Why does a cracking whip make a sound like a gunshot?
When a whip is snapped, why does its tip produce a sharp crack loud enough to sound like a gunshot, and what makes the tip move fast enough to cause it?
History and references
Settled in 1958 by B. Bernstein, D. A. Hall, H. M. Trent (theory); confirmed by high-speed photography (P. Krehl, S. Engemann, D. Schwenkel, 1998) and modeled in detail by A. Goriely & T. McMillen (2002).
Bernstein, Hall, and Trent showed in 1958 that a wave traveling down a tapering whip accelerates the tip past the speed of sound, producing a small sonic boom. This was long debated because early stroboscopic studies could not resolve the tip's motion well enough to confirm supersonic speed. In 1998 Krehl, Engemann, and Schwenkel used high-speed digital photography to directly correlate the tip's supersonic velocity with the moment of the shock-wave 'crack,' settling the question experimentally. Goriely and McMillen's 2002 PRL paper then produced a full continuum-mechanics model of the whip's tapering shape and wave dynamics.
A simple leather toy turns out to run a miniature, everyday sonic boom experiment on your porch.
Explanations that circulated
- The crack is caused by the leather 'snapping back' elastically
- The sound comes from the handle or grip, not the tip
References
- On the Dynamics of a Bull Whip, B. Bernstein, D. A. Hall, H. M. Trent, Journal of the Acoustical Society of America, 1958
- The puzzle of whip cracking uncovered by a correlation of whip-tip kinematics with shock wave emission, P. Krehl, S. Engemann, D. Schwenkel, Shock Waves, 1998
- Shape of a cracking whip, A. Goriely, T. McMillen, Physical Review Letters, 2002
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