Home > Courses > 1P21_Sternin > Dynamics 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