Coefficient of static friction

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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 [\mu_s]. The equation typically associated with it is:

[ f = \mu_s F_n ]

where f is the force required to start the object into motion, [\mu_s] is the coefficient of static friction, and [F_n] is the strength of the normal force in newtons.

The coefficient of static friction changes depending on material; smoother or more lubricated surfaces have a lower [\mu_s], 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.

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