Friction, the force or resistance that opposes the movement of one body or substance against another. Friction between solids is usually caused by irregularities in sliding surfaces, but sometimes by adhesion (sticking) or electrical attraction. Friction between fluids is usually caused by their viscosity (resistance to flowing).

If there were no friction, walking would be impossible and cars would spin their wheels without moving. Friction holds nails and screws in wood, and the operation of all brakes depends on friction. Friction between belts and pulleys is important in the operation of many machines. Friction between moving parts of machines, however, is undesirable. It wastes energy that could otherwise be used to perform work, produces heat, and can cause considerable wear. Friction can never be entirely eliminated, but it can be reduced by smoothing sliding surfaces or applying a lubricant such as oil.

There are three types of friction:

Sliding Friction

results when one solid moves across another, as in dragging a box across a floor.

Rolling Friction

is produced when a rolling body, such as a wheel or ball bearing, rolls over another Surface. On hard surfaces, rolling friction is generally much less than sliding friction.

Fluid Friction

is that produced by fluids in motion, or by contact between moving fluids and solids. Thin fluids flow more easily than thick fluids, and therefore produce less friction.

In general, more force is required in overcoming friction to start an object moving across a surface than to keep it moving. Once the object is moving, the friction acting on the object (that is, the force opposing its motion) is directly proportional to the force with which the object presses against the surface. For example, doubling the weight of an object sliding across a surface will double the force with which the object presses against the surface and therefore double the friction that must be overcome to keep the object moving.

The ratio of the friction acting on an object to the force with which the object presses against a surface is called the coefficient of friction. Its value depends on the materials in contact. The greater the friction between two materials, the higher is the coefficient. Knowledge of the coefficient of friction between materials is useful to engineers in designing the moving parts of machines and in calculating the amount of power necessary to operate them.