Static friction $\vec f_s$

Static friction $f_s$ applies solely when two touching surfaces do not slide over one another. It has an upper limit:

$$f_s\leq \mu_s n$$

$\mu_s$ is static friction coefficient, $n$ is normal force (the load).

External forces and other influences below this limit will not cause sliding, because $f_s$ will adjust to exactly balance them out.

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