Friction between two bodies in contact at a point was defined in 3.7 as that part of the mutual reaction between the bodies which lies in the tangent plane at the point of contact. When two bodies are in contact over a plane area on the surface of each body, the mutual reaction between them is in general a force acting at some point of this area, and its component, if any, in the plane of contact is called Friction.

The mutual reactions between bodies are passive forces in the sense that they only exist because of other forces applied to the bodies. Thus a body may stand alongside a vertical wall and in contact with it without any pressure between them until an external force presses the body against the wall and then there is a mutual reaction between the body and the wall and its amount is no more than is necessary to balance the externally applied force.

Friction, being a component of the mutual reaction, is a passive force. It prevents or tends to prevent the motion of one body across the surface of another, i.e. the relative motion of the points of contact.

Laws of Friction:

1. (i) Friction acts in the direction opposite to what would be the direction of relative motion if the friction did not exist; or, in the case of relative motion, the friction opposes the relative motion.

2. (ii) The magnitude of the friction is always just sufficient to preserve equilibrium or to prevent relative motion of the points of contact, provided that sufficient friction can be obtained.