conservation of energy influence the interna- tional peace, co–operation and security? Discuss. [Refer to TB page 183 Q9, Try These] SESSION 3. CONVERSION AND CONSERVATION OF ENERGY 105

A. i. As the world’s population is increasing, the consumption of energy sources both natural and fossil are increasing at a high rate. ii. The forest wealth and biodiversity are seriously dwindling and this scarcity has a serious effect on economically backward countries. iii. In the struggle for existence the well developed countries with their deadly weapons and armies may declare war against poor countries to plunder their nat- ural resources. iv. These would seriously influence the international peace, co–operation and security to a larger extent. v. Then the world will be in a hot tea pot and finally we may face a third world war. —— CCE Based Practice Questions —— AS1-Conceptual Understanding Very Short Answer Type Questions 1. State true or false. [Refer to Session 10.1 ] (i) The work done by a force is positive when the direction of force is opposite to the direction of displacement. [] (ii) If the position of a bag is changed by a person, then his muscular force is doing the work. [] (iii) If a ball is thrown upwards, the work done by gravity is negative. [ ] (iv) It is difficult to lift a heavy bag because friction acts against it. [ ] (v) W = –F s represents, force and displacement are in opposite directions. ] [ CHAPTER 10. WORK AND ENERGY 106

2. Match the following. [] Column B [(Session 10.1)] a. ms-2 Column A b. joule i. Force c. newton ii. Displacement [] d. watt iii. Work [] e. metre iv. Acceleration due to gravity [ ] v. Power [] 3. State true or false. [Refer to Session 10.2 ] (i) Energy can neither be created nor be destroyed but can be changed from one form to another. [] (ii) Object which does work gains energy and the object on which work is done loses energy. [] (iii) Energy possessed by a moving object is known as kinetic energy. [ ] (iv) Energy possessed by an object due to its position or deformed shape is known as mechanical energy. [] (v) The formula for potential energy is P.E = 1 mv2. 2 [] CHAPTER 10. WORK AND ENERGY 107

4. Match the following. Column B [(Session 10.2)] Column A i. Energy stored in water in dams [ ] a. Kinetic energy ii. Energy possessed by a moving cycle [] b. Mechanical energy iii. K.E + P.E [ ] c. Gravitation iv. Force acting against moving objects [ ] d. Potential energy v. Force acting on a falling object [ ] e. Friction 5. Fill in the blanks. [Refer to Session 10.2 ] (i) K.E = . (ii) Sum of kinetic energy and potential energy is called _____________________ _______________________ . (iii) Friction does work on the moving object. (iv) A moving object can do . (v) As the height of an object increases, its energy increases. 6. State true or false. [ ] [Refer to Session 10.3 ] [ ] [ ] (i) Potential energy can be converted into gravitational energy. (ii) Solar energy can only be converted into heat energy. (iii) Petroleum and coal are produced by dead plants and animals. CHAPTER 10. WORK AND ENERGY 108

(iv) When potential energy decreases kinetic energy also decreases. [ ] (v) In a pendulum, sum of potential energy and kinetic energy remains constant. ] [ 7. Match the following. [] Column B [(Session 10.3)] a. Potential energy to electrical energy Column A i. Photosynthesis ii. Hydroelectric plants [ ] b. Solar energy to heat energy iii. Batteries [ ] c. Electrical energy to mechanical energy iv. Solar cooker [ ] d. Chemical energy to electrical energy v. Fan [ ] e. Solar energy to chemical energy 8. Fill in the blanks. [Refer to Session 10.3 ] (i) Solar batteries can be used to convert solar energy to energy. (ii) In a windmill, wind energy is converted into energy when the blades of the windmill rotate. (iii) Water heaters generate heat by converting energy to heat energy. (iv) Dead plants and animals decompose to form petroleum and coal which have energy stored in them. (v) Total energy of free fall is calculated using the formula . CHAPTER 10. WORK AND ENERGY 109

Short Answer Type Questions 9. Answer the following questions in 3-4 sentences. (i) [(Session 10.1)] Define work. Give the expression for work done. (ii) [(Session 10.1)] Calculate the work done by a person in lifting the load of 25 kg from the ground and placing it on 1 m high table. 10. Answer the following questions in 3-4 sentences. (i) [(Session 10.2)] Define kinetic energy. Give equation for kinetic energy. (ii) [(Session 10.2)] Derive the expression of kinetic energy. Long Answer Type Questions 11. Answer the following questions in 6-8 sentences. (i) [(Session 10.1)] A ball of mass 0.5 kg thrown upward reaches a maximum height of 5 m. Calculate the work done by the force of gravity during this vertical displacement considering the value of g = 10 m/s2. (ii) [(Session 10.1)] Calculate the work done by a student in lifting a 4.7 kg wooden block from the ground and keeping it on a shelf at a height of 2.9 m. Take g = 9.8 ms-2. 12. Answer the following questions in 6-8 sentences. (i) [(Session 10.2)] What will happen to the kinetic energy of a body, if the mass of the body is doubled (when its velocity remains the same)? (ii) [(Session 10.2)] Define potential energy. A wood of 4 kg is lifted up through 3 m from ground. Calculate the potential energy of the block at that point. 13. Answer the following questions in 6-8 sentences. (i) [(Session 10.3)] State the principle of law of conservation of energy. A woman does 600 J of work in 3 minutes and a boy does 400 J of work in 2 minutes. Who delivers more power? CHAPTER 10. WORK AND ENERGY 110

AS2-Asking questions and making hypothesis Short Answer Type Questions 14. Answer the following questions in 3-4 sentences. (i) [(Session 10.1)] Rajeev said that it is easier to stop a car than a bus moving with the same speed. Why did he say that? AS3-Experimentation and field investigation Short Answer Type Questions 15. Answer the following questions in 3-4 sentences. (i) [(Session 10.2)] Write an activity to show that a moving object can do work. AS4-Information skills and projects Short Answer Type Questions 16. Answer the following questions in 3-4 sentences. (i) [(Session 10.2)] Complete the table by writing the formula. S.No. Quantity Formula 1 Kinetic energy 2 Potential energy 3 Work done 4 Force CHAPTER 10. WORK AND ENERGY 111

AS5-Communication through drawing and model making Long Answer Type Questions 17. Answer the following questions in 6-8 sentences. (i) [(Session 10.3)] Explain what you notice from the diagram in terms of potential energy and kinetic energy. AS6-Appreciation and aesthetic sense, Values Long Answer Type Questions 18. Answer the following questions in 6-8 sentences. (i) [(Session 10.3)] How do you appreciate the law of conservation of energy? AS7-Application to daily life, concern to bio diversity Short Answer Type Questions 19. Answer the following questions in 3-4 sentences. (i) [(Session 10.1)] Do you think a person standing on a railway platform carrying luggage is doing any work? Justify your answer. (ii) [(Session 10.1)] Work done by a boy to push a wooden block by 30 cm is 1.35 J. Calculate the force applied by the boy. CHAPTER 10. WORK AND ENERGY 112

Long Answer Type Questions 20. Answer the following questions in 6-8 sentences. (i) [(Session 10.1)] A person is standing by holding a suitcase and a person is lifting a suitcase from the ground. Between these two people who is doing work? Explain. 21. Answer the following questions in 6-8 sentences. (i) [(Session 10.2)] A ball is dropped from a height. Explain how energy is getting con- served in this example. 22. Answer the following questions in 6-8 sentences. (i) [(Session 10.3)] By giving an example from your daily life, explain how one form of energy is converted into other form. Also using the same example explain how energy is conserved. CHAPTER 10. WORK AND ENERGY 113

Objective Questions AS1-Conceptual Understanding 23. Choose the correct answer. (i) Work is said to be done when (A) force acts upon a body, but the body does not move (B) force acts upon a body and moves it in the direction of force (C)force acts upon a body, but does not move it in the direction of applied force (D)none of these (B) newton (ii) Force is measured in (A) joule (C) kelvin (D) mole (iii) Unit of work is (B) joule (A) newton metre (C) kelvin (D)both A and B (iv) Potential Energy, P.E = (B) m/gh (A) mg/h (C) mgh (D) g/mh (v) In equation, W=F s, if F=1 and s=1 then work done by force is equal to (A) 0 J (B) 1 J (C)3 J (D)2 J (vi) The correct formula for work done is (B) Force x Acceleration (A) Force x Speed CHAPTER 10. WORK AND ENERGY 114

(C)Force x Velocity (D)Force x Displacement (vii) An object raised to a height from the ground possesses (A) kinetic energy (B) potential energy (C)chemical energy (D)mechanical energy (viii) If work has positive value, the body on which work has been done would (A) lose energy (B) either gain or lose energy (C)gain energy (D)neither gain nor lose energy (ix) Energy can neither be created nor be destroyed. It can only be changed from one form to another. This is the (A) Law of constant energy (B) Law of gaining energy (C)Law of losing energy (D)Law of conservation of energy (x) If force acting on an object and displacement are in opposite directions, then the work done by the force is taken as (A) positive (B) negative (C) neutral (D)either positive or negative (xi) What is the unit of power? (B) joule per second (A) kilojoule (C) joule (D) watt CHAPTER 10. WORK AND ENERGY 115

AS2-Asking questions and making hypothesis 24. Choose the correct answer. (i) What is the main source of energy on earth? (A) Sun (B) Moon (C) Wind (D) Fire (ii) Which type of energy conversion takes place in an electric fan? (A) Mechanical energy to electrical energy (B) Electrical energy to chemical energy (C)Electrical energy to mechanical energy (D)Chemical energy to electrical energy AS3-Experimentation and field investigation 25. Choose the correct answer. (i) A spring is compressed. The potential energy of the compressed spring (A) increases (B) decreases (C)remains unchanged (D)becomes zero AS7-Application to daily life, concern to bio diversity 26. Choose the correct answer. (i) A vehicle is accelerated on a levelled road and attains a velocity 4 times its initial velocity. In this process the potential energy of the car (A) becomes twice that of initial (B) becomes 4 times of that of initial (C)does not change (D)becomes 16 times that of initial CHAPTER 10. WORK AND ENERGY 116

11. SOUND SESSION 1 INTRODUCTION AND PROPAGATION OF SOUND 1.1 Mind Map 1.2 Terminology i. Tuning fork – Device to generate particular frequency sound waves. ii. Longitudinal wave – A wave in which the direction of vibration of particles is along the direction of propagation of the wave. iii. Transverse wave – A wave in which the direction of vibration of particles is along the direction perpendicular to that of the propagation of the wave. iv. Compression – High pressure area of a sound wave. v. Rarefaction – Low pressure area of a sound wave. vi. Crest – It is the highest point the medium rises to in a transverse wave. vii. Trough – It is the lowest point the medium sinks to in a transverse wave. 1.3 Key Concepts i. A vibrating body produces sound. ii. Sound is a form of mechanical energy which produces sensation of hearing. iii. A tuning fork is used to produce sounds. It resonates at a constant pitch when set to vibrate. iv. If the particles of medium move to and fro along the direction of propagation of wave, the wave is called longitudinal wave. v. Sound travels in the form of longitudinal waves. SESSION 1. INTRODUCTION AND PROPAGATION OF SOUND 117

vi. The region of high density of particles in the medium during propagation of sound is called compression and low density region is called rarefaction. 1.4 Conceptual Understanding Q1. When we say sound travels in a medium: [Refer to TB page 202 Q1] (a) The medium travels (b) The particles of the medium travel (c) The source travels (d) The disturbance travels A. (d) The disturbance travels Q2. A sound wave consists of: [Refer to TB page 202 Q2] a) Number of compression pulses only b) Number of rarefaction pulses only c) Number of compression and rarefaction pulses one after the other d) Vacuum only A. c) Number of compression and rarefaction pulses one after the other Q3. Name two quantities that vary periodically at a place in air, as a sound wave travels through it. [Refer to TB page 203 Q8] A. Density and pressure of particles. Q4. What do you understand by a sound wave? [Refer to TB page 204 Q4, Try These] SESSION 1. INTRODUCTION AND PROPAGATION OF SOUND 118

A. Sound in air gets propagated in the form of longitudinal wave motion consisting of re- gions of compressions and rarefactions. Consider, for example, a tuning fork in a state of vibration. As prong moves towards right, it compresses the layer of air in contact with it. As air has elasticity the compressed air tends to relieve itself of its strain and moves towards the right. In wave form it has been shown in figure. At the point of compression, there is an increase of pressure and is shown in the form of crest C. At the point of rarefaction, the concentration of particles is least. 1.5 Communication Through Drawing and Model Making Q1. With the help of a diagram describe how compression and rarefaction pulses are pro- duced in air near a source of sound. [Refer to TB page 203 Q16] A. The regions where the coils become closer are called compressions (C) and the regions where the coils are further apart are called rarefactions (R). As we already know, sound propagates in the medium as a series of compressions and rarefactions. Now, we can compare the propagation of disturbance in a slinky with the sound propagation in the medium. These waves are called longitudinal waves. In these waves, the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth about their position of rest. This is exactly how a sound wave propagates; hence sound waves are longitudinal waves. SESSION 1. INTRODUCTION AND PROPAGATION OF SOUND 119

SESSION 1. INTRODUCTION AND PROPAGATION OF SOUND 120

SESSION 2 CHARACTERISTICS OF THE SOUND WAVE 2.1 Mind Map 2.2 Terminology i. Wavelength – The distance between the consecutive compressions or rarefactions. ii. Amplitude – The maximum displacement of a vibrating particle. iii. Frequency – Number of vibrations per seconds. 2.3 Solved Examples Q1. Example 1: Find the time period of the wave whose frequency is 500 Hz. [Refer to TB page 191 ] A. Time period = 1 Frequency T= 1 = 1 s = 0.002 s ν 500 SESSION 2. CHARACTERISTICS OF THE SOUND WAVE 121

Q2. Example 2: In a certain gas, a source produces 40,000 compression and 40,000 rar- efaction pulses in 1 s. When the second compression pulse is produced; the first is 1 cm away from the source. Calculate the wave speed. [Refer to TB page 191] A. We know frequency is equal to number of compression or rarefaction pulses travelled per second, hence frequency (ν ) = 40000 Hz Wave length ( ) = Distance between two consecutive compression or rarefaction pulses. = 1cm From v = ν = 40,000Hz x 1cm = 40,000 cm/s = 400 m/s 2.4 Key Concepts i. The distance between two successive compressions or rarefactions is called wave- length. ii. The maximum variation in density or pressure from the main value is called amplitude or the maximum disturbance of particles of a medium from their mean position is called amplitude. iii. The time taken to complete one oscillation in the medium is called time period of wave. iv. The number of oscillations of the medium at a place in unit time is called frequency. v. The distance by which a point on the wave such as a compression or rarefaction travels per unit time is called speed of sound. 2.5 Conceptual Understanding Q1. Hertz stands for oscillations per [Refer to TB page 202 Q3] (a) second (b) minute (c) hour (d) millisecond A. (a) second Q2. When we increase the loudness of sound of a TV, the property of sound that changes is [Refer to TB page 203 Q4] (a) amplitude (b) frequency (c) wavelength (d) speed SESSION 2. CHARACTERISTICS OF THE SOUND WAVE 122

A. (a) amplitude Q3. Explain the following terms. [Refer to TB page 203 Q7] (a) amplitude (b) wavelength (c) frequency A. a) Amplitude: Amplitude is the maximum disturbance in the medium on either side of the mean position. It is usually represented by ‘A’ and its SI unit is metre. b) Wavelength: The distance between two consecutive compressions or two consec- utive rarefactions is called the wave length (or) the distance between two consecutive crests or troughs is called wave length. c) Frequency: The number of waves or oscillations propagated in unit time is called the frequency. Q4. Deduce the relation between, wavelength, frequency and speed of sound. [Refer to TB page 204 Q1, Try These] A. Let the time period of a vibrating body be T. It means, the vibrating body completes one oscillation in time T. One oscillation of a body is represented by a wave of wavelength ń. Now, wave speed = distance travelled by the wave/ time taken Here distance travelled by the wave = wavelength (ń) Time taken = T Then wave speed = V = ń/T = ń × 1/T But, 1/T = ν (frequency of the wave) Therefore, V = ń ν Thus, speed of wave = wavelength × frequency SESSION 2. CHARACTERISTICS OF THE SOUND WAVE 123

Q5. Two sources A and B vibrate with the same amplitude. They produce sounds of frequen- cies 1 KHz and 30 KHz respectively. Which of the two waves will have larger power? [Refer to TB page 204 Q3, Try These] A. Though the two sources A and B vibrate with same amplitude and produce equal sound, the one with more frequencies, that is, 30 KHz has larger power because the sound with more frequency has more energy. Q6. Define the wavelength of sound waves. How is it related to the frequency and the wave speed? [Refer to TB page 203 Q9] A. Wavelength: The distance between any two consecutive compressions or rarefactions is called wave length. (or) The distance between any two consecutive crests or troughs is called wave length of the wave. The relationship between frequency and time period: Let the time taken for oscillations = 1 sec The time taken for one oscillations = 1/ν But the time taken for one oscillation is called the time period (T) and the number of oscillations per second is called the frequency (ν) . Hence Frequency and time period are related as T= 1/ν or ν = 1/T Relation between frequency and wave length with speed of wave; Let the distance travelled by a wave in T seconds = ń metres The distance travelled by a wave in 1 second = ń/ T metres––––––(1) Thus by definition, speed of sound wave v=ń/ T We know that frequency ν = 1/T ––––––––––––––––––––(2) From equation (1) and (2) we get v =ν ń Speed of a sound wave = frequency x wave length. Q7. Find the period of the source of the sound wave whose frequency is 400 Hz. [Refer to TB page 203 Q12] A. Frequency = 400 Hz T = Time period =1/400 = 0.0025 S Time period of source is 0.0025 Seconds. SESSION 2. CHARACTERISTICS OF THE SOUND WAVE 124

2.6 Application to Daily Life, Concern to Bio Diversity Q1. Given that sound travels in air at 340 m/s, find the wavelength of the waves in air pro- duced by a 20 kHz sound source. If the same source is put in a water tank, what would be the wavelength of the sound waves in water? (Speed of sound in water 1480 m/sec.) [Refer to TB page 204 Q19] A. Case I: Velocity = 340 m/s and Frequency = 20000 Hz Speed = wavelength × frequency Wavelength = 340/ 20000 = 0.017 m Case II: If same source is put in water, speed in water = 1480 m/s Wavelength = 1480/ 20000 = 0.074 m SESSION 2. CHARACTERISTICS OF THE SOUND WAVE 125

SESSION 3 CHARACTERISTICS OF A MUSICAL SOUND AND REFLECTION OF SOUND 3.1 Mind Map SESSION 3. CHARACTERISTICS OF A MUSICAL SOUND AND REFLECTI... 126

3.2 Terminology i. Pitch – Characteristic of sound that is dependent on the frequency. ii. Loudness – Characteristic of sound that is dependent on the volume. iii. Quality of sound – Characteristics of sound which allow the ear to distinguish sounds which have the same pitch and loudness. iv. Echo – Reflection of sound that arrives at the listener after a small interval of time of arrival of the original sound. v. Reverberation – The collection of reflected sounds from the surfaces in an enclosure. 3.3 Solved Examples Q1. Example 3: An echo is heard after 0.8 s when a boy fires a cracker, 132 m away from a tall building. Calculate the speed of sound. [Refer to TB page 196] A. Echo time, t = 0.8 s Total distance travelled by sound wave, 2d = 2 x 132 m = 264 m Therefore, speed of sound v = 2d = 264m = 330 m/s t 0.8s 3.4 Key Concepts i. Pitch is a characteristic of sound which distinguishes between a shrill sound and grave sound. ii. The degree of sensation produced on the ear is called loudness. iii. The quality or timbre of sound is the characteristic which enables us to distinguish between musical notes emitted by different musical instruments. iv. A reflection of sound arriving at the listener in more than 0.1 seconds after direct sound is called an echo. v. A reflection of sound arriving at the listener in less than 0.1 seconds after direct sound is called reverberation. SESSION 3. CHARACTERISTICS OF A MUSICAL SOUND AND REFLECTI... 127

3.5 Conceptual Understanding Q1. The characteristic of a sound that describes how the brain interprets the frequency of sound is called [Refer to TB page 203 Q5] (a) pitch (b) loudness (C) quality (d) sound A. (a) pitch Q2. In a stethoscope, the sound of heartbeats travels through stethoscope’s tube [Refer to TB page 203 Q6] (a) by bending along the tube (b) in a straight line (c) undergoing multiple reflect (d) all of the above A. (c) undergoing multiple reflections Q3. Does the sound follow same laws of reflection as light does? [Refer to TB page 204 Q2, Try These] A. Sound follows reflection, refraction and interference, dispersion just as light. But sound waves are longitudinal waves where as light waves are electromagnetic waves, which can travel even though vacuum. Q4. Explain how echoes are used by bats to judge the distance of an obstacle in front of them. [Refer to TB page 203 Q10] A. Bats search out prey by emitting and detecting reflections of ultrasonic waves. Very high frequency ultrasonic squeaks of the bat are reflected from prey and returned to the bat’s ear. Amount and nature of reflected wave helps the bat in estimating the size and distance of prey. SESSION 3. CHARACTERISTICS OF A MUSICAL SOUND AND REFLECTI... 128

3.6 Appreciation and Aesthetic Sense, Values Q1. How do you appreciate efforts of a musician to produce melodious sound using a mu- sical instrument by simultaneously controlling in frequency and amplitude of the sounds produced by it? [Refer to TB page 204 Q6, Try These] A. i. The sounds which produce a pleasing effect on the ear are called musical sounds. ii. Any instrument which produces musical sound is called musical instrument. iii. The person who plays a musical instrument to produce melodious sound is called a musician. iv. The musician must have control on breathing, concentration on the output of the sound, which is a very hard task. v. The quality of sound is that characteristic of a sound which enables us to distinguish between musical notes emitted by different musical instruments or voices even though they have same pitch and loudness. Pitch depends on frequency and loudness depends on amplitude. By simultaneously controlling frequency and amplitude the musician can vary quality and produce melodious notes. The musician should have control over this. vi. For this, the musician need a lot of practice. vii. With the musician’s practice and knowledge over musical notes only we can hear a melodious sound, otherwise it could only be a noise. Hence the efforts of musician are highly appreciable. 3.7 Application to Daily Life, Concern to Bio Diversity Q1. How are multiple reflections of sound helpful to doctors and engineers? [Refer to TB page 204 Q7, Try These] A. Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs. In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflections and by amplifying the sound. Doctors can see the images of patient’s organ like liver, gall bladder, uterus etc. to know the abnormalities in their functioning using ultrasounds. Engineers use the reflection of sound in designing concert halls and theatre halls. SESSION 3. CHARACTERISTICS OF A MUSICAL SOUND AND REFLECTI... 129

Q2. Why is soft furnishing avoided in concert halls? [Refer to TB page 204 Q18] A. Soft furnishing such as cushioned chairs, thermocol–articles, fluffy material, and door curtains are avoided in concert halls because they are good absorbers of sound because they are soft materials. Q3. A man is lying on the floor of a large, empty hemispherical hail, in such a way that his head is at the centre of the hall. He shouts “Hello!” and hears the echo of his voice after 0.2 s. What is the radius of the hall? (Speed of sound in air = 340 m/s) [Refer to TB page 204 Q9, Try These] A. Speed of sound in air, v = 340 m/s The total distance travelled by ‘Hello!’ and its echo = 2d Time taken by sound and its echo back to the man = 0.2 s Distance travelled = speed x time = 340 x 0.2 m = 68.0 m 2d = 68 m Radius of hemispherical hall = 34 m. Q4. “We know that sound is a form of energy. So, the large amount of energy produced due to the sound pollution in cosmopolitan cities can be used to our day to day needs of energy. It also helps us to protect bio diversity in urban areas.” Do you agree with this statement? If yes explain. [Refer to TB page 204 Q10, Try These] A. i. Sound is a form of energy. The large amount of energy produced due to sound pollution in cosmopolitan cities can assure us to meet the energy crisis in the world to some extent. ii. This sound energy originating from motor vehicles, jet, locomotives can be harnessed and used for converting into mechanical energy. This mechanical energy can be used to turn electric generators and electricity can be produced. iii. This electrical energy can be utilized in several ways. iv. The sound energy can also be made use to kill mosquitoes and harmful insects. v. Pitch depends on frequency and loudness depends on amplitude. By simulta- neously controlling frequency and amplitude the musician can vary quality and produce melodious notes. SESSION 3. CHARACTERISTICS OF A MUSICAL SOUND AND REFLECTI... 130

Q5. How do echoes in a normal room affect the quality of the sounds that we hear? [Refer to TB page 204 Q8, Try These] A. Room acoustics refer to the quality of sound maintained in the room, and they can affect your ability to hear effectively. Sound waves bounce off hard surface like windows, walls and hard floors. This creates sound reflections and echoes (called “reverberation”). The result of excess reverberation is distorted speech. Large gyms, cathedrals and open marble lobbies quickly come to mind when we think about reverberation. Reverberation can also occur in smaller spaces such as classrooms. We’ve all experienced how much easier it is to hear in rooms that are carpeted and have upholstered furniture (which absorbs noise) than in empty rooms with tile or cement floors. Anyone of these conditions (distance, noise or reverberation) can create listening prob- lems. More often than not, they occur together and have a debilitating effect on the ability to hear and process speech. HATS can help you overcome these listening difficulties. SESSION 3. CHARACTERISTICS OF A MUSICAL SOUND AND REFLECTI... 131

SESSION 4 RANGE OF HEARING AND APPLICATIONS OF ULTRASOUND 4.1 Mind Map SESSION 4. RANGE OF HEARING AND APPLICATIONS OF ULTRASOUND 132

4.2 Terminology i. Infrasonic sound – Sound of frequencies less than 20 Hz. ii. Sonic –Relating to a sound wave. iii. Ultrasonic sound – Sound of frequencies greater than 20 kHz. iv. Sonar – Sound Navigation And Ranging; a technique used to detect objects under water using sound waves. 4.3 Solved Examples Q1. Example 4: A research team sends a sonar signal to confirm the depth of a sea. They heard an echo after 6s. Find the depth of the sea, if the speed of sound in sea water is 1500.[Refer to TB page 200] A. Let the depth of the sea = d m Then total distance travelled by sonar signal (s) = 2d Speed of sound in sea water (u) = 1500 m/s Total time taken (t) = 6s From, s = ut, 2d = 1500 m/s x 6s ⇒ d = 9000 m = 4.5 km 2 4.4 Key Concepts i. The audible limit of human beings is usually 20 Hz to 20 KHz. ii. Sound of frequency less than 20 Hz is called infrasonic sound. iii. Sound of frequency higher than 20 KHz is known as ultrasonic sound. SESSION 4. RANGE OF HEARING AND APPLICATIONS OF ULTRASOUND 133

4.5 Conceptual Understanding Q1. Explain the working and applications of SONAR. [Refer to TB page 203 Q11] A. SONAR stands for Sound Navigation and Ranging; i. This is a method for detecting and finding the distance of objects under water by means of reflected ultrasonic waves. The device used in this method is also called SONAR. ii. SONAR system consists of a transmitter and a detector which are installed in the ‘Observation centre’ on the board of a ship. iii. From the observation centre on the frequencies, say 1,000 KHz, are sent in all directions into the water through transmitter. These waves travel in straight lines till they hit an object such as a submarine, sunken ship, a school of fish etc. The waves are then reflected and are received back by the receiver at the observation centre. The direction from which a reflected wave comes to the observation centre tells the direction in which the object is located. iv. From the time between sending the ultra sonic waves and receiving its echo and the speed of sound in sea water, the distance of the object from the observation center is calculated. Reflections from various angles can be utilized to determine the shape and size of the object. v. Let ‘d’ be the distance between the sonar and an underwater object, ‘t’ be time between sending an ultrasonic wave and receiving its echo from the object and ‘u’ is the speed of sound in water. The total distance covered by the wave from the sonar to the object and back is 2d. Using s = ut, or 2d = ut SESSION 4. RANGE OF HEARING AND APPLICATIONS OF ULTRASOUND 134

Application: Marine geologists use this method to determine the depth of sea and and to locate underwater hills and valleys. Q2. A sound wave travels at a speed of 340 m/s. If its wavelength is 2 cm, what is the frequency of the wave? Will it be in the audible range? [Refer to TB page 203 Q13] A. V = speed of sound wave = 340 m/s Wavelength = 2 cm = = 0.02 m Frequency of the wave = v / ń = 340 / 0.02 Frequency of the wave = 17000 Hz The sound wave is in the audible range (since it is less than 20,000 Hz.) 4.6 Asking Questions and Making Hypothesis Q1. Which has larger frequency – infrasonic sound or ultrasonic sound? [Refer to TB page 203 Q14] A. Ultrasonic have larger frequency than infrasonic. The frequency of ultrasonic is higher than 20 kHz. The frequency of infrasonic is less than 20 Hz. SESSION 4. RANGE OF HEARING AND APPLICATIONS OF ULTRASOUND 135

Q2. You might have observed that sometimes your pet dog starts barking though no one is seen near in its surroundings or no disturbance heard nearby. What questions would you have in your mind about the peculiar behaviour of dog after your understanding about ‘range of hearing of sound’? [Refer to TB page 203 Q15] A. Dogs can hear ultrasonic’s up to a frequency of 50,000 Hz. Hence when a dog barks we may not see anyone nearby but the pet dog might have heard a stranger at a distance whom we can’t perceive and so it starts behaving peculiarly. 4.7 Information Skills and Projects Q1. Find out the names of the animals (and their photographs from internet) which commu- nicate using infrasonic or ultrasonic sound and prepare a scrap book. [Refer to TB page 204 Q5, Try These] A. Students’ Activity. 4.8 Application to Daily Life, Concern to Bio Diversity Q1. The grandparents and parents of a two–year old girl are playing with her in a room. A sound source produces a 28 kHz Sound. Who in the room is most likely to hear the sound? [Refer to TB page 203 Q17] A. The human ears are able to hear sounds in a frequency range of 20 Hz —20 kHz. For the elders, the upper limit often falls to 10–12 kHz. In the case of children they can hear sounds of frequencies up to 30 kHz. So the girl of two years can hear 28 kHz sounds but her grandparents and parents cannot hear such sounds. SESSION 4. RANGE OF HEARING AND APPLICATIONS OF ULTRASOUND 136

—— CCE Based Practice Questions —— AS1-Conceptual Understanding Very Short Answer Type Questions 1. Match the following. Column B a. Dresses and utensils [(Session 11.4)] b. Metal rods and construction materials Column A c. Breaking kidney stones d. Glass i. Drilling holes [] e. Imaging of organs and foetus ii. Ultrasonic cleaning [] iii. Ultrasonic detection [] iv. Ultrasonography [] v. Ultrasound surgery [] 2. Answer the following questions in one sentence. [Refer to Session 11.4 ] (i) What is the range of frequency of sound that the human ear can hear? (ii) Name the technique which uses ultrasonic waves to form an image of the heart. (iii) What does SONAR stand for? (iv) What is the use of SONAR? (v) What do you call sound of frequency less than 20 Hz? 3. Fill in the blanks. bodies. [Refer to Session 11.1 ] . (i) Sound is produced by (ii) Sound is a form of CHAPTER 11. SOUND 137

(iii) Pulse of a vibrating string where the density of air is more is known as . (iv) Pulse of a vibrating string where the density of air is less is known as . (v) If the particles of medium vibrate along the direction of wave, the wave is known as . 4. State true or false. ] [Refer to Session 11.3 ] (i) Laws of reflection of sound are different from the laws of reflection of light. [ (ii) Hard surfaces reflect sound better than soft surfaces. [] (iii) Reflected sound arriving at the position of listener more than 0.1 s after the direct sound is called reverberation. [] (iv) Pitch distinguishes between a shrill sound and a growling sound. [ ] (v) Incident sound and reflected sound make unequal angles with the normal. ] [ 5. Answer the following questions in one sentence. [Refer to Session 11.3 ] (i) What are the characteristics by which we can distinguish a musical note? (ii) When is the pitch of a musical note said to be high? (iii) What is loudness? (iv) What happens if the peak of a sound wave is high? CHAPTER 11. SOUND 138

(v) In what units is loudness of sound measured? 6. Fill in the blanks. reflections of sound. [Refer to Session 11.3 ] (i) Stethoscope works on the principle of (ii) If reflected sound reaches our ears in less than 0.1 s then it is known as . (iii) To reduce the echo in auditoriums, walls are covered with materials. (iv) In horns, conical surface helps to guide the . (v) Music arises through guitar because of vibration of _____________________ . 7. Answer the following questions in one sentence. [Refer to Session 11.2 ] (i) What four quantities describe the nature of the wave? (ii) What happens to density and pressure of air when sound travels through it? (iii) During a thunderstorm why do you think we first see the flash and then hear sound? (iv) In whose honour was the SI unit of frequency, Hertz, given? (v) If a noise is made in an open atmosphere and in water, in which case is the sound heard first? 8. Fill in the blanks. [Refer to Session 11.2 ] (i) The distance between two consecutive crests or troughs is denoted by . CHAPTER 11. SOUND 139

(ii) Frequency of the sound wave is denoted as ________________ . . (iii) Speed of sound wave = (iv) Sound travels fastest in media. (v) Unit of amplitude in terms of pressure is . Short Answer Type Questions 9. Answer the following questions in 3-4 sentences. (i) [(Session 11.4)] List out the industrial applications of ultrasonic waves. (ii) [(Session 11.4)] A research team sends a SONAR signal to confirm depth of sea. They heard an echo after 10 s. Find the depth of sea water.(Speed of sound in water is 1500 m/s.) 10. Answer the following questions in 3-4 sentences. (i) [(Session 11.1)] How does sound travel from source to our ears? (ii) [(Session 11.1)] Define longitudinal and transverse waves. How do they affect the medium? 11. Answer the following questions in 3-4 sentences. (i) [(Session 11.3)] What do you mean by pitch? (ii) [(Session 11.3)] An echo is heard 0.75 s after a boy shouts from a distance of 150 m away from a building. Calculate the speed of sound. 12. Answer the following questions in 3-4 sentences. (i) [(Session 11.2)] What is the wavelength of a sound wave? (ii) [(Session 11.2)] On what basis is the loudness of sound determined? Long Answer Type Questions CHAPTER 11. SOUND 140

13. Answer the following questions in 6-8 sentences. (i) [(Session 11.4)] Explain the working of SONAR. 14. Answer the following questions in 6-8 sentences. (i) [(Session 11.3)] What is the difference between a reverberation and an echo? 15. Answer the following questions in 6-8 sentences. (i) [(Session 11.2)] In hydrogen gas, a tuning fork produces 25,000 compressions and 25,000 rarefactions in 1 s. When the second compression pulse is produced, the first is 1 cm away from the source. Calculate wave speed. AS2-Asking questions and making hypothesis Short Answer Type Questions 16. Answer the following questions in 3-4 sentences. (i) [(Session 11.3)] We can distinguish the musical notes produced by different musical instruments though they have same pitch and loudness. How do you think we can identify that? AS3-Experimentation and field investigation Short Answer Type Questions 17. Answer the following questions in 3-4 sentences. (i) [(Session 11.1)] Write an activity to show sound is a form of energy. AS4-Information skills and projects Short Answer Type Questions 18. Answer the following questions in 3-4 sentences. (i) [(Session 11.3)] Complete the table. S.No. Characteristic of sound The property that varies 1 Frequency 2 Loudness CHAPTER 11. SOUND 141

AS5-Communication through drawing and model making Long Answer Type Questions 19. Answer the following question. (i) [(Session 11.1)] Draw the diagram showing compressions and rarefactions of a vibrat- ing membrane. AS6-Appreciation and aesthetic sense, Values Long Answer Type Questions 20. Answer the following questions in 6-8 sentences. (i) [(Session 11.3)] How do you appreciate the uses of sound? AS7-Application to daily life, concern to bio diversity Short Answer Type Questions 21. Answer the following questions in 3-4 sentences. (i) [(Session 11.3)] What happens if we listen to loud music? (ii) [(Session 11.3)] What range of sound is considered as normal to humans? 22. Answer the following questions in 3-4 sentences. (i) [(Session 11.2)] On what properties does speed of a sound wave depend? (ii) [(Session 11.2)] What is sonic boom? How does it damage buildings? Long Answer Type Questions 23. Answer the following questions in 6-8 sentences. (i) [(Session 11.4)] How have ultrasonic waves helped doctors? Explain with the help of an example. 24. Answer the following questions in 6-8 sentences. (i) [(Session 11.3)] You are going to design an auditorium.You have been told that you need to take care in avoiding reflection of sound waves. What would you do to minimize the reflection of sound waves? CHAPTER 11. SOUND 142

25. Answer the following questions in 6-8 sentences. (i) [(Session 11.2)] Why is the sound produced from a stretched string different, when it is plucked lightly than when plucked strongly? Objective Questions AS1-Conceptual Understanding 26. Choose the correct answer. (i) The SI unit of wavelength is (B) metre (A) centimetre (C) pascal (D) kg/m3 (ii) Maximum disturbance of the particles in the medium on either side of its mean position is called the (A) oscillation (B) frequency (C) dispersion (D) amplitude (iii) One megahertz (MHz) is equal to (B) 103 Hz (D)108 Hz (A) 102 Hz (C)106 Hz (B) electromagnetic in nature (iv) Sound waves are (A) transverse in nature (C)magnetic in nature (D)longitudinal in nature (v) The distance between two consecutive compressions or rarefactions is called (A) wavelength (B) amplitude (C) frequency (D)time period (vi) The number of oscillations completed in one second is called CHAPTER 11. SOUND 143

(A) amplitude (B) frequency (C) pitch (D) tone (vii) The range of audibility for human ears is (B) 20 KHz –200 KHz (A) 2 Hz–20 KHz (C)2 KHz –20 KHz (D)20 Hz –20 KHz (viii) A reflection of sound, arriving at the listener in more than 0.1s after direct sound is called (A) reverberation (B) echo (C) loudness (D) pitch AS2-Asking questions and making hypothesis 27. Choose the correct answer. (i) Which is the characteristic of sound that distinguishes between a shrill sound and a deep low sound? (A) Wavelength (B) Pitch (C)Time period (D) Frequency (ii) Sound wave of which frequency is an ultrasonic sound? (A) 300 Hz (B) 30 Hz (C)30,000 Hz (D)3000 Hz (iii) Which is the characteristic of sound that enables us to distinguish between musical notes emitted by different musical instruments? (A) Loudness (B) Quality (C) Pitch (D) Frequency CHAPTER 11. SOUND 144

AS4-Information skills and projects 28. Choose the correct answer. (B) pascal (i) The loudness of the sound is measured in (A) hertz (C) decibel (D) kilo–hertz AS5-Communication through drawing and model making 29. Choose the correct answer. (i) Identify the instrument shown in the following diagram. (A) SONAR (B) RADAR (C)Tuning fork (D) Stethoscope CHAPTER 11. SOUND 145

12. UNITS AND GRAPHS SESSION 1 UNITS i. Unit: A unit is a standard measure used for comparing measurements. ii. Fundamental Quantities: Physical quantities that cannot be expressed in terms of other quantities iii. Derived Quantities: The quantities that are obtained by combining fundamental quantities either by multiplication or division or both operations iv. MKS- metre kilogram second v. CGS- centimetre gram second SESSION 1. UNITS 146

vi. FPS – foot pound second vii. SI units - The International System of units of measurement i. The quantities are understood only when they are attached with a measuring term. ii. The physical quantities that cannot be expressed in terms of other quantities are called fundamental or basic quantities. iii. Fundamental quantities are independent quantities. iv. The quantities that are obtained by combining fundamental quantities either by multiplication or division or both operations are derived quantities. v. The measuring terms are called units. vi. Different units can be used for the same quantity depending on the quantity of the material. vii. Units are generally classified as fundamental units and derived units. viii. MKS, CGS, FPS and SI are the different measuring systems. ix. To compare any two quantities, their units must be common units. x. Some units are expressed using the name of some scientists. (E.g., newton(N)) No Questions in the Textbook SESSION 1. UNITS 147

SESSION 2 GRAPHS i. Graph – A pictorial form of representation that shows the relation between two quantities ii. Axes – The vertical and horizontal lines on the graph iii. Range – The difference between the highest and the lowest value iv. Scaling – Pointing of the values in the table on x-axis and y-axis v. Independent variable – The variable whose values are pre-determined vi. Dependent variable – The variable depending on the independent variable vii. Odometer – An instrument that measures the distance travelled viii. Slope of a straight line – The ratio between the change in y-coordinates and the x-coordinates SESSION 2. GRAPHS 148