Why dragging a bow makes a steady note: stick-slip and Helmholtz's travelling kink
30seconds
A bow moves at a steady speed, yet the string under it vibrates in an exact rhythm hundreds of times a second. The secret is stick-slip: rosin grips the string, drags it sideways, and lets go when the string's tension wins.
Hermann von Helmholtz found in 1862 that the string is really two straight lines meeting at a sharp kink that runs around it once per cycle. Drag the bow in slow motion, change its speed and distance from the bridge, and hear the ramp-and-snap sawtooth it makes.
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