# NCERT Solutions for Class 8 Science Chapter 12 - Friction

##### Question 1:

Friction opposes the __________ between the surfaces in contact with each other.

relative motion

##### Question 2:

Friction depends on the __________ of surfaces.

nature

##### Question 3:

Friction produces __________.

heat

##### Question 4:

Sprinkling of powder on the carrom board __________ friction.

decreases

##### Question 5:

Sliding friction is __________ than the static friction.

less

##### Question 6:

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.

1. Rolling, static, sliding
2. Rolling, sliding, static
3. Static, sliding, rolling
4. Sliding, static, rolling

Static, sliding, rolling

##### Question 7:

Alida runs her car on a 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

1. wet marble floor, dry marble floor, newspaper and towel.
2. newspaper, towel, dry marble floor, wet marble floor.
3. towel, newspaper, dry marble floor, wet marble floor.
4. wet marble floor, dry marble floor, towel, newspaper.

wet marble floor, dry marble floor, towel, newspaper.

##### Question 8:

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.

The frictional force acting on it is in the upward direction.

##### Question 9:

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

Soapy water will make the walking more difficult. Soapy floor acts as a smooth surface as it has no irregularities in it and soap is also very slippery in nature.

##### Question 10:

Explain why sportsmen use shoes with spikes.

Sportsmen use shoes with spikes to get a better grip on the ground. Spikes act as irregularities and help in increasing friction.

##### Question 11:

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?

Seema will experience more frictional force since the heavier box applies greater pressure on the floor as compared to the lighter box.

##### Question 12:

Explain why sliding friction is less than static friction.

When a body is at rest, then the force acting between the two surfaces in contact is more due to irregularities on the two surfaces locked into one another. However, when a body is moving or sliding, the contact points do not get enough time to lock into one another. So sliding friction is always less than static friction.

##### Question 13:

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

Friction is considered a friend. It is because of friction that:
(i) We can write with a pen or pencil.
(ii) A teacher is able to write with chalk on the blackboard.
(iii) We can walk on the floor.
(iv) We can fix a nail on the wall.
Friction is considered a foe. It is because of friction that:
(i) Things experience wear and tear.
(ii) Heat is produced in machines which damages its parts.
(iii) The speed of the vehicle reduces due to friction.

##### Question 14:

Explain why objects moving in fluids must have special shapes.