# NCERT Class 8 Science Chapter 9 ‘Friction’: Notes and Solutions (Free PDF)

The understanding of a chapter becomes easy when there are well-written chapter notes. Such notes, not only cover the chapter completely but also provide a way for the students to revise the same quickly. Hence, keeping in mind the requirements of students, we have prepared NCERT Solutions and Notes for Class 8 Science Chapter 9: Friction. These notes are prepared in a point-wise manner and hence will greatly serve their purpose during the exam season. After the notes, there are also Solutions available for the NCERT exercises. The PDF of those exercises is also available for you to download. Let’s explore this blog.

## Class 8 Science Chapter 9: Friction Notes

In this chapter, we will be studying several topics related to friction such as what is friction?, Factors affecting Friction, Increasing and Reducing Friction, Fluid Friction, etc. Friction is a pretty interesting thing to explore. Hence, let’s start with the notes.

Friction is defined as the resistance that one surface or object encounters while moving over the other. It is caused because of the irregularities that are present between the two surfaces. These irregularities lock over one another and hence, friction comes into effect.

### Factors affecting Friction

Let us now understand what are the factors that affect friction and how they do so.

• As mentioned before, friction is caused because of irregularities that are present on the surfaces. The more regularities, the more the force of friction or in simple words we can say that the more rough the surface, the more will be friction.
• If the two surfaces are pressed harder, friction between them will increase.
• To overcome this force of friction we have to apply force to move an object.

There are three types of friction:-

1. Static friction:- The force required to overcome friction at the instant an object starts moving from rest is known as static friction.
2. Sliding Friction:- The force that is required to keep an object moving is known as sliding friction
3. Rolling Friction:- When one body rolls over the surface of another body, the resistance to its motion is called rolling friction.

When a box is already in motion then the surfaces do not get time to interlock, hence, this makes it easy to move the box that is already in motion compared to the one that is stationary.

From this, we can conclude that the value of sliding friction is less than that of static friction.

Wheels reduce friction. Hence it is easier to pull a luggage bag with wheels because rolling is much easier as compared to sliding this is the reason that there are ball bearings used in industrial machines.

The order of the comparative measures of different frictions is as follows:

Static Friction>Sliding Friction>Rolling friction

## Class 8 Science Chapter 9:Friction A Necessary Evil Notes

In this part of the chapter, we will see why friction is important, what is fluid friction, etc.

All the daily activities we do involve friction some of the scenarios of the same are listed below:-

• We would not be able to walk if there was no friction.
• Holding smooth objects such as glass, etc would not be possible if there was no friction present.
• We are able to write with a pen or pencil because of friction.
• A moving object would not stop at all if there was no friction.
• We are able to steer our vehicles in a direction because of the friction present between their tyres and the road.
• Fixing of nails in the walls is possible due to friction.

Although friction is necessary, it still is an evil force. Some of the reasons for the same are mentioned below:-

• It wears out the materials whether they are screws, ball bearings or soles of shoes.
• Friction leads to heat generation causing the machines to get overheated and leading to the waste of energy.

### Increasing and Reducing Friction

There are some instances where friction is required and then there are other instances where reduction of friction is required. Let us see how to increase and decrease friction.

Increasing Friction

In some instances, We need to increase friction. The ways to do the same are mentioned below:-

• Soles of shoes, sandals, etc and tyres of cars, trucks, etc are treaded to increase the friction and provide better grip.
• In bicycles and motorbikes, brake pads are used to bring to a stop. These brake pads grip the rim of the tyres and stop the bicycle or bike.
• Kabaddi players rub their hands with soil for a better grip on their opponents.
• Gymnasts apply some coarse substance on their hands to increase friction for better grip.

Decreasing Friction

There are situations where friction is undesirable, hence, it needs to be reduced. The ways for the same are mentioned below:

• Oil is used on the hinges of the doors to make their movements smooth.
• In moving parts of vehicles grease is used to reduce friction.
• During the game of carrom. The powder is sprinkled on the board to reduce friction.
• Some substances reduce the friction in machinery and are known as lubricants.

In all the above-mentioned ways, oil, powder, etc make a layer over the irregularities of the surfaces and prevent interlocking of the same hence reducing friction.

### Fluid Friction

In science, the common name of gases and liquids is fluids.

Air, water and other fluids exert the force of friction when objects move through them. This friction is known as fluid friction or drag.

This fluid friction depends on:-

• The speed of the object with respect to the fluid.
• On the shape of the object.
• The nature of the fluid.

Reducing Fluid Friction

Fluid friction acts on aeroplanes, boats and other objects moving through fluids, which leads to them losing energy, hence fluid friction has to be reduced. Efforts are made to reduce the same and the inspiration for that is derived from nature.

Fishes and birds have to go through fluid friction and the streamlined shape of their bodies helps in reducing the fluid friction acting on them. Hence, drawing inspiration from them, the bodies of planes, boats, etc are made in aerofoil or streamlined shapes to reduce fluid friction.

Also Read: Branches of Physics (Free PDF)

## Important Definitions in NCERT Class 8 Science Chapter 9: Friction

Some important definitions covered in the chapter are mentioned below:

• Friction:  Friction is defined as the resistance that one surface or object encounters while moving over the other.
• Static friction: The force required to overcome friction at the instant an object starts moving from rest is known as static friction.
• Sliding Friction: The force that is required to keep an object moving is known as sliding friction
• Rolling Friction: When one body rolls over the surface of another body, the resistance to its motion is called rolling friction.
• Fluid Friction: Air, water and other fluids exert the force of friction when objects move through them. This friction is known as fluid friction or drag.

## NCERT Solutions of Class 8 Science Chapter 9: Friction

Let us now explore NCERT Solutions of Class 8 Science Chapter 9 to get answers to all writing

questions in the lesson Friction.

Exercise Questions

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.

Soln:

(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.

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

Soln:

The answer is (c) static, sliding, rolling

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

Soln:

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

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?

Soln:

We are able to walk on the floor because of the friction present between our shoes and the floor. Spilling the soapy water on the marble floor would make it difficult for us to walk on the same floor because the friction between the shoes and the floor would get reduced because of the soapy water. Hence, we will slip on the floor.

6. Explain why sportsmen use shoes with spikes.

Soln:

Sportsmen use shoes with spikes for better grip. The spikes in the shoes lead to a better grip which in turn increases the friction, hence, making it easier for them to perform their sport.

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?

Soln:

Seema will have to apply a larger force to move the box. When boxes are placed on the floor and force is applied to move them then friction comes into play because of the irregularities in the two surfaces. Now, because Seema’s box is heavier, this will lead to the surface of that box and the floor being pushed harder against each other hence increasing the friction. This will require Seema to apply a larger force to push that box as compared to Iqbal.

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

Soln:

When an object is already in motion then the surfaces do not get time to interlock. Hence, this makes it easy to move the object that is already in motion compared to the one that is stationary.

As a result, the value of sliding friction is less than that of static friction.

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

Soln:

All the daily activities we do involve friction some of the scenarios of the same are listed below:-

• We would not be able to walk if there was no friction.
• Holding smooth objects such as glass, etc would not be possible if there was no friction present.
• We are able to write with a pen or pencil because of friction.
• A moving object would not stop at all if there was no friction.
• We are able to steer our vehicles in a direction because of the friction present between their tyres and the road.
• Fixing of nails in the walls is possible due to friction.

Although friction is necessary, it still is an evil force. Some of the reasons for the same are mentioned below:-

• It wears out the materials whether they are screws, ball bearings or soles of shoes.
• Friction leads to heat generation causing the machines to get overheated and leading to the waste of energy.

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

Soln:

Air, water and other fluids exert the force of friction when objects move through them. This friction is known as fluid friction or drag.

This fluid friction depends on:-

• The speed of the object with respect to the fluid.
• On the shape of the object.
• The nature of the fluid.

Fluid friction acts on aeroplanes, boats and other objects moving through fluids, which leads to them losing energy, hence fluid friction has to be reduced. Efforts are made to reduce the same and the inspiration for that is derived from nature.

Fishes and birds have to go through fluid friction and the streamlined shape of their bodies helps in reducing the fluid friction acting on them. Hence, drawing inspiration from them, the bodies of planes, boats, etc are made in aerofoil or streamlined shapes to reduce fluid friction.

## FAQs

Q1. What is friction 8th science notes?

Ans: Friction is defined as the resistance that one surface or object encounters while moving over the other. It is caused because of the irregularities that are present between the two surfaces. These irregularities lock over one another and hence, friction comes into effect.

Q2. How to reduce friction?

Ans: There are situations where friction is undesirable, hence, it needs to be reduced. The ways for the same are mentioned below:
-Oil is used on the hinges of the doors to make their movements smooth.
-In moving parts of vehicles grease is used to reduce friction.
-During the game of carrom. The powder is sprinkled on the board to reduce the friction.
-Some substances reduce the friction in machinery and are known as lubricants.

Q3. What are the different types of friction?

Ans: Here are the three types of friction:
a. Static friction: The force required to overcome friction at the instant an object starts moving from rest is known as static friction.
b. Sliding Friction: The force that is required to keep an object moving is known as sliding friction
c. Rolling Friction: When one body rolls over the surface of another body, the resistance to its motion is called rolling friction.

Follow Leverage Edu for complete study material on CBSE Notes of Class 8 Science