Lesson: Friction
Topic 1: Types of Friction

# Question: 1

What is friction and what are the factors on which friction depends?

## Solution:

Friction is the force that opposes the relative motion between two surfaces of objects in contact. The force of friction always acts in a direction opposite to that of the applied force. Nature of surfaces in contact and mass of the body are the two main factors on which friction depends.

# Question: 2

What causes sliding friction between two surfaces?

## Solution:

The interlocking of irregularities on surfaces sliding over each other gives rise to the force of friction.

# Question: 3

In which direction, does friction work?

## Solution:

Friction always acts in a direction opposite to that of the applied force.

# Question: 4

Explain why sliding friction is less than static friction.

## Solution:

Friction comes into play when the surfaces of the two objects in contact get interlocked with each other. While sliding one surface over another, the time for interlocking is very small and hence interlocking is not as rigid as it would be if the two surfaces were stationary. Therefore, less force is required to overcome the interlocking when the surfaces are moving, and sliding friction is less than static friction.

# Question: 5

What is fluid friction?

## Solution:

The resistance to an object's motion through a particular fluid may be termed as fluid friction. For example, vehicles moving at high speed are opposed by air molecules.

# Question: 6

What is rolling friction?

## Solution:

Rolling resistance, sometimes called rolling friction or rolling drag, is the force resisting the motion when a body (such as a ball, tire, or wheel) rolls on a surface.

# Question: 7

Is sliding friction the same for all surfaces?

## Solution:

No, friction depends on the roughness/ smoothness of the surface.

# Question: 8

Why is the force required to make a stationary object start sliding more than the force required to keep the same object sliding?

## Solution:

In the static position, the irregularities of the two surfaces in contact are interlocked rigidly due to which more force is required to overcome the interlock. When an object is already sliding, the interlocking is not as deep, so a relatively smaller force is required to overcome the kinetic friction.

# Question: 9

How does the friction get affected by the nature of the surface?

## Solution:

We know that friction occurs due to the irregularities of the surfaces in contact. A rough surface has more irregularities than a smooth surface, so the sliding friction will be greater in the rough surface.

Topic 2: Effects of friction

# Question: 1

Name any four methods by which we can reduce friction.

## Solution:

Friction between surfaces can be reduced by the following methods:

Applying lubricants to minimize the interlocking of irregularities

Using a soap solution to make the surface slippery

Spreading fine powder for parts that cannot have oil applied

Polishing surfaces to reduce surface irregularities

Streamlining shapes

Converting sliding to rolling friction

# Question: 2

What role does friction play when someone writes with a piece of chalk on a blackboard?

## Solution:

When someone writes with chalk on the blackboard, the friction between the chalk piece and the blackboard causes some chalk particles to get separated from the piece of chalk, which stick to the blackboard.

# Question: 3

What is streamlining?

## Solution:

Streamlining is a design or a form that minimises fluid friction. It provides very little resistance to the flow of air or water, thereby increasing speed and ease of movement. Birds and fish have naturally streamlined bodies, which make it easier for them to fly, swim or move with greater speeds.

# Question: 4

Why do objects that need to move in fluids have a streamlined shape?

## Solution:

The objects moving in the fluids have a streamlined shape because this shape helps in reducing the friction between the objects and the fluids which they are moving through.

# Question: 5

Why do kabaddi players rub their hands with mud?

## Solution:

Kabaddi players rub their hands with soil to get a better grip of their opponents. The mud increases the friction on their hands and provides a better grip..

# Question: 6

If you take two wooden boxes, wrap one with jute and other with a polythene, which box would be easier to drag?

## Solution:

It would be easier to drag the box which is covered with polythene because polythene has a smooth surface and experiences less friction as compared to jute. Hence, it requires less force to drag it.

# Question: 7

Why do we slip when stepping on a banana peel?

## Solution:

A banana peel has a smooth surface which reduces the friction between the ground and our feet, making us slip.

# Question: 8

What are the advantages of friction?

## Solution:

Following are the advantages of friction:

Helps in walking comfortably

Helps in writing

Helps to minimise the speed or stop a moving vehicle by applying brakes

Helps in gripping objects

# Question: 9

What would happen if there was no friction and an object started to move?

## Solution:

If an object started moving in the absence of friction, it would never stop on its own.

# Question: 10

How can we reduce friction?

## Solution:

Following are the ways to reduce friction:

Polishing the surface in contact

Lubrication between the surfaces

Use of ball bearing/roller bearings that change sliding to rolling friction

Streamlining the shape of objects that are moving through fluids

# Question: 11

How can we increase friction?

## Solution:

Following are the ways to increase friction:

Make the contact surfaces rough

Increase the weight of the moving body

Using soft materials

Remove lubricating substances from the contact surfaces

# Question: 12

Why friction is called a necessary evil?

## Solution:

Friction is very important force that helps us in various activities despite showing some ill effects. We would not be able to move or stop without friction. We would not be able to grip an object, fix a nail on the wall or tie a knot without friction. That is why it is called a necessary evil.

# Question: 13

What are the disadvantages of friction?

## Solution:

Following are the disadvantages of friction:

Reduces the speed of moving objects

Generates heat

Causes wear and tear of the objects subject to friction

Reduces the efficiency of machines