coefficient of friction

coefficient of friction (plural coefficients of friction)

1. (physics) The ratio between (1) the magnitude of the force of friction which a surface produces on an object (moving along the surface or being pressed statically against it) and (2) the magnitude of the normal force which is produced by the surface on the object and which is perpendicular to that surface.

• There are two kind of coefficients of friction. One is the coefficient of static friction, ${\displaystyle \mu _{s}}$ , whose product with the normal force yields the maximum possible static frictional force. (The static frictional force may be a fraction of its maximum possible value in such a way so as to exactly counterbalance an opposite force acting on the object in a direction parallel to the surface.) The other is the coefficient of kinetic friction, ${\displaystyle \mu _{k}}$ , which is applicable when the object is moving along the surface. In such case the force of friction points in a direction parallel to the surface and directly opposing the direction of motion of the object along the surface.
• The normal force is a reaction against a component, perpendicular to the surface, of a force which acts on the object in a direction towards the surface. An example of such a force could be the force of gravity on the object.
• When the static friction begins to surpass the magnitude allowable by the coefficient of static friction, the static friction no longer applies and is replaced by kinetic friction and its corresponding coefficient.