Types of Tests

Friction

The coefficient of friction is a dimensionless quantity symbolized by the Greek letter μ and is used as a measure of force of friction, whether that frictional resistance is static or kinetic.

The coefficient of static friction is defined as the ratio of the maximum static friction force (F) between the surfaces in contact to the normal (N) force, whereas the coefficient of kinetic friction is defined as the ratio of the kinetic friction force (F) between the surfaces in contact to the normal (N) force:

Both static and kinetic coefficients of friction depend on the surfaces in contact with each other and for a given pair of surfaces, the coefficient of static friction is usually larger than that of kinetic friction.

The static friction force is best demonstrated as the force required to initiate the movement of a static object; the kinetic friction force would be the force required to maintain that movement, if friction is the only opposing force. It is important to note that in all cases, Newton's first law of motion holds.

It is often stated that the coefficient of friction (COF) is a "material” property, however it is better categorized as a "system” property since it depends on another surface and can vary significantly between other contact materials.