NCERT Solutions for Class 8 Science Chapter 9 Friction

NCERT
SOLUTIONS
CLASS - 8TH

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Book : Science Ncert Solutions | Chapter-12 Science

Class : 8th
Subject : Science
Chapter : 12
Chapter Name : FRICTION

Q1 Fill in the blanks.
(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.

Answer. (a) Friction opposes the motion between the surfaces in contact with each other.
(b) Friction depends on the nature of surfaces.
(c) Friction produces heat
(d) Sprinkling of powder on the carrom board reduces
(e) Sliding friction is less than the static friction.

Q2 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

Answer. (c) static, sliding, rolling
Friction comes into play when irregularities present in the surfaces of two objects in contact get
interlocked with each other. Static friction comes into play when we try to move an object which is
at rest. Sliding friction comes into play when an object slides over the surface of another object. In
sliding friction, the time given for interlocking is very small. Hence, interlocking is not strong.
Therefore, less force is required to overcome this interlocking. Because of this reason, sliding
friction is less than static friction. Similarly, the area of contact in case of rolling friction is
smallest as compared to static or sliding friction. This area of contact changes gradually because of
rolling. Hence, rolling friction is lesser than both static and sliding friction. Thus, the correct
sequence is—static, sliding, rolling.

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Q3 Alida runs her toy car on dry marble oor, wet marble oor, newspaper and towel spread on the
oor. The force of friction acting on the car on different surfaces in increasing order will be.

Answer. Alida runs her toy car on dry marble oor, wet marble oor, newspaper and towel spread
on the oor. The force of friction acting on the car on different surfaces in increasing order will be
(a) wet marble oor, dry marble oor, newspaper and towel.
(b) newspaper, towel, dry marble oor, wet marble oor.
(c) towel, newspaper, dry marble oor, wet marble oor
(d) wet marble oor, dry marble oor, towel, newspaper

Q4 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.

Answer. When a book slides on the writing desk, a frictional force acts between the book and the
surface of the desk. The direction of frictional force on the book is opposite to the direction of its
motion and acts in upward direction, as shown in the following gure.

Q5 You spill a bucket of soapy water on a marble oor accidently. Would it make it easier or more
dif cult for you to walk on the oor? Why?

Answer. 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 oor. Hence, it becomes dif cult to
walk on the soapy oor.

Q6 Explain why sportsmen use shoes with spikes.

Answer. Sportsmen use shoes with spikes because these shoes give them a better grip while
running. This is because the force of friction between the shoes and the ground increases with the
help of spikes.

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Q7 Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same oor.
Who will have to apply a larger force and why?

Answer. Force of friction arises because of interlocking of irregularities on the two surfaces in
contact. When a heavy object is placed on the oor, the interlocking of irregularities on the
surfaces of box and oor 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.

Q8 Explain why sliding friction is less than static friction.

Answer.Friction comes into play when irregularities present in the surfaces of two objects in
contact get interlocked with each other. In sliding, the time given for interlocking is very small.
Hence, interlocking is not strong. Therefore, less force is required to overcome this interlocking.
Because of this reason, sliding friction is less than static friction.

Q9 Give examples to show that friction is both a friend and a foe.

Answer. Advantages of friction:
(i) We are able to walk because of friction.
(ii) Friction between the tip of the pen and a paper allows us to write.
Disadvantages of friction:
(i) Tyres and soles of shoes wear out because of friction.
(ii) Friction between the different parts of machines produces heat. This can damage the
machines.

Q10 Explain why objects moving in uids must have special shapes.

Answer. When a body moves through a uid, it experiences an opposing force which tries to
oppose its motion through the uid. This opposing force is known as the drag force. This frictional
force depends on the shape of the body. By giving objects a special shape, the force of friction
acting on it can be minimised. Hence, it becomes easier for the body to move through the uid.

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