Friction is the force which resists relative motion between surfaces in contact with each other.
Friction is categorized by the nature of the surfaces in contact and the conditions under which they are interacting. There are many different types of friction, some of which are listed below.
Dry friction, which is the force that opposes one solid surface sliding across another solid surface.
Rolling friction is the force that opposes motion of a rolling wheel or ball.
Fluid friction is the friction between layers of a viscous fluid in motion.
Skin friction, also called drag, is the friction that occurs between a fluid and a moving surface.
Internal friction is the force resisting the internal deformation of a solid material.
An understanding of friction is important factor in the design, functionality and performance of many mechanical systems. In this text we will only consider dry friction out of the many types of friction listed above. So far, we have only looked at situations where friction could be safely ignored; however, there are many cases where the effect of dry friction is significant. These include slipping or tipping, blocks, wedges, screws, belts, bearings, and rotating discs. These are the subject of this chapter.