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