Friction
the act of rubbing two bodies together, or rubbing one body on another; in physics, the resistance which any body meets with in moving over another body. No body is quite smooth; all have elevations and depressions, and when one moves over another some of the projecting points of the one are sure to enter the cavities of the other, and render movement more difficult. Friction is greater when a body, previously at rest, first begins to move. The larger and heavier the body, the greater the friction. Friction is a retarding force in nature. It gives stability to bodies which else would be easily moved.
It generates heat, and is one of the chief means of developing electricity.
Co-efficient of Friction for Two Surfaces. The ratio between the force required to move one of these surfaces over the other, and the pressure between the two surfaces.