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NCERT Solutions for Class 8 Science chapter 12 – Friction

Back Exercise

Question 1:
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.

Answers
(a) motion
(b) Nature
(c) Heat
(d) Reduces
(e) Smaller

Question 2:
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

Answers
(c) static, sliding, rolling

Question 3:
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

Answers
(a) wet marble floor, dry marble floor, newspaper and towel.

Question 4:
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.

Answers
Frictional force is acting opposite to the movement of the book i.e. upwards.

Question 5:
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?

Answers
It would make it more difficult for us to walk on a soapy floor because layer of soap makes floor smooth. The coating of soapy water reduces the friction and the foot can not make a proper grip on the floor and we may get to slip on the floor.

Question 6:
Explain why sportsmen use shoes with spikes.

Answers
The sportsmen use shoes with spikes,in order to increase the force of friction between the ground and the shoe. Increased force of friction,provides more grip.This prevent sportsman from slipping or skidding during play.

Question 7:
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?

Answers
Friction force is directly proportional to mass of object. Heavier box will put more force on the surface as compared to lighter box and therefore will experience more friction force. Therefore seema will have to apply larger force.

Question 8:
Explain why sliding friction is less than static friction.

Answers
Sliding friction is less than static friction as the frictional force between two surfaces depends on the interlocking between the irregularities on the surfaces. This interlocking in case of sliding decreases as the contact points does not get enough time to interlock properly thus reducing the frictional force.

Question 9:
Give examples to show that friction is both a friend and a foe.

Answers
Friction is friend (Advantages) :
(i) Friction allows us to grip and catch different objects.
(ii) It helps us to walk comfortably on the surface.
(iii) It helps to minimise the speed or to stop the moving objects.
(iv) It helps us to write on paper or blackboard.
(v) The things do not move from their places due to friction.

Friction is foe (Disadvantages) :
(i) Friction causes wear and tear in objects.
(ii) It causes damage the parts of machines.
(iii) The machines or tools require regular maintenance due to which a lot of money is wasted.
(iv)It reduces the speed of moving objects so more force is required.
(v)It does not allow the free movement of objects.

Question 10:
Explain why objects moving in fluids must have special shapes.

Answers
When a body moves through a fluid, it experiences an opposing force which tries to oppose its motion through the fluid. The force of friction exerted by fluids is called drag or fluid friction. To overcome or to minimize drag, the body of object moving in fluids is given a special shape called streamlined shape. Aeroplane, jets, rain drops have streamlined shape.