Friction flashcards

Friction between surfaces that are not slipping. It adjusts to exactly balance the push trying to cause slipping, up to a maximum $f_{s,max}={\mu }_{s}N$. A 30 N push on a crate at rest meets 30 N of friction.

$f_k={\mu }_{k}N$: kinetic coefficient (no unit) times normal force (N). It is roughly constant regardless of sliding speed and points opposite the slipping, along the surface.

Usually ${\mu }_s>{\mu }_k$, so starting an object moving takes more force than keeping it sliding. Both are dimensionless ratios, $\mu =f/N$, describing a pair of surfaces.

From the force balance perpendicular to the surface: N = mg on level ground; a downward push adds to N, an upward pull subtracts; on a ramp $N=mg\cos \theta$. N equals weight only in the simple level case.

Down-slope part $mg\sin \theta$, into-ramp part $mg\cos \theta$. It stays put if $\tan \theta \le {\mu }_s$; the angle of repose is where $\tan \theta ={\mu }_s$, independent of weight.

Deceleration $a={\mu }_{k}g$ (mass cancels, so heavy and light objects slow alike). Stopping distance $d=\frac{v_0^2}{2\mu g}$; e.g. 20 m/s, μ = 0.8, g = 10 gives 25 m.

No. It opposes relative slipping between the surfaces. A box on an accelerating conveyor, a walker's foot and a driven tire all get forward static friction.

No. A brick slides with the same friction on its broad face or narrow edge, because friction depends only on μ and the normal force.