Types of friction: 

1.     Static friction: It is experienced by a body, when at rest. 

2.     Dynamic friction: It is friction experienced by a body when in motion. 

a.     Sliding friction: It is friction experienced by a body when it slides over another body.

b.     Rolling friction: It is friction experienced between the surfaces which the balls or rollers interposed between them. 

c.      Pivot friction: It is the friction experienced by a body due to motion of rotation.

 Further classified 

1. Friction between unlubricated surfaces 

2. Friction between lubricated surfaces. Laws of dry or solid friction:

·         The force of friction directly proportional to the normal load between the surfaces.

·         The force of friction is independent of the area of the contact surface for a given normal load.

·         The force of friction depends upon material which the contact surfaces or made.

·         The force of friction is independent of the velocity of sliding of one body relative to other body.

Coefficient of friction (µ):

 It is as the ratio the limiting friction (F) to the normal reaction (RN) between the two bodies.

 µ= F/ RN

 Angle of friction:

 It may be defined as the angle which the resultant reaction R makes with normal reactions

tan ϕ= F/ RN