Illustrated Physics
physics, drawn
Concept

Kinetic friction — where it appears

The friction between two surfaces that are already sliding over each other. It is usually a fixed fraction of the force pressing them together, somewhat smaller than the most static friction can hold, and that gap is what makes contacts stick and slip.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

What friction returns, against what it is asked for. The friction force on a block under a 50 N normal load, against the force applied to it. Below 30.0 N — the static limit μs·N — friction returns exactly what is asked for and nothing moves, so the curve is the 45° line and the coefficient never appears. At that point the surface gives way and the force drops to μk·N = 22.5 N, where it stays however hard the block is pushed. The gap above the flat line is the surplus that accelerates it: 22.5 N at the right-hand edge of the axis.

The force that takes what it needs

Static friction has no value of its own. It supplies exactly what equilibrium demands and not a newton more, right up to the moment it cannot — which is the only instant in the whole business at which a coefficient of friction means anything at all.

mechanics · Friction Two fingers that take turns and arrive together. Two fingers under a metre rule, started 30 cm and 10 cm from its balance point and slid towards each other, with static friction 0.6 and sliding friction 0.45. Only one finger slides at a time: the farther one, which carries less of the weight. It slides until its sliding friction matches what the near finger can hold statically, at a distance 0.75 times the other's, and then the roles change. 14 changes of role close the gap to under half a centimetre, each stage a fixed fraction of the one before, and both fingers end under the balance point.

The fingers that always meet in the middle

Rest a metre rule across two outstretched fingers, one near an end and one near the middle, and slide the fingers together. However the hands try, the fingers take turns: one slides while the other sticks, then they swap, and they arrive together under the rule's balance point. Pull them apart instead and only one ever moves. Both halves come from one inequality between two friction coefficients, and the turns are a measurement of their ratio.

mechanics · Friction

Named alongside it

The objects these essays reach for when they reach for this one.

FrictionNormal forceStatic frictionStick-slipCentre of massCoefficient of frictionContact areaEquilibrium

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