### Friction - Revision Notes

CBSE Class: 8 Science
Revision Notes
Chapter – 12
Friction
Friction is a 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.
Causes of Friction: Friction exists between two surfaces due to irregularities on the surfaces of the objects in contact, interlocking of micro-level irregularities of the two surfaces and ploughing of harder surfaces into smoother surfaces.
Factors That Affect Friction
•  Roughness of the surface
•  The extent to which the two surfaces press together
•  Nature of the surface
· Types of Friction:
(i)  Static Friction: When a body is at rest, the force of friction is called the static friction and is always equal and opposite to the applied force. The force of friction which acts when the body is just at the verge of sliding on the surface is called limiting friction.
(ii)  Sliding friction: The friction force which opposes the actual relative sliding motion between two contact surfaces. Sliding friction is smaller than static friction.
(iii)  Rolling Friction: The frictional force that exists between two surfaces when a body rolls over the other. Rolling friction is smaller than sliding friction.
Effects of Friction
•  Friction produces heat.
•  Friction causes wear and tear.
•  Friction opposes motion.
•  Friction between pen and paper enables us to write on the paper.
•  Friction between our feet and the ground allows our movements like standing, walking and running.
•  Friction between the surface of the road and tyres of our vehicles allow the vehicles to move without slipping.