Shaping a Theory on Whip Cracks
Math Prof Says Sonic Boom Creates That Snapping Sound

Listen to Scott Simon's interview with Alain Goriely.

Creating sonic boomlets in Australia Photo: courtesy The Man from Snowy River Bush Festival

Some songs to crack a whip by: "Whip It" by Devo "When the Whip Comes Down" by the Rolling Stones "Tied to the Whipping Post" by the Allman Brothers "Theme from Rawhide" by Frankie Laine "Theme from Rawhide" by The Dead Kennedys

June 1, 2002 -- In the past, it was assumed that the sound made by the crack of a whip was caused by the tip either slapping against the body of the whip or slapping against the air.

But University of Arizona mathematics professor Alain Goriely gave the past the slip recently when he proved that the cracking sound actually comes from the mini sonic boom that's created by the arc traveling along the length of the whip. Goriely and his colleague Tyler McMillen presented their findings in a paper -- "The Shape of a Cracking Whip" -- appearing in a journal published by the American Physical Society. The paper essentially supports the mathematical equation that Goriely says explains the phenomenon.

"Nobody has really seriously studied this equation in the context of a whip," Goriely tells Scott Simon for Weekend Edition Saturday. Researchers have, however, been studying whip-cracks for many decades. A 1958 paper in the Journal of the Acoustical Society of America described an experiment that proved that the tip of a bullwhip travels faster than the speed of sound. And one study conducted a few years back by some students at the North Carolina School of Science and Mathematics proved the same thing about snapped towels.

If all this seems a bit silly, consider that Goriely says his findings could help researchers in all sorts of fields, from biology to engineering. It might, for instance, help scientists understand the propagation of waves along a DNA molecule, he says. There's no indication that the snapped-towel study has borne scientific fruit.

The professor is more than just whip-smart -- he's also handy with a whip, as he proves during the interview with several startling lashes. "It does bring a certain level of respect in the classroom and among my colleagues," he says.


Other Resources

Goriely's homepage at the University of Arizona includes some cool animations.

The Straight Dope on snapped towels.

Everything you always wanted to know about whips, but were afraid to ask.

More than you probably care to know about whip enthusiasts.

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