coefficient of friction

(mechanics) The ratio of the frictional force between two bodies in contact, parallel to the surface of contact, to the force, normal to the surface of contact, with which the bodies press against each other. Also known as friction coefficient.

A unitless number indicating the mechanical or molecular interaction between two surfaces in contact. Assuming the normal reaction force is constant, the lower the coefficient of friction, the easier it is for two surfaces to slide over each other. Surfaces with a coefficient of friction of zero, are perfectly smooth and frictionless. Factors affecting the coefficient of friction include the roughness and hardness of the surfaces in contact and the type of molecular interaction between them. The coefficient takes different values depending on whether the bodies with their surfaces in contact are motionless or moving. See also coefficient of limiting friction, coefficient of kinetic friction.