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 \[ f_k = \mbox{const} = \mu_k F_N \]


  • $f_s$ 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