Kinetic friction $\vec f_k$ (Amontons’ law)

Kinetic friction $\vec f_k$ applies solely when there is sliding between two surfaces. It is proportional to the normal force $n$ (the load) at not-too-large normal forces per area (does not depend on contact area or sliding speed).

$$f_k=\mu_k n$$

This is a macro-scale friction model. At larger normal forces per area, other kinetic friction models take over.

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