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Excerpt
Actually two different, but related, constants of proportionality, relating frictional force to the associated normal force

The coefficient of static friction , represented by the symbol μ_s, when multplied by the Normal Force, is the maximum value of the force of friction opposing motion across a frictional surface. No motion will occur until the applied force parallel to the surface exceeds this value.

the coefficient of kinetic friction, represented by the symbol μ_k, when multipled by the associated Normal Force, is the frictional force acting against the applied force parallel to the frictional surface opposing motion when the body is moving.

See friction for a fuller explanation.