Contact forces
Normal force is $\bot$ to the contact surface
Static friction is tangential ($\|$ surface), opposes applied force $\vec{F}$ and
matches it:
\[
\vec{f}_s = - \vec{F}
\]
up to its maximum value: $f_s\leq f_s^{(max)} = \mu_s F_N$
Once in motion, kinetic friction opposes motion, with
\[
f_k = \mbox{const} = \mu_k F_N
\]
Normally
- $f_s$ and $f_k$ do not depend on contact area
- $f_k$ does not depend on speed (for small speeds)
- $\mu_k < \mu_s$, and so $f_k < f_s^{(max)}$
Tension in a massless rope is a perfect `transmitter' of force:
$T=\mbox{const}$ at all points in the rope
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