Friction is the natural resistance to motion caused by surfaces that are in contact with each other. The purpose of lubrication is to reduce the harmful effects of friction by changing sliding friction to fluid friction. Before discussing the characteristics of sliding and fluid friction, we must first define the two main categories of fric-tion: static and kinetic. The friction that exists between a body at rest and the surface upon which it is resting is called STATIC friction. The friction that exists between the surfaces of moving bodies (or between one moving body and a fixed body) is called KINETIC friction. Static friction is greater than kinetic friction. To put a body in motion, you must overcome static friction and inertia. To keep a body in motion, you must overcome kinetic friction.

There are three types of kinetic friction: sliding, rolling, and fluid. Sliding friction occurs when the surface of one solid body slides across the surface of another solid body. Rolling friction occurs when the surface of a curved body, such as a cylinder or a sphere, rolls across another surface. Fluid friction is the internal resistance to motion exhibited by a fluid.

Fluid friction occurs because of two proper-ties of a lubricant: cohesion and adhesion. COHESION is the attraction between the molecules of a substance that tends to hold the substance together. ADHESION is the attraction between molecules that tends to cause unlike surfaces to stick together. Consider a paddle, for example, that is being used to stir a liquid. Cohesion between the molecules of the liquid tends to hold the liquid together. This tendency retards the motion of the liquid. However, adhesion between the molecules of the liquid and those of the paddle cause the liquid to stick to the paddle. This tendency further causes friction between the paddle and the liquid. In the theory of lubrication, cohesion and adhesion are important properties of a liquid. Adhesion is the property of a lubricant which, in liquid form, causes the lubricant to stick (adhere) to the parts being lubricated. Cohesion is the property which holds the lubricant together and enables it to resist breakdown under extreme pressure. Later in this chapter, we will discuss other important properties of a lubricant.

Different materials have varying degrees of cohesion and adhesion. In general, solid bodies are highly cohesive but only slightly adhesive. Most liquids are highly adhesive but only slightly cohesive.