## Topic outline

• ### Friction

##### Q57. Why will a pencil write on a paper, but not on a glass?

Ans. We are able to write on a paper because there is friction between the tip of the pencil and paper. We are not able to write on a glass because the glass surface is very smooth due to which friction between the tip of the pencil and glass surface is less. This friction is not enough to rub off black graphite particles from the tip of pencil.

##### Q58. When we try to push a very heavy box kept on ground, it does not move at all. Which force is preventing this box to move forward? Where does this force act?

Ans. When we push a very heavy box kept on the ground, it does not move at all because frictional force balances the force that we apply. So, there is no net movement of this box. It acts in a direction opposite to the force we apply.

##### Q59. Explain why objects moving in fluids must have special shapes.

Ans. Fluids exert force of friction on objects in motion through them. The frictional force on an object in a fluid depends on its speed with respect to the fluid. The frictional force also depends on the shape of the object and the nature of the fluid. It is obvious that when objects move through fluids, they have to overcome friction acting on them. In this process they lose energy. Efforts are, therefore, made to minimize friction.

##### Q60. Give examples to show that friction is both a friend and a foe.

i.        Friction enables us to walk.

ii.        Friction enables us to write with pen or pencil.

i.        Friction wears away the soles of our shoes.

ii.        The tyres of vehicles wear out gradually due to friction.

##### Q61. 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. Static, sliding, rolling

##### Q62. Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force 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) Wet marble floor, dry marble floor, newspaper and towel.

##### Q63. You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why?

Ans. We are able to walk because of the friction present between our feet and the ground. In order to walk, we push the ground in the backward direction with our feet. The force of friction pushes it in the forward direction and allows us to walk. The force of friction between the ground and feet decreases when there is soapy water spilled on the floor. Hence, it becomes difficult to walk on the soapy floor.

##### Q64. Which is easier to hold in hand: a kulhar (earthen pot) or a glass tumbler? Why?

Ans. It is easier to hold a kulhar (earthen pot) in our hand than a glass tumbler. This is because due to rough surface of kulhar, the friction between kulhar and our hand is much more which makes it easier to hold it. On the other hand, due to the smooth surface of glass tumbler, the friction between glass tumbler and hand is much less which makes it comparatively difficult to hold it.

##### Q65. 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 a larger force and why?

Ans. Force of friction arises because of interlocking of irregularities on the two surfaces in contact. When a heavy object is placed on the floor, the interlocking of irregularities on the surfaces of box and floor become strong. This is because the two surfaces in contact are pressed harder. Hence, more force is required to overcome the interlocking. Thus, to push the heavier box, Seema has to apply a greater force than Iqbal.

##### Q66. How do rough and smooth surfaces affect friction?OrWhy do smooth surfaces have less friction?OrWhy do smooth surfaces have less friction than non-smooth surfaces?OrWhy does a rough surface have greater friction than a smooth surface?OrFriction depends on the nature of two surfaces in contact. Explain.OrIs the friction the same for all the surfaces? Does it depend on the smoothness of the surfaces?

Ans. Friction is caused by the irregularities on the two surfaces in contact. Even those surfaces which appear very smooth have a large number of minute irregularities on them. Irregularities on the two surfaces lock into one another. When we attempt to move any surface, we have to apply a force to overcome interlocking. On rough surfaces, there are a larger number of irregularities. So the force of friction is greater if a rough surface is involved.