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Friction Notes for Class 8, Chapter 12, Revision Material Based on CBSE, NCERT

Important topics Covered in Friction Notes for Class 8:

  • Introduction
  • Friction
  • Factors affecting friction
  • Friction: A Necessary Evil
  • Increasing and reducing friction
  • Fluid Friction
  • Rolling Friction
  • Sliding Friction
  • Static Friction


  • In our day to day life, we see several situations which use application of friction.
  • A vehicle slows down when a brake is applied.
  • A ball rolling on the ground stops after sometime.
  • The tire of vehicle wears down after its regular usage.
  • These are some applications of friction.


friction force

  • When one surface is moving over another surface, a force which opposes the relative motion between the two is called friction.
  • The force of friction always opposes the force applied to it.
  • There will be no friction if there is no contact between the surfaces.

Factors affecting friction

cause of friction

  • The irregularities of the two contact surfaces are the cause of friction.
  • No surface is perfectly smooth in this universe.
  • Even some surfaces that look smooth to our naked eye actually look rough or bumpy when we see it under a microscope.
  • Smooth surfaces cause lesser friction whereas rough surfaces cause greater friction.
  • If the force applied is more, greater will be the friction.
  • Greater is the mass of object greater will be the friction.
  • The force of sliding friction is always less than static friction. Rolling friction is less than sliding friction.

Friction: A Necessary Evil

harms of friction

Some disadvantage of frictions is given by:

  • More heat energy is dissipated from engine due to friction between surfaces in motion.
  • A large amount of energy is lost in pulling objects on a rough surface.
  • Shoes soles and tires wear after usage.
  • It causes moving objects to stop or slow down.
  • It also causes wear and tear of moving machinery parts.

Increasing and reducing friction

increasing and reducing friction

  • In different cases, we are required either to increase or decrease friction.
  • To increase friction the roughness of surface should be increased.
  • To increase friction we tread shoes and tires.
  • When friction is undesirable we need to reduce the friction.
  • By using lubricants like oil and grease and by using ball bearing between machine parts we can minimize friction.
  • Wheels reduce friction.
  • Shoes soles and tires are threaded to increase friction.
  • By aiding suitable shapes to objects moving in the fluids reduces friction, like shape of the plane.

Fluid Friction

fluid friction

  • The friction produced by the fluid surface on any other surface is said to be fluid friction.
  • Fluid friction is also said to be drag.
  • The fluid friction depends upon the speed of objects through the fluid.
  • Friction force depends upon the speed of the object with respect to the fluid.
  • It can be reduced by aiding suitable shape to the object.

Rolling friction

rolling friction

  • The resistance produced in the motion of an object which rolls on a surface.
  • It is less than that of sliding friction so it is preferred.
  • Wheels are mostly preferred over other modes of transport on the land.
  • Ground transportation that uses wheels are bicycles, cars, 4-wheelers, skateboards etc.
  • Ball bearings are another use of rolling friction which converts sliding friction to rolling friction.

Sliding friction:

sliding friction

  • The resistance produced in the motion of objects which are sliding over the surface are said to be sliding surface.
  • Sliding friction is much weaker than static friction.
  • It is easier to slide the table over some distance.
  • Measure of sliding friction is the force required to keep the object moving with the same speed.
  • The sliding friction is useful as writing with a pencil uses application of sliding friction.

Static friction:

static friction

  • The frictional force that acts on objects when they are resting on a surface.
  • Static friction comes into play when we try to move an object at rest.
  • Glass on glass either lifting is an example of static friction.

FAQ on Friction Notes for Class 8

What is the frictional force?

Frictional force is the force which opposes the relative motion when two objects are moving one over the other with their contact surfaces touching each other. There are several kinds of frictional force. If an object slides over the other then it is the case of sliding friction when an object is rolling over the other then it is an example of rolling friction and when an object is moving in fluid then the resistance provided in motion of object is fluid friction.

What causes friction?

There are several factors which are responsible for friction. They are:

  • Irregularity or roughness of the surfaces in contact causes friction.
  • Weight of the body also leads to high friction.
  • Magnitude of the forces holding the bodies together.

Which friction is strongest?

Out of the four types of friction, static friction is the strongest. Static friction is frictional force which acts on objects when they are resting on a surface. Rolling friction is the weakest which applies between different solid surfaces.

Suggest some methods to reduce friction.

Some methods to reduce friction are as follows:

  • To reduce friction between moving parts, lubricating oil and grease are used in machines.
  • Use of oil is not ideal in some machines air cushion is used for reducing friction.
  • To reduce friction in some machines graphite powder is used.
  • Ball bearing is used in most machines because converts sliding friction into rolling friction.

Discuss some evils of friction?

Some evils or harms about friction are:

  • The wear and tear of shoes and tires, clothes, machine parts etc. are some evils of friction.
  • By the above factors, we can conclude that it also cause monetary loss.
  • Excess of heat generated by friction gets wasted which results in wastage of energy.
  • Moreover, we require a significant amount of energy in overcoming friction.

Explain why sportsmen use shoes with spikes.

Shoes with spikes give a better grip to sportsman while running. The reason for better grip is, with the help of spikes the force of friction between the shoes and the ground increases. If the shoe has no grip over the soil the sliding friction and will cause sportsman to slide which may harm their body. The shoes are specially designed for them to save them from such harm.

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