A. 1. In first case, friction acts along the right. In second case, friction acts along the left.

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1 Chapter 12 Friction Topicwise Assignment Force of Friction A. 1. In first case, friction acts along the right. In second case, friction acts along the left. 2. Friction opposes the motion of one surface over the other. 3. No, friction is more for the rough surfaces and less for the smooth surfaces. Factors Affecting Friction A. 1. spring balance 2. Weight 3. spring, force, graduated 4. Force B. 1. The cell covers more distance when rolled on a smooth table. 2. The distance covered is different because the different surfaces offer the different amount of friction to the objects moving on them. c. The force of friction will be zero as the net force acting on the box is zero. D. The iron block 2 will require a larger force to move it as its weight is more. Friction: A Necessary Evil A. (a).b. 1. Necessity 2. Evil 3. Necessity Increasing and Reducing Friction A. She should apply some mud on her hand. This increases the friction between the palm and the rope that helps to make a better grip. B. 1. Increased 2. Increased 3. Decreased 4. Increased. C. Using rollers will help them to make this task easier due to the rolling friction as it is less than the sliding friction. Wheels Reduce Friction A. 1. Sliding friction: The friction which comes into play when a body slides over the surface of another body, is called sliding friction. 2. Rolling friction: The friction which comes into play when a body rolls over the surface of another body, is called rolling friction. B. (c) Static, sliding, rolling. C. The direction of frictional force acting on a sliding book is opposite to the direction of its motion, i.e. upward direction.

2 Fluid Friction A. 1. Fluids: The gases and liquids are known as fluids. 2. Drag: The frictional force exerted by fluids is called drag. 3. Streamlined shape: The shape of a body which is thicker at the middle and tapers at the ends is g called streamlined shape. Or A special shape of the objects that reduces the friction of fluid is called streamlined shape. Integrated Assignment Objective Type Exercises A. 1. (d),' 2. (b) 3. (a) 4. (b) 5. (b) 6. (d) 7. (c) 8. (d) B. 1. motion 2. roughness 3. heat 4. reduces 5. less c. 1. Talcum powder 2. Oil 3. Polishing 4. Roller D. 1. Treaded tyres 2. Grooved shoes Subjective Type Exercises A. 1. The rolling ball stops due to the friction caused by the ground in the opposite direction of the motion. 2. We slip on stepping over a banana peel because it is smooth and there is less friction between the peel and the ground. 3. Smooth and wet floor has less friction with object/person moving on it and he is likely to slip. 4. The vehicle slows down because the brake pads arrest the motion of the rim of the wheel due to the friction. 5. Friction is reduced if the surface is made smooth. If the surface is rough, the friction is more. 6. Rough surfaces have greater friction because they have more surface irregularities which get interlocked with those of surfaces placed on them. 7. Yes, friction occurs between the glass window pane and the raindrop. 8. The one with old tyres is likely to skid because the treading of old tyres gets worn and they offer the less friction with the road. 9. It will become difficult to walk because the irregularities on the marble get filled with the soapy water and the surface becomes very smooth reducing friction and one is likely to slip.

3 10. Friction is caused by the interlocking of irregularities between the two surfaces. 11. Friction is increased when two surfaces are pressed together. When mat is loaded, the irregularities on its surface get interlocked harder with those of the ground making it difficult to drag. 12. The force required to overcome the friction at the instant when object starts moving from rest is a measure of static friction. 13. The applied force which keeps the object sliding on a surface with same speed is equal and opposite to sliding friction. So force needs to be applied continuously. 14. An earthen pot has rough surface and has more friction with our hand. So, it does not slip and is easier to hold than glass or steel. 15. No, it is difficult to hold a tumbler whose surface is greased because it offers less friction to hand and is likely to slip. 16. Rough handle increases the friction with the hand of the player and ensures a better grip while playing. 17. The wearing off of cartilage increases the friction at joints. This causes joint pain and makes the movement difficult. 18. No, the object does not experience any friction as there is no medium or surface in contact with it. 19. No, it would not be possible to hold anything or even walk if there was no friction to hold two surfaces together When the chalk is moved over the blackboard, its rough surface rubs off some mall: particles on the board which stick to it. If the surface was not rough it would slide off without making any mark on the board. 21. The rubbing of palms makes one feel warm because the friction between the palms produces heat. 22. The jar of a mixer becomes hot because friction produces heat. 23. The sole of a shoe is grooved to provide it a better grip on the floor so that one can move safely without slipping. 24. Kabaddi players rub their hands with the soil to increase the friction for a better grip of their opponents. 25. We sprinkle fine powder on the carrom board to reduce the friction between the surfaces of carrom board and carrom coins so that carrom coins move smoothly. 26. The substances such as oil and grease which reduce the friction are called lubricants. 27. Grease is applied to moving parts of machines which forms a layer and the moving surfaces don't rub against each other making movement smooth.

4 28. Friction can't be completely eliminated because no surface can be completely smooth. Some irregularities are always there. 29. Luggage bags have rollers because rolling reduces friction and makes the bags easier to move from one place to another. 30. Streamlined shape helps birds and fish to lose less energy in overcoming friction. B. 1. The surfaces have irregularities on them which get interlocked into one another. If we apply force, we have to overcome the interlocking to move a surface. Since the rough surfaces have more irregularities, friction is more. 2. It is easier to move an object when it is already in motion because the sliding friction is less than the static friction. 3. The sliding friction is less than the static friction. This is because when the load starts sliding, the contact points on its surface do not get enough time to lock into the contact points of the other surface. 4. Seema has to apply a larger force because friction is more if the surfaces are pressed together which is the case with heavier box. 5. Initially, they applied force to set the car in motion. Once the car started rolling, they had to apply force to just balance the rolling friction which is less. 6. This helps to increase the friction for effective grinding as the surfaces become smooth after prolonged use. 7. The surfaces of soles become smooth after prolonged use. This reduces the friction between the sole and the floor making the slippers slippery. 8. Yes, it is rolling friction. On introducing air cushion, the friction is largely decreased. ' 9. If there was no friction between the tyres of automobile and the road, the vehicle could not be moved or stopped or turned to change the direction of motion. 10. Due to the force of friction, following actions are possible: holding objects constructing buildings writing on a paper or board fixing nail on the wall tying a knot walking without slipping 11. Friction is an evil due to the following reasons: wearing out of materials such as screws, ball bearings etc. tearing of shoe soles and tyres of vehicles. wastage of energy due to heating of electrical appliances.

5 12. The seat cover is very smooth and the friction between our body and the seat is very less. Thus as the brakes are applied, we tend to slip. 13. Use of ball bearings between hubs and axles of fans and bicycle. Use of rollers under luggage bags. 14. (a) Striking a matchstick against the rough surface of matchbox to burn and produce heat. (b) Due to the friction between rolling stones in the mountainous area the spark can cause fire in dry leaves in the forest which could result in forest tires. 15. The marble moves the shortest distance on the layer of sand because it offers the maximum frictional resistance to the marble. Be a Genius 1. (a) It is easier to move the book on rollers because the rolling friction is less than the sliding friction. (b) Rolling friction is less because less irregularities get interlocked between the surfaces as the entire surface is not in contact with the other surface. (c) Heavy machinery is moved by placing log-rollers under it as it is easier to move it on rollers. 2. (a) The friction between the rotating stone and the knife edge generates heat. The large friction results in the generation of large amount of the heat and sparks are produced. (b) Sandpaper and metal sheet have large amount of friction between them. So sand paper removes the outer dull layer from the metal surface on rubbing it and metal shines brightly. The ordinary paper has less friction and the outer side layer is not removed by it. 3. The objects moving through fluids have to overcome friction acting on them. This is done by giving them special streamlined shapes. 4. If friction suddenly vanished, the following instances would occur: We would not be able to walk. Vehicles would not move on the road and slip. The fixing of nail on the wall would be impossible. The holding of things would not be possible. The construction of buildings would never happen. No moving object would ever stop or change direction. 5. Due to the force of friction the following actions are possible: holding objects constructing buildings writing on paper or board fixing nail on the wall

6 tying a knot walking without slipping Friction is an evil due to the following reasons: it causes wearing out of materials such as screws, ball bearings etc. tearing of shoe soles and tyres of vehicles. wastage of energy due to heating of electrical appliances. 6(a) The force of friction between the brick and the floor. (b) The reading of spring balance reduces because the friction between the polythene and the floor is lesser. (C) The reading of spring balance increases because the friction increases. 7. Friction plays the following roles in sports: Sportsmen's shoes are made rough at sole to increase friction. This helps them to run, jump and play without slipping. Handles of bats and racquets are made rough to increase friction and ensure a firm grip. Gymnasts and weight lifters apply coarse powder on the hands for a firm grip. Talcum powder is sprinkled on the carrom board to reduce friction.

(A) Lubrication. (B) Warning signs. (C) Eliminate the motion. (D) None. (A) Increase. (B) Both. (C) Decrease. (D) None.

(A) Lubrication. (B) Warning signs. (C) Eliminate the motion. (D) None. (A) Increase. (B) Both. (C) Decrease. (D) None. FRICTION 1.When does a body start motion? 2.What is the cause of friction? 3.What is rolling friction? 4.What is sliding friction? 5.What do you mean by the term drag? 6.What is the direction of frictional