12 FRICTION TEXTBOOK QUESTIONS AND THEIR ANSWERS Q.1. Why a vehicle slows down when brakes are applied? Ans. A vehicle slows down when brakes are applied because the shoes of the brakes rub against the wheels causing a friction between them which in turn reduces the speed of the vehicle. Q.2. Why does a moving ball on the ground stops after some time? Ans. A moving ball on the ground stops after some time because of frictional force between the surface of the ball and the surface of ground. Q.3. Why do we slip when we step on a banana peel? Ans. We slip when we step on a banana peel because banana peel reduces the friction between your foot and the surface on which you are moving Q.4. Why is it difficult to move on a smooth and wet floor? Ans. There is a very little friction on a smooth and wet floor. That is why it is so difficult to move on a smooth and wet floor. 1
Q.5. Is the friction same for all the surfaces? Ans. No, the friction is not same for all the surfaces. Q.6. Does it depend on the smoothness of the surfaces? Ans. Yes, it depends on the smoothness of the surfaces. For a smooth surface, the friction offered by it is less and for a rough surface, the friction offered by it is more. Q.7. What is easier to move the box from rest or move it when it is already in motion? Ans. It is easier to move the box when it is already in motion. Q.8. Is it easier to hold an earthen pot or a glass tumbler? Ans. It is easier to hold a kulhar earthen pot due to more friction between its rough surface and fingers. Q.9. If the outer surface of the tumbler is greasy, would it become easier or more difficult to hold it? Ans. It the out surface of the tumbler is greasy, it would become more difficult to hold it. Q.10. Would it be possible to hold the glass at all if there is no friction? 2
Ans. If won t be possible for us to hold the glass at all if there is no friction. Q.11. Can you imagine being able to walk at all if there were no friction? Ans. No, we won t be able to walk if there were no friction. Q.12. When the teacher is writing with chalk on the blackboard, its rough surface rubs off some chalk particles which stick to the blackboard. Could it happen if there were no friction between the chalk and the board? Ans. No, it will not happen if there were no friction between the chalk and the blackboard. Q.13. Why are the sole of your shoes grooved? Ans. The sole of our shoes are grooved to provide the shoes a better grip on the floor so that we can move safely. Q.14. Why do you sprinkle fine powder on the carrom board? Ans. We sprinkle fine powder on the carrom board in order to reduce the friction so that the striker and the carrom coins are able to move smoothly on the carrom board. 3
Q.15. Can we reduce friction to zero by polishing surfaces or using large amount of lubricants? Ans. Friction can never be eliminated. No surface is perfectly smooth. Some irregularities are always there. Q.16. How can a child pull such pieces of luggage that is fitted with rollers? Ans. Even a child can pull such pieces of luggage that is fitted with rollers because rolling friction is less than sliding friction. So, it always easier to roll than side a body. Q.17. Why wheel is said to be one of the greatest inventions of mankind? Ans. Wheel is said to be one of the greatest inventions of mankind as its invention made transportation of goods and other heavy objects much easier. Q.18. Do you find any similarity in the shape of an aeroplane and that of a bird? Ans. The shape of an aeroplane and that of a bird are streamlined. Q.19. Fill in the blanks : 4
(a) Friction opposes the between the surfaces in contact with each other. (b) Friction depends on the of surfaces. (c) Friction produces. (d) Sprinkling of powder on the carrom board friction. (e) Sliding friction is than the static friction. Ans. (a) motion, (b) smoothness, (c) heat, (d) reduces, (e) less. Q.20. Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements are given below. Choose the correct arrangement : (a) Rolling, static, sliding (b) Rolling, sliding, static (c) Static, sliding, rolling (d) Sliding, static, rolling Ans. (c) Q.21. Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force 5
of friction acting on the car on different surfaces in increasing order will be : (a) Wet marble floor, dry marble floor, newspaper and towel. (b) Newspaper, towel, dry marble floor, wet marble floor. (c) Towel, newspaper, dry marble floor, wet marble floor. (d) Wet marble floor, dry marble floor, towel, newspaper. Ans. (a) Q.22. Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it. 6
Ans. The direction of frictional force is equal and opposite to the direction of motion of the sliding book kept on a tilted table. Q.23. You spill a bucket of soapy water on a marble floor earlier or accidentally. Would it make it more difficult for you to walk on the floor? Why? Ans. We fill a bucket of soopy water on a marble floor accidentally. It would make it more difficult for us to walk on such a floor because with a film of soupy water, marble floor will offer very less friction and hence, we will have less grip on the floor and eventually, will fall. Q.24. Explain why sportsmen use shoes with spikes? Ans. Sportsmen use shoes with spikes to get better grips on the ground so that they don t slip and fall while running. The spikes in the sportsmen s shoes increases the friction between surface of their shoes and the ground. Q.25. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply larger force and why? 7
Ans. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Seema will have to apply a larger force because a heavier box will offer a higher frictional force than a lighter box and hence, a greater force will be needed by Seema to push the heavier box. Q.26. Explain why the sliding friction is less than static friction. Ans. The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction. On the other hand, the force required to keep the object moving with the same speed is a measure of sliding friction. When a body starts sliding on the floor, the contact points on its surface, do not get enough time to lock into the contact points on the floor. So, sliding friction is slightly less than the static friction. Q.27. Give examples to show that friction is both a friend and a foe. Ans. Examples to show that friction is a friend are : (a) Friction helps us to walk. (b) With the help of friction, we are able to write on a paper with a pen. 8
(c) We require friction to burn a matchstick. Examples to show that friction is a foe : (a) Soles of our shoes wear down because of friction. (b) Friction wears out ball-bearings of machineries. (c) When a machine is operated, heat generated due to friction causes much wastage of energy. Q.28. Explain why objects moving in fluids must have special shapes. Ans. Objects moving in fluids must have special shapes in order to overcome resistance offered by air pressure along with minimum loss of wea andr tear due to friction of air. 9