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# 202110241-TRIUMPH-STUDENT-WORKBOOK-PHYSICAL_SCIENCE-G08-PART1

## Description: 202110241-TRIUMPH-STUDENT-WORKBOOK-PHYSICAL_SCIENCE-G08-PART1

Physical Science Workbook_8_P_1.pdf 1 11/5/19 6:22 PM Name: ___________________________________ Section: ________________ Roll No.: _________ School: __________________________________

Table of Contents 1 26 1 FORCE 57 2 FRICTION 94 3 SYNTHETIC FIBRES AND PLASTICS 123 4 METALS AND NON-METALS 154 5 SOUND 184 6 REFLECTION OF LIGHT AT PLANE SURFACES 216 7 COAL AND PETROLEUM 245 8 COMBUSTION, FUELS AND FLAME 270 9 ELECTRICAL CONDUCTIVITY OF LIQUIDS PROJECT BASED QUESTIONS TABLE OF CONTENTS

SESSION 1 1. FORCE 1.1 Mind Map FORCE AND TYPES OF FORCES 1. Forces SESSION 1. FORCE AND TYPES OF FORCES 1

2. Contact forces 3. Non–contact forces SESSION 1. FORCE AND TYPES OF FORCES 2

4. Effects of force 1.2 Terminology i. Friction – Friction is the resistance to the movement of a body over the surface of another body. ii. Muscular force – The force that we exert by using our body muscle is known as muscular force. iii. Magnetic force – Attraction or repulsion of a magnet. The magnetic force attracts magnetic material such as iron. iv. Tension – When you try to stretch a rope or a string, the tightness of rope or string is called tension. v. Gravitational force – The force of attraction exerted between any two objects is known as gravitational force. The force of attraction exerted by the earth on all the bodies on the earth’s surface is the gravitational force of Earth. 1.3 Key Concepts i. Force is responsible for the changes taking place in nature. ii. Pull or push is called force. iii. Forces are of different types. iv. A force can be exerted on an object either in contact or at a distance. v. Contact force is a force which results when there is a direct physical contact between two interacting objects. Normal force, friction, muscular force, and tension are exam- ples of contact forces. SESSION 1. FORCE AND TYPES OF FORCES 3

vi. The force which occurs between two objects when the bodies are not in contact with each other is called force at a distance. Magnetic force and electrostatic force are the examples of force at a distance. vii. Muscular forces are responsible for the various actions taking place inside the body. viii. Force of friction always acts in a direction that opposes the direction of motion of the object. ix. Normal force is the contact force exerted on an object perpendicular to the surface on which it lays and balances the gravitational force, which works vertically downwards. x. The gravitational force is balanced by the upward tension force of a string. xi. Field is a region in which a force can be experienced by another object kept at any point in that region. xii. Magnet exerts magnetic force on the magnetic objects. xiii. Electrostatic force is the force exerted by charged bodies on other uncharged or charged bodies. xiv. All the objects on the earth or close to the earth will experience a gravitational pull. xv. Gravitational force is a force of attraction that exists between any two objects. 1.4 Reﬂection on Concepts Q1. [AS1] What is a force? What changes can be produced by a force? [Refer to TB page 16 Q1] A. A push or pull is called force. Force can change the shape or size of an object, state of motion of an object and its velocity. Q2. [AS1] Give two examples each for a contact force and a force at a distance. [Refer to TB page 16 Q2] A. Contact force : i. When we press a toothpaste tube against a brush, toothpaste comes out – muscular force. ii. Writing with a chalk on a board – muscular and frictional forces. Force at a distance : i. Needle of a magnetic compass moves when we place it in a magnetic ﬁeld – magnetic force. SESSION 1. FORCE AND TYPES OF FORCES 4

ii. Balloon attracts pieces of paper when rubbed with a woollen cloth – electrostatic force. Q3. [AS1] Explain Gravitational forces by giving a suitable example. [Refer to TB page 17 Q3] A. If an object is thrown upwards, there exists a force which pulls it down towards the earth, because of this it falls down to the ground. We call this force as gravitational force. Every object on the Earth or close to Earth, will experience a gravitational pull.This force of gravity is not just due to the attraction of the Earth. It is a force of attraction that exists between any two bodies (or masses) everywhere in the universe. Few examples of gravitational force are : i. Gravitational force causes an apple to fall downward from an apple tree. ii. Gravitational force keeps us walking on Earth instead of ﬂoating away into space. 1.5 Application of Concepts Q1. [AS1] How can you differentiate between contact force and force at a distance? [Refer to TB page 17 Q1] A. Contact force is a force where there is a direct physical contact between two interacting objects. For example, the tyres of a bicycle has direct contact with the surface of the road. The force which occurs without any physical contact between two objects is known as force at a distance or ﬁeld force. For example, the attraction of two magnets. Q2. [AS6] How do you appreciate the role played by friction in facilitating us for doing various activities? [Refer to TB page 17 Q5] A. Friction is necessary in our daily life. Friction is necessary for walking, driving vehicles, applying brakes, brushing teeth, running machines, writing on a black board etc. 1.6 Higher Order Thinking Skills Q1. [AS2] Imagine that friction has disappeared from the earth. What will happen? Explain. [Refer to TB page 17 Q3] SESSION 1. FORCE AND TYPES OF FORCES 5

A. If there is no friction, it will be difficult to walk and the moving objects will not slow down. Vehicle, especially two wheelers will slip on the road causing accidents and the brakes of vehicles will not work if friction is absent. 1.7 Suggested Experiments Q1. [AS3] Conduct an experiment to ﬁnd the limiting forces that can be borne by different strings. [Refer to TB page 18 Q1] A. Aim : To ﬁnd the limiting force that a string can bear. MATERIALS USED: Spring balance, weights, light strings, weight hanger. i. Take a spring balance and hang it from ceiling. Arrange the system as shown in ﬁgure. Put some small weights like 50 g on the weight hanger and note the reading of the spring balance. Now add some more weights on the weight hanger and note the reading of the spring balance. Do the same till the string is broken. Also note the reading of the balance when the string is broken. ii. Separate the whole system from the ceiling, and tie the string to the weight hanger and now slowly pull up the whole system with your hand when there is a small weight on the hanger. Note the reading and do the same when you bring it down. SESSION 1. FORCE AND TYPES OF FORCES 6

Observation : • Tension is a force that acts along the string and it counters gravity of the suspended object. • The string does not break as long as the gravitational force does not exceed the ten- sion of the string. • The magnitude of gravitational force that just exceeds the tension in the string is the value measured by the spring balance at the instant the string breaks. This value is the limiting force borne by the string. Conclusion : The force required to overcome the gravitational pull on the stone acts in the upward direction and it is called as the tension of the string. The minimum amount of force, acting in the direction opposite to the direction of tension of the string, required to break the string is called the limiting force borne by the string. 1.8 Suggested Projects Q1. [AS4] Collect pictures to illustrate contact forces, forces at a distance and prepare a report. [Refer to TB page 18 Q1] A. Students’ activity. Q2. [AS4] Classify the actions in your daily life in to – [Refer to TB page 18 Q2] i. Actions where we exerts forces which appear as a push. ii. Actions where we exert force which appear as a pull. iii. Actions which involve both push and pull. A. Students’ activity. Few examples are given as a reference. i. Actions where we exerts forces which appear as a push. a. Pushing a door. b. Pushing a baby stroller. ii. Actions where we exert force which appear as a pull. SESSION 1. FORCE AND TYPES OF FORCES 7

a. Hoisting a ﬂag. b. Opening a drawer. iii. Actions which involve both push and pull. a. Digging a bore well. Q3. [AS4] Observe the situations of electrostatic forces in your life and prepare a report. [Refer to TB page 18 Q3] A. Students’ activity. Few examples are given for reference. i. Plastic bags sometimes stick to our clothes. ii. Lightning. iii. While taking off woollen clothes there is always a sound because of electrostatic force. Objective Questions (1) Hoisting a ﬂag is related to (Pg 17; TB Q 1) (A) push (B) pull (C) push and pull both (D) pressure Correct Answer: B (2) A person is pulling water from well. Which type of force is it ? ( Pg 17; TB Q 2) (A) Muscular force (B) Magnetic force (C)Friction force (D)Electrostatic force Correct Answer: A (3) The force that a solid surface exerts on any object in the normal direction is called (Pg 18; TB Q 3) (A) Muscular force (B) Normal force (C)Tension force (D)Magnetic force Correct Answer: B 8 SESSION 1. FORCE AND TYPES OF FORCES

SESSION 2 NET FORCE AND PRESSURE 2.1 Mind Map 1. Net force 2. Pressure 2.2 Terminology i. Net force – The algebraic sum of all the forces acting on object is called net force. ii. Applied Force – An applied force is a force that is applied to an object by a person or another object. iii. Equilibrium – Equilibrium is the state of being completely at rest or applied forces being balanced by one another. iv. Pressure – The force acting perpendicularly on a unit area is called pressure. SESSION 2. NET FORCE AND PRESSURE 9

v. Free body diagram – Free–body diagrams are the diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation. 2.3 Key Concepts i. The algebraic sum of all the forces acting on an object is called net force. ii. The direction of the net force acting on an object depends on the direction of forces acting upon it. iii. The strength of a force is represented by its magnitude. iv. Equilibrium is the state of being completely at rest or applied forces being balanced by one another. v. Force can change the shape of an object, direction of moving object or it can bring the moving object to rest. It can change the velocity of an object. vi. Effect of force depends on the area of contact. vii. The force acting on a unit area is called pressure. 2.4 Reﬂection on Concepts Q1. [AS1] Why do tools meant for cutting always have sharp edges? [Refer to TB page 17 Q5] A. The lesser the area of contact, the greater will be the pressure. Hence, sharp edges will be more effective in cutting. Q2. [AS5] Draw and explain a free body body diagram (FBD) to show all the forces acting on a car? [Refer to TB page 17 Q4] A. Considering the car is moving, coasting to the right, and slowing down. [ Neglect air resistance ] SESSION 2. NET FORCE AND PRESSURE 10

Considering the car at rest. Here in all the above Free Body Diagrams Fnorm = Normal force Fgrav = Gravitational force Ffrict = Frictional force 2.5 Application of Concepts Q1. [AS1] Find the net forces from the following diagrams. [Refer to TB page 17 Q2] A. a) Net force = 8 + 10 – 12 = 6 N, movement towards the right. b) Net force = 8 – 8 = 0 N, no movement. c) Net force = 8 + 6 = 14 N, movemet towards the right. d) Net force = 9 – 8 = 1 N, movement upward. Q2. [AS5] A man is standing still on a level ﬂoor. What forces act on him? Draw a free body diagram (FBD) to show all forces acting on him. [Refer to TB page 17 Q3] SESSION 2. NET FORCE AND PRESSURE 11

A. Gravitational and normal forces are acting on a man standing on a level ﬂoor. Where FN1 and FN2 are the normal forces acting on the man’s legs Fg is the force of gravity. And gravitational pull is balanced by the normal forces acting on the legs. Q3. [AS7] The surface area of an object is 20m2 and a force of 10N is applied on it, then what is the pressure? [Refer to TB page 17 Q4] A. Here given A = 20m2 F= 10N We know that P = F A = 10 20 SESSION 2. NET FORCE AND PRESSURE 12

= 0.5 N /m2 2.6 Higher Order Thinking Skills Q1. [AS1] If you push on a heavy box which is at rest, you must exert some force to start its motion. However, once the box is sliding you apply a lesser force to maintain that motion. Why? [Refer to TB page 17 Q1] A. The amount of force to move an object which is at rest should be more than the static friction acting on the object. When an object starts moving it is easy to maintain move- ment because the frictional force becomes less, i.e. the sliding friction (friction when an object is already in motion) becomes less than the static friction (friction that needs to be overcome to move an object from rest). Its all because sliding friction is lesser than static friction. Q2. [AS1] How do you increase the pressure by keeping [Refer to TB page 17 Q2] a) Area Unchanged b) Force Unchanged A. a) By increasing the force applied. b) By decreasing the area on which the force is applied. SESSION 2. NET FORCE AND PRESSURE 13

2.7 Suggested Experiments Q1. [AS3] Design and conduct experiments to test a few ways in which friction may be re- duced? [Refer to TB page 18 Q2] A. A few experiments that can be tried: 1. Polishing: a. Polishing the surface reduces the irregularities and hence reduces friction. 2. Use of ball–bearing or roller bearing: a. Roller bearing helps to convert sliding friction into a milder form – rolling friction. Rolling friction is lesser than sliding friction. 3. Lubrication: a. It provides a layer of the lubricant on which the bodies can easily slide. Lubricants like oil are used in bicycle chains, pulleys or in machinery parts. Experiment –The rusted pulley makes a rattling noise. Apply some oil in between the wheel and axle. The sound will be reduced. Q2. [AS3] Conduct an experiment to determine the changes in effect of force with an area of contact. [Refer to TB page 18 Q3] A. Take a pencil. Just push its rounded end on your palm. Now push from the other side of the pencil so that the sharp end is on your palm. SESSION 2. NET FORCE AND PRESSURE 14

We observe and experience that there is a difference in the force experienced. The blunt end does not hurt, but the sharp end hurts. The reason of this difference is because the effect of force depends on the area of contact on which the force is acting. When there is a decrease in the area of contact of the force or load then the effect of force increases and vice versa. Objective Questions (1) Let the forces F1 and F2 act on the table in the opposite directions, ( Pg 18; TB Q 4) F1 > F2, the Fnet = (A) F1 − F2 (B) F1 + F2 (C) 0 (D)2F2 − F1 Correct Answer: A (2) A situation for effect of force leads to a permanent change in shape of object is (Pg 18; TB Q 5) (A) Stretching rubber band (B) Squeezing sponge (C) Spring (D)Breaking glass Correct Answer: D —— CCE Based Practice Questions —— AS1-Conceptual Understanding Very Short Answer Type Questions 1. Answer the following questions in one sentence. [Refer to Session 1.1 ] (i) What type of change is produced by the following actions? (shape, direction or speed) i. Goal keeper stops the ball – ii. Rubber band is stretched – iii. Applying force on pedals of bicycle – iv. Tearing a piece of paper – CHAPTER 1. FORCE 15

(ii) Deﬁne the force that exists when a stone suspended with the help of a string is pulled downwards by the gravitational force. 2. Answer the following questions in one sentence. [Refer to Session 1.2 ] (i) If two forces with a magnitude A and magnitude B act on an object in the same direc- tion, what would be the net force? Short Answer Type Questions 3. Answer the following questions in 3-4 sentences. (i) [(Session 1.2)] Why does the sharper side of a knife cut more easily than the blunt side of it? CHAPTER 1. FORCE 16

(ii) [(Session 1.2)] What is FBD? How do you calculate the net force acting on a body? Long Answer Type Questions 4. Answer the following questions in 6-8 sentences. (i) [(Session 1.2)] What are the different effects of force? AS2-Asking questions and making hypothesis Short Answer Type Questions 5. Answer the following questions in 3-4 sentences. (i) [(Session 1.1)] Roopa’s mother said to Roopa not to run on the muddy road. Why do you think her mother said like that? CHAPTER 1. FORCE 17

6. Answer the following questions in 3-4 sentences. (i) [(Session 1.2)] The tyres of a motor bike, auto rickshaw, car and truck differ in their breadth. Your friend says that the breadth of the tyres does not matter to the load carried by the vehicle. Do you agree with your friend? Give two reasons to support your answer. CHAPTER 1. FORCE 18

AS3-Experimentation and ﬁeld investigation Long Answer Type Questions 7. Answer the following questions in 6-8 sentences. (i) [(Session 1.1)] Describe an experiment to ﬁnd the limiting force that can be borne by a string. State the aim, apparatus, procedure and the observation. CHAPTER 1. FORCE 19

AS4-Information skills and projects Very Short Answer Type Questions 8. Answer the following questions in one sentence. [Refer to Session 1.1 ] (i) Complete the table. S.No. Action Push/pull/both 1 Brushing teeth 2 When same poles of two magnets are brought together 9. Answer the following questions in one sentence. [Refer to Session 1.2 ] (i) The following table shows the effect of force on the shape of an object. Identify the changes and write in the table. CHAPTER 1. FORCE 20

S.No. Action of force Change in shape 1 (Permanent/ 2 Squeezing of a Temporary) sponge Tearing a paper Long Answer Type Questions 10. Answer the following questions in 6-8 sentences. (i) [(Session 1.1)] Read the information about different forces in the table. Complete the missing information. CHAPTER 1. FORCE 21

Situation Type of Direction of Name force force Normal Force exerted Perpendicular by an object on Contact Magnetic the table Towards the Tension force magnetic ﬁeld Gravitational force Contact Towards the Electrostatic force centre of the force earth Force between an inﬂated balloon and pieces of paper AS5-Communication through drawing and model making Short Answer Type Questions 11. Answer the following question. (i) [(Session 1.2)] Raman ﬂies a kite in the sky. Draw the diagram of all the forces acting on the kite. CHAPTER 1. FORCE 22

Long Answer Type Questions 12. Answer the following questions in 6-8 sentences. (i) [(Session 1.1)] Rahul and Rajeev are pushing a box with equal forces. Will the box move in case (a)? What is the direction of the frictional force and how can they make it move easily in case (b)? a) b) CHAPTER 1. FORCE 23

AS6-Appreciation and aesthetic sense, Values Short Answer Type Questions 13. Answer the following questions in 3-4 sentences. (i) [(Session 1.1)] Give four examples of muscular force. AS7-Application to daily life, concern to bio diversity Long Answer Type Questions 14. Answer the following questions in 6-8 sentences. (i) [(Session 1.1)] A wall lizard is crawling on the ceiling. Name the forces exerted on the lizard. What will happen if one of the forces overcomes the other? CHAPTER 1. FORCE 24

15. Answer the following questions in 6-8 sentences. (i) [(Session 1.2)] Pressure on a body is inversely proportional to the area of contact. Give three examples from your daily life to demonstrate this principle. CHAPTER 1. FORCE 25

2. FRICTION SESSION 1 FRICTION 1.1 Mind Map 1.2 Terminology i. Frictional force – The force which opposes the relative motion of two surfaces of bodies which are in contact is called frictional force. ii. Static friction – Static friction is the friction which comes into play when surfaces of the objects are at rest relative to each other. iii. Sliding friction – Sliding friction is the friction which comes into play when the surface of one object moves relative to the surface of another object. iv. Rolling friction – Rolling friction is the opposing force that slows down the motion of a rolling ball or wheel. 1.3 Key Concepts i. Frictional force opposes the relative motion between two surfaces in contact. It acts on both the surfaces. ii. Frictional force acts in the direction opposite to that of the applied force. iii. Frictional force depends on the nature of surfaces in contact and on the areas of contact. SESSION 1. FRICTION 26

iv. Frictional force is of three types: static friction, sliding friction, rolling friction. v. Sliding friction is smaller than static friction but greater than rolling friction. vi. Friction is important for many of our activities. 1.4 Reﬂection on Concepts Q1. [AS1] Give few examples of sliding friction? [Refer to TB page 30 Q3] A. Few examples of sliding friction are – i. Rubbing both hands together to create heat. ii. A person sliding down a slide. iii. A sled sliding across snow or ice. Q2. [AS1] What kind of friction do you know and explain with suitable example? [Refer to TB page 30 Q1] A. I know about Static friction, Sliding friction, Rolling friction and Fluid friction. Static friction –Static friction is the friction which comes into play when surfaces of the objects are at rest relative to each other. A simple example for static friction is a wooden block kept on a table . Unless sufﬁcient force is exerted, the block will not slide down the table because static friction holds it in its place, and it resists sliding. Sliding friction – Sliding friction comes into play when an object is sliding over another object. As for an example: when a man skiing through on the mountain or when a man is skiing down a mountain, sliding friction comes into play. Rolling friction – When one body rolls over the surface of another body, the friction of- fered is called rolling friction. As for example a car with its wheels moving or the roller skates which we use for skating. Fluid friction – Fluid friction is the resistance to an object’s motion through a liquid or gas. A simple example for ﬂuid friction is air resistance and a swimmer swimming through water exhibits ﬂuid friction. Q3. [AS5] A book placed on the surface of a table. The book is pushed in one direction. Draw the forces acting on the book and explain. [Refer to TB page 30 Q2] SESSION 1. FRICTION 27

A. The forces acting on the book are Gravitational force, Normal force and Frictional force. Two forces act on the book in the vertical direction as shown in the above ﬁgure. They are: (i) Weight of the book (W) or gravitational force acting vertically down. (ii) Normal force (N) or reaction force applied by the ﬂoor vertically up. As there is no change in motion of the book along the vertical direction, the net force acting on the book in the vertical direction is zero. That is, W − N = 0 ⇒ W = N Here we are considering the book is pushed in horizontal direction. The speed of the book gradually decreases and after some time it stops.The surface of the table applies a force on the book against its motion. Similarly the book also applies the same amount of force on the surface of the table in the opposite direction. Here it is clear that the surface of the table is at rest. Hence the net force acts in the direction of the force applied by the surface of the table on the book. The force applied by the surface of the table on the book is called “frictional force” or friction. The forces acting on the book are Gravitational force, Normal force and Frictional force. SESSION 1. FRICTION 28

1.5 Application of Concepts Q1. [AS1] Explain why sportsmen use shoes with spikes? [Refer to TB page 30 Q1] A. Spikes provide grip on the ﬁeld by making the sole of the shoe a rough surface. This increases friction and enables the sportsmen to move safely without slipping. Q2. [AS1] Would it be easier or more difficult for you to walk on a marble ﬂoor with soapy water? Why? [Refer to TB page 30 Q2] A. It is difﬁcult to walk on a marble ﬂoor with soapy water because layer of soap acts as a lubricant and makes the ﬂoor smooth. The coating of soapy water reduces the friction due to which our foot may slip on the ﬂoor. 1.6 Suggested Experiments Q1. [AS3] Conduct an experiment to understand the nature of friction and concept of static friction. [Refer to TB page 31 Q1] A. Aim – Understanding the nature of friction and the concept of static friction. Materials required: Toy trolley, small wooden block, inextensible string, weight hanger, weights, pulley and table. SESSION 1. FRICTION 29

Procedure: Take a small toy trolley and keep a small wooden block on it as shown in above ﬁgure. Tie an inextensible string to the trolley and pass it over a pulley. The other end of the string is ﬁxed to the weight hanger. Take a small weight and keep it on weight hanger and observe the changes in motions of block and trolley. You will notice that the trolley with the block on it moves towards left with an acceleration. The block is at rest with respect to the surface of the trolley, but it is in motion with respect to the surface of the table. Now keep on increasing the weight on the hanger. Observe the motions of both trolley and block. The surface of the trolley tries to keep the block at rest here with respect to its surface. Thus, the force of friction by the surface of the trolley acts on the block in the direction of motion. At the same time the block also applies a force on the trolley in opposite direction and tries to move towards the right. SESSION 1. FRICTION 30

We can increase the trolley’s acceleration by increasing the weight on the hanger. If we increase the acceleration of trolley gradually, at certain limiting acceleration or limiting weight, the block comes into motion in the reverse direction. This means that now there exists relative motion between the surface of the trolley and the block. From the above activities we may deﬁne friction as follows : The force which opposes the relative motion of two surfaces of bodies in contact, is called ‘frictional force’. In this activity, the block is at rest relative to the surface of the trolley up to a certain limiting acceleration. The friction exists at this stage is a static friction. Sliding friction is the friction which comes into play when the surface of one object moves relative to the surface of another object. So, static friction is the friction which comes into play when surfaces of the objects are at rest relative to each other. In the above activity we observe that there exists two types of frictional force at a time. One is sliding friction between surface of the table and trolley, and the other is the static friction that exists between the surface of trolley and wooden block kept on it. Objective Questions (1) Which of the following friction has self adjusting force (Pg 31; TB Q 5) (A) sliding friction (B) static friction (C)rolling friction (D)fluid friction Correct Answer: B SESSION 1. FRICTION 31

SESSION 2 FACTORS AFFECTING FRICTION 2.1 Mind Map 2.2 Terminology i. Irregularities – Ups and downs on a surface are called irregularities of the surface. Irregularities create friction. ii. Normal force – Any force coming from the surface and acting at a right angle to the surface is called the normal force. Friction is proportional to the normal force. iii. Area of contact – That part of the surface in contact with the other surface where friction occurs between the two surfaces. 2.3 Key Concepts 32 i. All surfaces have minute irregularities. ii. These irregularities cause friction. iii. The roughness or smoothness of a surface inﬂuences the friction. SESSION 2. FACTORS AFFECTING FRICTION

iv. Friction is independent of the area of contact. 2.4 Reﬂection on Concepts Q1. [AS1] Explain with one example that frictional force is proportional to the normal force. [Refer to TB page 30 Q5] A. Keep a brick on the horizontal ﬂoor and pull it with the spring balance attached to it and measure the frictional force. Now put another brick over the brick tied to the spring bal- ance or press it vertically with your hand and then measure the force of friction. We will observe that when we add a brick to the existing brick or apply a force by pressing it ver- tically, the normal force increases and hence, we ﬁnd there is an increase in the frictional force. So, Friction is proportional to the Normal force i.e., Friction ∝ Normal Force 2.5 Application of Concepts Q1. [AS5] Draw a free body diagram to show various forces acting on a body which is sliding on an inclined plane. [Refer to TB page 30 Q5] A. In the free body diagram shown, the weight ‘mg’ is acting vertically downwards. The normal, N is perpendicular to the surface of the inclined plane. The force F1 is the applied force and the force F2 is the frictional force that is acting at the surface of contact and opposite to the direction of motion. 2.6 Suggested Experiments Q1. [AS3] Conduct an experiment to ﬁnd out the effect of roughness on frictional force. [Refer to TB page 31 Q2] SESSION 2. FACTORS AFFECTING FRICTION 33

A. Set up an inclined plane on the horizontal ﬂoor. Use a wooden board as inclined plane. Put a mark at any point “A” on the inclined plane. Now let a pencil cell or ball move down from this point. Note the distance covered by the pencil cell from the bottom of the inclined plane to point where it comes to a stop. Now, spread a cloth over the ﬂoor. Make sure that there are no wrinkles in the cloth. Try again with pencil cell. Now note down the distance. You can conclude that smoothness / roughness of the surfaces of both the ﬂoor and the pencil cell could affect the distance travelled by it. Though many surfaces look like perfect planes, there exists many ups and downs (irreg- ularities of surface) on them. Friction is caused by the irregularities on the two surfaces which are in con- tact. Irregularities on the two surfaces lock into one another, when we attempt to move on any surface. We have to apply a force to overcome interlocking. On rough sur- faces, there exists a large number of irregularities (ups and downs). Hence, the force of friction is greater if a rough surface is involved. SESSION 2. FACTORS AFFECTING FRICTION 34

2.7 Suggested Projects Q1. [AS4] Collect information about various new techniques being adopted by human beings to reduce energy losses due to friction. Preparea note on it .[Refer to TB page 31 Q1] A. Students’ activity. Few examples are given for reference. i. Ball bearings. e.g. roller skates. ii. Streamline objects. e.g. Aeroplane iii. Air cushioning. e.g. Hovercraft. iv. Alloys – alloys exhibit less friction across their surfaces. v. Lubricants. e.g using graphite, talcum powder, oil etc. With research in materials science, we have better lubricants that give almost zero friction. vi. Vacuum – It is seen that vacuum reduces aerodynamic friction. Objective Questions (1) The change in the length of a spring is to the applied force. (Pg 31;TB Q 2) (A) Indirect proportional (B) Equal (C)Direct proportional (D)Not depends on the force Correct Answer: C (2) When the speed of the body moving in a straight line changes continuously then the body acquires (Pg 31; TB Q 3) (A) Acceleration (B) Speed (C) Velocity (D) Displacement Correct Answer: A SESSION 2. FACTORS AFFECTING FRICTION 35

SESSION 3 IS FRICTION NECESSARY? 3.1 Mind Map SESSION 3. IS FRICTION NECESSARY? 36

3.2 Terminology i. Lubricants – Lubricants are substances such as oil or grease that are used to reduce friction in machineries. ii. Rollers – Rollers are cylinders that rotate on a central axis that are used in various machines to move. iii. Ball bearings – Ball bearings are devices containing a ring of freely rotating metal balls used to reduce friction between rotating shafts of machine tools. iv. Grip – Take and keep a firm hold of; grasp tightly. v. Rolling friction – When one body rolls over the surface of another body, the friction offered is called rolling friction. 3.3 Key Concepts i. Friction can be increased or decreased by making the surfaces in contact rough or smooth. ii. Friction is sometimes undesirable. iii. A bearing is a machine element that constrains relative motion between moving parts. SESSION 3. IS FRICTION NECESSARY? 37

iv. Lubricants are used to reduce friction. v. Rollers are used for heavy luggage and shifting since rolling friction is lesser than sliding friction. 3.4 Reﬂection on Concepts Q1. [AS1] Explain how does lubrication reduce friction? [Refer to TB page 30 Q4] A. The substances which reduce friction are called lubricants. When oil, grease or graphite is applied between the moving parts of a machine, they occupy the gaps due to the irregularities in the surface of the machine parts, reduce the interlocking of the surfaces thereby reducing friction. Interlocking of irregularities between the body and surface or two bodies is avoided to a great extent by the application of lubricants. Hence the movement becomes smooth. 3.5 Application of Concepts Q1. [AS2] What happens if we do not reduce friction in machine? [Refer to TB page 30 Q3] A. The less friction generated on the machine’s part, the more efficient the machine is in do- ing work. Increasing a machine’s efficiency by reducing friction makes the maintenance easier. Friction is responsible for overheating and wearing out of machinery parts. So if we do not reduce the friction, the efficiency of machine will get reduced and there will be wearing out of machinery part. Q2. [AS7] What purposes are served by bearings in machines? Explain with daily life exam- ples. [Refer to TB page 30 Q4] A. They reduce friction by making use of the principle that rolling friction is lesser than sliding friction. To reduce friction between rotating shafts of machine tools, we use “ball bearings”. It is always easier to roll a body than to slide it over a surface. So it is convenient to pull the luggage ﬁtted with rollers. SESSION 3. IS FRICTION NECESSARY? 38

3.6 Higher Order Thinking Skills Q1. [AS1] Do you agree with the statement, “Friction is both good and evil.” Explain with examples. [Refer to TB page 30 Q1] A. We cannot walk or run without friction. We will not have any cars, bicycles or scooters. All of them move only because of friction. Even if somebody pushes a car, we will not be able to stop it by applying brakes. Carpenters will not be able to smoothen surfaces. You will not be able to hold any appliances such as hammer, soap etc. It will not possible to write with pen or pencil if there is no friction. You would not be able to ﬁx a nail on the wall. No building could be constructed if there were no friction. All the above examples tell that friction is necessary. It is an evil because friction is responsible for overheating that leads to wear and tear of moving parts. Q2. [AS7] Reducing friction to the lowest possible level in machine and tools solves the problem of energy crisis and saves “biodiversity”. How do you support the statement? Explain. [Refer to TB page 30 Q3] A. i. Reducing friction in machines improves their efﬁciency. ii. When machines become more efﬁcient, the consumption of energy is less. iii. This can solve the problem of energy crisis. iv. It also decreases pollution. 3.7 Suggested Projects Q1. [AS4] Collect information about the substances using to reduce the friction in different machines and prepare a note on it. [Refer to TB page 31 Q2] A. Students’ activity. Few examples are given as reference. Method of reducing friction: SESSION 3. IS FRICTION NECESSARY? 39

• Ball–bearing: As the rolling friction is less than the sliding friction consequently in rotating machinery, the shafts are ﬁxed on the ball bearing so that the friction can be reduced considerably. For example–the free wheel of a cycle, the axle of amotorcar, the shafts of the motor dynamo etc. are provided with ball–bearing. • Lubricants: Friction between two surfaces can be decreased by using lubricants. A lubricant is a substance (a solid or a liquid) which forms a thin layer between the two surfaces in contact. It also ﬁlls the depressions present in the surfaces of con- tact and reduces friction considerably. In light machinery thin oils with low viscosity are used, in heavy and fast moving machinery thick oils and solutes (grease) are used. • Streamline: The friction due to air is reduced by making the automobiles stream- line. Objective Questions (1) Which material do gymnasts apply on their hands to increase friction for better grip? (Pg 31; TB Q 1) (A) Oil (B) Soap (C)Coarse substance (D) Water Correct Answer: C SESSION 3. IS FRICTION NECESSARY? 40

SESSION 4 FLUID FRICTION 4.1 Mind Map SESSION 4. FLUID FRICTION 41

4.2 Terminology i. Fluid friction – Water and other liquids exert ﬂuid friction when objects move through them. ii. Drag – Frictional force exerted by liquids is also called as drag. iii. Streamline – It is a type of body shape that provides little resistance to air or ﬂuid ﬂow. Example–Body shapes of birds and ﬁshes. iv. Viscosity – Viscosity is a measure of a ﬂuid’s resistance to ﬂow. It describes the internal friction of a moving ﬂuid. 4.3 Key Concepts i. Friction in liquids and gases is known as ﬂuid friction or drag. ii. Fluid friction describes the friction between layers within a viscous ﬂuid that are mov- ing relative to each other. iii. Fluid friction can be reduced by streamlining bodies. 4.4 Higher Order Thinking Skills Q1. [AS6] Scientists designed the shape of aeroplane using the technique of reducing ﬂuid friction by observing birds shape. How would you appreciate. [Refer to TB page 30 Q4] A. It is highly appreciated that scientists have observed and analysed the mechanism of bird’s ﬂying and designed the shape of the aeroplane using the technique of reducing ﬂuid friction by observing bird’s shape. It is obvious that when objects move through ﬂu- ids, they have to overcome friction acting on them. So efforts are made to minimize friction. Objects are given special streamlined shape to reduce friction. Streamlining re- duces the area of the front of the object as it moves forwards, so the air ﬂows around the object rather than being pushed forward by it. Objective Questions (1) The bodies of birds and ﬁshes must have evolved to shapes which would make them not to loose much energy in overcoming friction.(Pg 31; TB Q 4) (A) Rolling friction (B) Static friction (C)Sliding friction (D)Fluid friction Correct Answer: D SESSION 4. FLUID FRICTION 42

—— CCE Based Practice Questions —— AS1-Conceptual Understanding Very Short Answer Type Questions 1. Answer the following questions in one sentence. [Refer to Session 2.4 ] (i) Write any one method to reduce ﬂuid friction? 2. Fill in the blanks. [Refer to Session 2.3 ] (i) We use to reduce friction. (ii) Friction can be increased by making the surface . (iii) Sprinkling a powder on the carromboard the friction. (iv) In many machines, friction is reduced by using . (v) Threads are provided in tyres to friction. (vi) The substances which reduce friction are called . (vii) Soapy ﬂoor is slippery due to decreased . (viii) In undesirable cases we friction. (ix) Matchstick catches ﬁre due to . (x) Friction causes of energy. CHAPTER 2. FRICTION 43

3. Answer the following questions in one sentence. [Refer to Session 2.2 ] (i) Why do we use grease between moving parts of motor vehicles? Short Answer Type Questions 4. Answer the following questions in 3-4 sentences. (i) [(Session 2.1)] “Friction is independent of weight, but depends on normal force be- tween surfaces of contact where friction exists.” Do you agree with this statement? Discuss. When normal force is doubled, what happens to friction? CHAPTER 2. FRICTION 44

(ii) [(Session 2.1)] What is static friction and when does it come into play? Give an exam- ple. Long Answer Type Questions 5. Answer the following questions in 6-8 sentences. (i) [(Session 2.3)] Do you agree with the statement, “friction is both good and an evil”? Explain with examples. CHAPTER 2. FRICTION 45

AS2-Asking questions and making hypothesis Short Answer Type Questions 6. Answer the following questions in 3-4 sentences. (i) [(Session 2.2)] We have two sacks of books on the ﬂoor. One is wrapped in jute and the other is wrapped in polythene. Which one will be easy to drag and why? (ii) [(Session 2.2)] By looking at Rohith’s sports shoes, Vijaya asks why do his shoes have spikes. What may Rohith reply? CHAPTER 2. FRICTION 46

AS3-Experimentation and ﬁeld investigation Long Answer Type Questions 7. Answer the following questions in 6-8 sentences. (i) [(Session 2.2)] Write an activity to show the effect of normal force on friction. AS4-Information skills and projects 47 Very Short Answer Type Questions 8. Answer the following questions in one sentence. [Refer to Session 2.2 ] (i) Fill in the right information in the boxes given below. CHAPTER 2. FRICTION

Situation Types of friction When a block of wood kept on table just tends to move. When a block of wood kept on table moves (or slips). (ii) Abdul and Joseph tried to push a heavy box but could not succeed. Abdul suggested Joseph to ﬁt wheels to the box so that they could move it easily. Do you think the information suggested by Abdul to Joseph is correct? Brieﬂy describe your response. CHAPTER 2. FRICTION 48