Class 8 Friction Notes

Friction Notes


It is a type of 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.
  • Friction is a force that comes into play when two surfaces come in contact with each other.

Causes of friction :-

Friction exists between two surfaces due to irregularities on the surfaces of the object in contact, interlocking of micro-level irregularities of the two surfaces and ploughing of harder surfaces into smoother surfaces.

Factors that affect friction :-

  • Nature of the two surfaces in contact (Roughness or smoothness of the surfaces).
  • Force (weight) pressing the two surfaces together.

Note:- Friction between two solid surfaces does not depend upon:

  • their apparent area of contact
  • their speed relative to each other after the start of motion

Types of friction:-

  • Static friction
  • Sliding/Kinetic friction
  • Rolling friction

Static Friction:-

The force of friction that comes into play when the object is at rest is known as static friction.

  • Static friction is a self-adjusting force and is always equal and opposite to the applied force. 
    However, it can adjust and increase itself only upto a limit.

Limiting friction:-

The maximum value of the force of static friction is known as limiting friction.

Sliding/Kinetic Friction:-

The friction that comes into play when objects are in motion is called as sliding/kinetic friction.

  • When the object starts sliding, the contact points on its surface, do not get enough time to lock into the contact points on the floor. So, the sliding friction is slightly smaller than the static friction.

Rolling Friction:-

When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction.

  • Static friction > Sliding friction > Rolling friction

Drag (Fluid friction):-

The frictional force exerted by fluids is called drag.

Drag/Fluid friction depends on the:-

  • nature of the fluid
  • shape of the object (area of contact)
  • speed of the object

Friction – A Necessity:-

  • Friction is responsible for writing, walking and transmitting energy.
  • Friction is responsible for starting or stopping.
  • Friction is used in gripping or holding an object with our hands.

Friction is important for many of our activities.

Friction – An Evil or Nuisance:-

  • Friction is responsible for wear and tear of various parts of machines.
  • Friction decreases the efficiency of machines as a part of the machine’s energy is lost in extra friction.

Friction can be reduced by using:

  • grease, oil, powder, ball bearing, and cushion of dry air between the moving surfaces.
  • using anti-friction alloys.
Image result for lubricants image

Friction can be increased by making a surface rough, e.g.:-

  • The tyres and shoes have treads and grooves to improve their grip on road.
  • Sports shoes have spikes/cleats on their soles to get a better grip on the ground.
  • Gymnasts apply some coarse substance/chalk powder on their hands for a better grip on uneven bars.
  • Kabaddi players rub their hands with soil for having a better grip on their opponents.


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