Friction

Friction, in mechanics, resistance to the sliding, rolling, or flowing motion of a body in relation to another body with which it is in contact.

In any solid the molecules display internal friction. This form of friction is the force that causes any oscillating object, such as a piano string or a tuning fork, to stop oscillating. Internal friction in liquids and gases is called viscosity.

External friction is of two kinds, sliding friction and rolling friction. In sliding friction, the resistance is caused by the interference of irregularities on the two surfaces. In rolling friction the resistance is caused by the interference of small deformations or indentations formed as one surface rolls over another. In both forms of friction molecular attraction between the two surfaces causes some resistance. The frictional resistance in either case is directly proportional to the force pressing the two objects against each other. Friction between any two surfaces is measured in terms of the coefficient of friction, the ratio between the force required to move two surfaces in contact with each other and the force holding the two surfaces together. If a weight of 25 kg (about 55 lb) is resting on a flat surface and it requires a force of 5 kg (about 11 lb) to move the weight along the surface, the coefficient of friction between the weight and the surface is 5 divided by 25, or 0.2. The coefficient of friction between well-oiled metallic surfaces is about 0.01 to 0.05, and between ball bearings and a bearing race in rolling contact about 0.002. The friction between two objects is at maximum just before they begin to move in relation to each other and less when the objects are in motion. The maximum value of friction is called static friction, or friction of rest, and the value of friction between moving objects, kinetic friction or friction of motion. The sliding motion of two bodies in contact is discontinuous, and kinetic friction may be regarded as produced by a series of episodes in which static friction dominates.

The angle of friction is the angle to which a surface must be tilted before an object placed on it will slide steadily down. This angle measures the effectiveness of friction in overcoming the force of gravity, which tends to make the object slide.