Friction for class 8 CBSE

Here we come with Friction for class 8 CBSE. In previous post we learn Force and Pressure. Now we learn Friction for class 8 CBSE. For that we have to clear some questions regarding Friction.

  • What is Friction?
  • How friction originate?
  • What are the parameter friction depends on?
  • Types of friction?
  • Why is dynamic friction slightly lesser than the limiting friction?
  • What are the advantages and disadvantages of friction?
  • How to increase and reduce friction?

What is Friction?

Friction is the opposing force that is set up between the two surfaces of contact, when one body slides or rolls or tends to do on the surface of another body. And we can define friction as follows.

“Friction is the resistance to motion experienced when two surfaces in contact move with respect to each other.”

 “The force acting along the two surfaces in contact which opposes the motion of one body over the other is known as the force of friction or frictional force.”

Friction for class 8 CBSE

How friction originate?

  • Friction is very closely related with the roughness or unevenness of the surfaces in contact. The frictional force increases with the increase in roughness of the surfaces.
  • A closer look through a microscope clearly reveals that the surfaces have crests and troughs, cracks and bumps or hills and valleys. These are known as irregularities or imperfections. Friction is caused by these irregularities found on the two surfaces in contact with each other. It is due to the interlocking of irregularities of the two surfaces that produce friction between them.

Friction for class 8 CBSE

  • The force of friction also arises due to forces of adhesion. When two rough surfaces come in contact, many contact point are formed. the atoms or molecules present at such points of contact give rise to attractive force of adhesion which oppose the relative motion of the bodies. Thus, the force of adhesion between the two surfaces give rise to friction.
  • Friction depends upon the nature of the surfaces in contact. Smoothness of the contacting surfaces does affect the force of friction between two surfaces in contact.
  • The force of friction is directly proportional to the normal force. When a body is moving over a horizontal surface, it presses down against the surface with a force equal to its weight i.e. to the pull of gravity upon it. An increase in the weight of the body causes an increase in the amount of resistance offered to the relative motion of the surface in contact.
  • Friction does not depend on the amount of surface area in contact between the moving bodies.

What are the parameter friction depends on?

As per your syllabus of Friction for class 8 CBSE, the force of friction not only tends to stop a moving object, it also tends to stop a stationary object from moving. That is why it is difficult to push a stone (or a brick) to get it to move on a rough surface. The force of friction depends upon two parameters.

 (i) Mass.

 (ii) Nature of the surface.

The force of friction is almost independent of the velocity of the moving body with respect to the surface, as long as the velocity is not too high.

Types of friction?

On the basis of its nature frictional force can be divided into three types.

 (i) Static Friction

(ii) Sliding Friction

(iii) Rolling Friction

(i) Static Friction: The friction that exists between the two surfaces in contact when there is not relative motion between them is called Static friction.

(ii) Sliding Friction: The friction that exists between a surface sliding on another surface is called the Sliding friction.

(iii) Rolling Friction: When a body rolls on the surface of another, the friction that exists between the surfaces is known as Rolling friction.

Experimentally, it has been proved that, Static friction > Sliding or Dynamic Friction > Rolling Friction

Why is dynamic friction slightly lesser than the limiting friction?

You know that friction is caused due to the interlocking of the irregularities on the surface of two bodies in contact with each other. Now, more force is required to unlock the irregularities between the surfaces of two stationary bodies as compared to the force required to keep the irregularities unlocked between the surfaces of two sliding bodies. Thus, the limiting friction is always slightly more than the dynamic friction or dynamic friction is slightly less than the limiting friction. Fig. shows a graph between the applied force on the spring balance and the force of friction. The region AB in the graph shows static friction, which is self-adjusting. BC shows the magnitude of limiting friction. DE shows the magnitude of dynamic friction or sliding friction.

What are the advantages and disadvantages of friction?


 (i) No two bodies will stick to each other without friction.

(ii) Adhesives will lose their purpose without friction.

(iii) When a person pushes the ground backward. The rough surface of the ground reacts and exerts a forward force due to friction.

(iv) It plays an important role in our daily life. It would be impossible for us to walk if there was no friction between the soles of our shoes and the ground.

(v) Without friction, there would be no reaction from the ground in a forward direction. As a result, we would not be able to move forward.

 (vi) It is because of friction that we are able to hold a pen. It is because of the friction between a paper and a pen that we are able to write.

(vii) Cars and buses are able to move on the road because of friction between the tyres and the road. It is because of the friction between brake-shoes and wheels of automobiles that aoutomobiles slow down and stop when brakes are applied.

(viii) It is the force of friction that holds screws and nails in wooden furniture.

 (ix) A matchstick lights due to the force of friction.


(i) Friction always opposes the motion so extra energy has to spent in over coming the friction.

(ii) Tyres of vehicles and soles of shoes wear out in a few months due to friction.

 (iii) Frictional force results in the production of heat, which causes damage in machinery.

 (iv) The heat produced in the moving parts of machinery due to friction result in wear and tear of the parts.

(v) Friction causes wastage of energy.

 (vi) Friction reduces (retards) the speed of moving vehicles to a great extent.

How to increase and reduce friction?


Air and water offer the maximum friction. Objects moving in air or water have streamlined bodies to reduce friction. Birds, cars, aeroplanes and rockets have special streamlined bodies to reduce air resistance. Ships, fishes, boats and submarines are broad in the middle and narrow in front and at the back.

Friction for class 8 CBSE


Polishing rough surfaces also reduces friction. If the surfaces that are in contact are rough, there is more friction. Hence to reduce friction, the surfaces in contact are kept smooth and polished. It is worth nothing that if the two surfaces in contact are of the same metal, the friction increases on polishing the two surfaces. This is why the bearings and the axle are made of different metals.


If a suitcase is fitted with wheels, it is easier to move it because the friction between the wheels and the ground is less.


Use of lubricants makes the surface smooth. In machines, friction is reduced by using lubricants such as oil, grease or graphite powder. The lubricant fills the minute unevenness of the two surfaces and separates them by forming a very thin layer in between. This layer offers very little resistance and as a result, the friction gets reduced.


When steel slides over an alloy of lead, the friction is less than when steel slides on steel. Bearings are sometimes packed with such an alloy.


Ball bearing changes sliding friction to rolling friction. This is a very useful thing to do since rolling friction is much smaller than sliding friction. Ball bearings are used in most mechanical structures which have moving parts.


But in spite of what is given above, there can be times when more friction is better. As we have seen, sometimes it is better for us if friction is greater (as we would like to have when walking on a slippery floor).

There are two methods of increasing friction.

(a) By increasing the roughness of the surface.

(b) By increasing the mass of the object.

  • Types have designs and patterns (treads) with grooves on the surface to increase friction (these treads also channel away water on the road that, acting as a lubricant, would decrease the friction between tire and road surface, and cause the car to skid off the road).
  • Spikes are provided in the soles of the shoes of athletes to increase the friction and prevent them from slipping. • Sand and gravel are strewn on slippery ground during rainy season to increase friction.
  • The soles of your shoes also have treads, as tires do. These grooves are there to increase friction between the sole of the shoe and the floor. You might have noticed that when these grooves wear out, your shoes become slippery.
Let see how you learn Friction for Class 8 CBSE ?

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Force for Class 8 CBSE

Let’s start with very important and interesting topic Force for Class 8 CBSE students. When we start reading any chapter first we should read introduction

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Hi myself Subham Dutta, having 15+ years experience in filed of teaching. I love to teach and try to build foundation of students. Try to make them imagine what they learn.

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