Coefficient of static friction

The coefficient of static friction is a physical concept that determines how much force is required before an inert object, of a given material, at rest on another known substance, can be put into motion.

The coefficient of static friction is typically represented by the symbol <math>\mu_s<math>. The equation typically associated with it is:

<math> f = \mu_s N <math>

where f is the force required to start the object into motion, <math>\mu_s<math> is the coefficient of static friction, and N is the weight of the object in Newtons.

The coefficient of static friction changes depending on material; smoother or more lubricated surfaces have a lower <math>\mu_s<math>, and vice versa for rougher surfaces. Ice on ice has a coefficient of static friction of roughly 0.1, whereas rubber on concrete is 1.0.

Once the force f has been reached in the above equation, the object is put into motion, and the coefficient of static friction is no longer relevant – rather, the coefficient of kinetic friction is used in the equation.