NCERT Solutions for Class 8 Science Chapter 12 Friction

Q.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) The 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 friction.
(e) Sliding friction is lesser than the static friction

EXPLANATION

Friction opposes the motion between the surfaces in contact with each other because it creates interlocking between the two surfaces in contact in such a way that the relative motion of the object slows down and direction of frictional force is shown in the opposite direction to the motion of the object.

Friction depends on the nature of surfaces because friction is all about interlocking which arises because of the roughness of a surface. If the surface is more rough then it undergoes more interlocking with the other surface in contact, thus resulting in more friction. And if the surface is smooth or less rough then it undergoes less interlocking with the other surface in contact, thus resulting in less friction.

Friction produces heat when an object forcibly tries to overcome the interlocking between its own surface and the surface that is in contact with it.

Sprinkling of powder on the carrom board reduces friction because the powder fills the roughness on the surface of the carrom board thus reducing interlocking and making it easy for the dots and striker to move on the board.

Sliding friction is lesser than the static friction because sliding motion allows less time for interlocking, thus making the interlocking less strong, while static motion allows more time for interlocking, thus resulting in a stronger interlocking and providing more friction than the sliding one.

Q.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

Answer :

Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements will be as follows:-

(c) static, sliding, rolling.

EXPLANATION

In rolling friction, two surfaces never comes in contact for even a little time period as the object gets rolled and continuously changes its surface of contact within a span of seconds. So in rolling friction, the time given for interlocking is almost negligible, and thus there is no interlocking at any point of the time, as a result of which rolling provides very less friction as compared to sliding friction and static friction.

While in case of sliding friction, the time given for interlocking is small as compared to that of static friction but as interlocking takes place for some moments, so the resulting interlocking is not so strong. With the application of less external force, one can overcome this type interlocking. Hence sliding provides less friction as compared to static friction.

In case of static friction, the interlocking between the two surfaces of the two objects is much more stronger than the interlocking provided by the sliding friction. So this type of interlocking requires more external force to overcome the friction created because of this interlocking.

Q.3 Alida runs her toy 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
(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.

Answer :

Alida runs her toy 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

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

EXPLANATION

The force of friction will be less in marble floor as compared to the newspaper and the towel because the marble has very less roughness as compared to the other two things.

Now between dry marble and wet marble, wet marble will have the lowest force of friction because it has negligible roughness or no roughness. This is because the water spread on wet marble acts as a lubricant and fills the roughness present on the surface of the marble, thus making it more smooth and slippery. As a result, when the toy car moves on the wet marble, there is no interlocking between the wet marble surface and the toy car's wheel surface. Thus there is no friction acting between the two and the toy car slips easily on the wet marble surface.

Now between dry marble and newspaper, dry marble has less roughness and thus less interlocking takes place between the dry marble surface and the toy car's wheel surface, resulting in the easy movement of the toy car on the dry marble as compared to the newspaper.

Now between newspaper and towel, newspaper has less roughness, thus there is less interlocking between the newspaper surface and the toy car's wheel surface, as a result toy car moves easily on newspaper as compared to the towel.

Q.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.

Answer :

When a book slides down on the desk, a frictional force acts between the lower surface of the book that is in contact with the desk and the upper surface of the desk that is in contact with the book. That means the frictional force comes into play when the minute roughness on the surface of the book gets interlocked with the minute roughness on the surface of the desk.

The direction of the frictional force on the book is opposite to the direction of its motion of the book. As the book is sliding downwards so the frictional force acts in the upward direction.

It acts as shown in the diagram below :-

Q.5 You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?

Answer :

Walking on the ground is possible due to friction. As we step forward using one of our feet, then the ground surface and the foot surface gets interlocked between each other's minute roughness, as a result of which that particular foot gets a support to let the other foot come forward, thus making the walking process more easier.

The layer of soap makes the floor smooth due to which the friction is reduced. This means that the soap acts as a lubricant when spilled on the floor. As the floor has some minute roughness which making the walking process easier, but due to the presence of soap, it fills the roughness, thus making the floor smooth and slippery.

So when a person tries to walk on such a floor then he/she loses grip or balance due to lack of support on any foot which is a result of less interlocking or even no interlocking. Therefore it is difficult to walk on a soapy floor.

Q.6 Explain why sportsmen use shoes with spikes.

Answer :

Sportsmen use shoes with spikes to increase the friction between shoes' surface and the surface of the ground.

Because if the shoes are used without spikes during sports, then the shoes will have comparatively less roughness, and thus they won't get interlocked properly with the surface of the ground, making it difficult for the sportsperson to walk and run on the ground.

If there is no interlocking or less interlocking then the person may slip and injure themselves.

Q.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?

Answer :

Iqbal has to push a lighter box, so the interlocking between the surface of the lighter box and the surface of the floor is comparatively less than that of the heavier box. Because lighter box has less mass, so its' surface's roughness gets less pressed into the surface's roughness of the floor, thus making the interlocking less stronger and easier to overcome the friction using an external force of push.

While Seema has to push a similar heavier box, so the interlocking between the surface of the heavier box and the surface of the floor is comparatively more than that of the lighter box. Because heavier box has more mass, so its' surface's roughness gets more pressed into the surface's roughness of the floor, thus making the interlocking more stronger and harder to overcome the friction using an external force of push.

So Seema will have to apply a larger force.

Q.8 Explain why sliding friction is less than static friction.

Answer :

Friction comes into play when irregularities or the minute roughness present in the surfaces of two objects, get interlocked upon coming in contact with each other.

In sliding friction, the time given for interlocking is very small and thus the resulting interlocking is not so strong. Therefore with the application of less external force, one can overcome this type interlocking.

While in case of static friction, the interlocking between the two surfaces of the two objects is much stronger as compared to that of sliding friction. So this type of interlocking requires more external force to overcome the friction created because of that interlocking. Hence, sliding friction is less effective than static friction.

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

Answer :

Advantages of friction:

(i) Walking on the ground is possible due to friction. As we step forward using one of our feet, then the ground surface and the foot surface gets interlocked between each other's minute roughness, as a result of which that particular foot gets a support to let the other foot come forward, thus making the walking process more easier.

(ii) Friction between the tip of the pen and a paper allows us to write as the tip surface of the pen and the surface of paper both have minute roughness by which they tend to get interlocked and thus the pen tip doesn't slip easily on the paper surface and this makes the writing process more easier.

Disadvantages of friction:

(i) Tyres and soles of shoes undergoes wear and tear because of friction as the surface of tyres and shoes have to overcome the friction provided by the ground while walking by preventing the roughness on the surface of tyres and shoes to get interlocked with the roughness on the surface of ground.

(ii) Friction between different parts of machines produces heat. In machines, there is sometimes rubbing of two surfaces of two different parts, due to which there is a rapid process of overcoming each other's roughness on the surfaces and this produces heat. Continuous heating at a particular surface may melt or result in wear and tear of those parts.

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

Answer :

When a body moves through a fluid, it experiences an opposing force which opposes the relative motion of the body through the fluid.

The opposing force is known as the drag force which is a result of friction between the surface of the body and the fluid.

This frictional force depends on the shape of the body. Objects with a special shape can minimize the effect of frictional force, thus making it more easier for movement of the body through the fluid.