Friction

When a solid body slides over a stationary solid body, a force is exerted at the surface of contact by the stationary body on the moving body. This force is called as the force of friction and is always acting in the direction opposite to the direction of motion. Friction is a property of the bodies by virtue of which a force is exerted by a stationary body on the moving body to resist the motion of the moving body. Friction acts parallel to the surface of contact and depends on the nature of surface of contact.

The different contact surfaces are i) dry ii) partially lubricated (greasy) iii) completely lubricated (film lubricated).

Force of friction should be reduced or increased. Reducing the force of friction is needed when the power transmitted is partially lost due to friction, the friction must be reduced by using lubricated surfaces.

Terminology
Consider a solid body placed on a horizontal plane surface.