• When we throw a ball on the floor it starts moving with some velocity.
  • But once it is rolled ideally no force is acting in the direction of motion and according to Newton’s first lawthe ball should keep on rolling but this does not happen.
  • The ball stops after moving a certain distance so a force must be acting on it.
  • That force is known as friction.

What is Friction?

Friction is defined as:

  • The resistance offered by the surfaces that are in contact with each other when they move over each other.
  • Friction works in the opposite direction in which the body is moving making the body slow down.
  • Friction is useful in most of the cases.
  • Friction is also dependent on external factors.

Factors Affecting Friction

Following are the two factors on which friction depends:

  • On the nature of the two surfaces that are in contact

Friction is dependent on the smoothness or roughness of the two surfaces that are in contact with each other. When the surface is smooth, the friction between the two reduces as there is no much interlocking of irregularities taking place. While the surface is rough, friction increases.


  • On the force that is acting on these surfaces

When force is applied along with the irregularities, friction increases.

What Causes Friction?

  • When we see any object, we can see the smooth surface but when the same object is viewed under a microscope, it can be seen that even the smooth appearing object has rough edges.
  • Tiny hills and grooves can be seen through the microscope, and they are known as irregularities of the surface.
  • So, when one object is moved over the other, these irregularities on the surface get entangled giving rise to friction.
  • More the roughness, more will the irregularities and greater will be the force applied.
Friction can be reduced by using:
  • grease, oil, powder, ball bearing, and cushion of dry air between the moving surfaces.
  • using anti-friction alloys.
Friction can be increased

Friction can be increased by making a surface rough.

The sole of the shoes and the tyres of the vehicle are treated to increase friction.


Lubricants: The substance which reduce the friction are called lubricants. Examples PE waxes, paraffin, metal soaps, esters (high esterification), amides, and fatty acids.

Types of Friction

There are four types of friction:

Static friction:
  • Static friction:Static friction is defined as the frictional force that acts between the surfaces when they are at rest with respect to each other.

The magnitude of the static force is equal in the opposite direction when a small amount of force is applied. When the force increases, at some point maximum static friction is reached.

Static Friction Examples

Following are the examples of static friction:

  • Skiing against the snow
  • Creating heat by rubbing both the hands together
  • Table lamp resting on the table
Sliding friction:
  • Sliding friction:Sliding friction is defined as the resistance that is created between any two objects when they are sliding against each other.
Examples of Sliding Friction

Following are the examples of sliding friction:

  • Sliding of the block across the floor
  • Two cards sliding against each other in a deck
Rolling friction:
  • Rolling friction:Rolling friction is defined as the force which resists the motion of a ball or wheel and is the weakest types of friction.
Examples of Rolling Friction

Following are the examples of rolling friction:

  • Rolling of the log on the ground
  • Wheels of the moving vehicles
Fluid friction:
  • Fluid friction:Fluid friction is defined as the friction that exists between the layers of the fluid when they are moving relative to each other.
Examples of Fluid Friction

Following are the examples of fluid friction:

  • The flow of ink in pens
  • Swimming

Applications of Friction

  • Friction finds application when matchsticks are ignited.
  • Motion of pistons in a cylinder is an application of friction.
  • It is possible to write on books and board as there is friction between pen and the board.


1.Kinetic energy is converted into heat because of friction, how?

Kinetic energy is converted into heat in friction because during friction there will be an interaction of atoms with each other from both the surfaces. When there is an interaction taking place, there will be production of heat.

  1. what is friction drag?

Friction drag is also known as skin friction drag. It is caused by the friction of fluid layers. It is proportional to the area with which it is contact and increases with square of velocity.

  1. What are the laws of friction?

Following are the laws of friction:

  • Friction depends on the roughness or smoothness of the surfaces that are in contact.
  • Friction is independent of the area that is in contact.
  • Friction is proportional to the force acting on the contacting surfaces.
  • Fluid friction increases with increase in the relative speed.
  1. Can we  reduce  friction  to zero  by  polishing  surfaces or  using  large  amount  of lubricants?

         Answer: Friction can never be entirely eliminated.  No surface is perfectly smooth. Some irregularities are always there.