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
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
Q60. Give examples to show that friction is both a friend and a
Ans. Advantages of friction:
Friction enables us to walk.
Friction enables us to write with pen or
Friction wears away the soles of our shoes.
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
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
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
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
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?
Why do smooth surfaces have less friction?
Why do smooth surfaces have less friction than non-smooth
Why does a rough surface have greater friction than a smooth
Friction depends on the nature of two surfaces in contact.
Is 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.