## Topic outline

• ### Friction

##### Q24. Which type of friction comes into play when a book on cylindrical pencils is moved by pushing?

Ans. Rolling friction

##### Q25. Name the device which is attached to heavy luggage (such as a heavy suitcase) to move it easily by pulling.

Ans. Rollers

##### Q26. Give one example from everyday life where wheels (or rollers) are used to reduce friction.

Ans. Suitcase

##### Q27. Why a bicycle and a motor mechanic use grease between the moving parts of these machines?

Ans. They do so to reduce friction in order to increase efficiency.

##### Q28. Why are cars, aeroplanes and rockets streamlined?

Ans. Cars, aeroplanes and rockets are streamlined to reduce the fluid friction.

##### Q29. What prevents us from slipping every time we take a step forward?

Ans. Frictional force prevents us from slipping every time we take a step forward.

Ans. The treaded tyres of cars, trucks and bulldozers provide better grip with the ground.

##### Q31. What are fluids? Name two common fluids.

Ans. In science, the common name of gases and liquids is fluids. Example: air and water.

##### Q32. Why do gymnasts apply a coarse substance to their hands?

Ans. Gymnasts apply some coarse substance on their hands to increase friction for better grip.

##### Q33. Why is it difficult to climb up an oily or greasy pole?

Ans. An oily or greasy pole has much less friction due to which it is difficult to climb up an oily or greasy pole.

##### Q34. Why do tyres of cars wear out gradually?

Ans. The tyres of cars wear out gradually due to friction between the surface of tyres and the surface of road.

##### Q35. What is static friction?

Ans. The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction.

##### Q36. What is sliding friction?

Ans. The force required to keep the object moving with the same speed is a measure of sliding friction.

##### Q37. What is rolling friction?

Ans. When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction.

##### Q38. Why is it difficult to walk on a well-polished floor?

Ans. A well-polished floor offers very less resistance. Therefore, it is difficult to walk properly on a well-polished floor.

##### Q39. Why is it difficult to walk on a smooth and wet floor?

Ans. A smooth and wet floor offers very less resistance. Therefore, it is difficult to walk properly on a smooth and wet floor.

##### Q40. Can we reduce friction to zero by polishing surfaces or using large amount of lubricants?

Ans. Friction can never be entirely eliminated. No surface is perfectly smooth. Some irregularities are always there.

##### Q41. Which device is used between the hubs and axles of bicycle wheels to reduce friction?

Ans. Ball bearing is used between hubs and the axles of ceiling fans and bicycles to reduce friction.

##### Q42. What happens when you rub your hands vigorously for a few seconds? Why does this happen?

Ans. If we rub our hands together for several seconds, then our hands feel warm. That warmth is caused by a force called friction.

##### Q43. Explain why sportsmen use shoes with spikes.

Ans. Sportsmen use shoes with spikes because spikes increase the force of friction between the shoes and the ground and give them a better grip while running.