ABSTRACT
When a body slides on a surface the minute uneven and interlocking surface asperities on the contacting surfaces oppose the sliding motion. Friction is usually considered wasteful, as in automobiles, trains, and machines where it absorbs power and causes wear; in these circumstances efforts are made to reduce it. However, it can also be useful, such as in brakes, belt drives, and clutches. Some of the asperities weld together and then shear. In fact, without friction one could not even walk or ride a bike because there would not be the necessary interaction between the floor and the shoe or the road and the tire. Without friction a bicycle wheel would just spin and skid; without friction a simple knot could not grip, a nail would be squeezed out of wood, and a nut would not stay on the end of a bolt.
