Whip crack sonic boom

Settled in 2002 by Alain Goriely, Tyler McMillen (University of Arizona).

Whip-cracking folklore long held that the loud crack simply came from the whip 'hitting itself' or the material snapping, and it was not until mid-20th-century high-speed photography (Bernstein, Hall and Trent, 1958) confirmed the tip does briefly move faster than sound that the sonic-boom idea gained real evidence. In 2002 Goriely and McMillen built a full dynamical model of the whip as a tapering elastic rod and showed mathematically how tension, taper and the initial cracking motion combine to accelerate a bending wave from the thick handle to the thin tip, amplifying its speed past Mach 1 by the time it reaches the end. Their model reproduced the whip's characteristic loop shape during a crack and quantitatively explained why tapering is essential to reaching supersonic tip speed, settling both what causes the sound and confirming the tip genuinely breaks the sound barrier.

A whip is essentially a mechanical amplifier that takes a slow flick of the wrist and turns the very tip into the only part of your body's motion that ever exceeds the speed of sound.

Explanations that circulated

• The crack is simply the whip material snapping or the leather popping like a toy noisemaker, with no real sonic boom involved — high-speed studies (building on earlier work by Bernstein, Hall and Trent in the 1950s and formalized by Goriely and McMillen's 2002 model) confirmed the tip genuinely exceeds the speed of sound and the crack is a true miniature sonic boom, produced by the whip's tapering shape accelerating a wave from handle to tip, not by any part of the whip breaking.

References

• Shape of a Cracking Whip, A. Goriely, T. McMillen, Physical Review Letters, 2002