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Skill 7 of 10
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Static friction

We all know the childhood issue of sitting on a slide and not sliding. After a little “kink” with the body, sliding starts. But what keeps us from sliding in the first place?

Kinetic friction was a sliding friction only taking place during sliding. Now we rather have some stationary version of friction. A kind of friction that does not just try to stop sliding but tries to prevent it. Let’s call it static friction and symbolize it: $\vec f_s$.

The two rough surfaces still “grip” into each other with their “peaks” and “valleys”. Since there is no motion, we can imagine that they have time to “sink fully in”. So, we’d expect static friction $\vec f_s$ to be larger than kinetic friction $\vec f_k$ due to a deeper “grip”.[1] This explains the necessary small “kink” to get started: Static friction holds back, but a quick “kink” of extra force overcomes it and sliding starts, now with kinetic friction taking over.

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