True Cause of Whip's Crack Uncovered

Join Our Community of Science Lovers!

On supporting science journalism

If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

It takes a dexterous hand to coax a whip to crack. Now researchers report that they have discovered the mechanism responsible for the startling sound. It has long been thought that the crack results from the tip of the whip traveling fast enough to break the sound barrier and create a sonic boom. But the new findings suggest otherwise. Apparently, it's the loop in a whip that is the real noisemaker.

Though by no means a master whip cracker, Alain Goriely of the University of Arizona was nonetheless intrigued by the phenomenon and set out to study it at a theoretical level. Together with Tyler McMillen, a graduate student in applied mathematics, he modeled the behavior of the leather strips in a paper to be published in Physical Review Letters. Previous whip work (one of just three papers on the subject in the past century) had resulted in the puzzling observation that the sonic boom occurs when the tip of the whip is traveling at about twice the speed of sound. But if the tip were truly the cause of the crack, why wasn't the sound heard earlier, when the tip first reached the speed of sound? Goriely and McMillen's calculations have revealed the answer. "The crack of a whip comes from a loop traveling along the whip, gaining speed until it reaches the speed of sound and creates a sonic boom," Goriely says. He notes that even though some parts of the whip travel at greater speeds, "it is the loop itself that generates the sonic boom."

Although the whip's tip has lost the distinction of being the source of the menacing crack, it is still a force to be reckoned with: according to Goriely's calculations, "the tip can reach speeds more than 30 times the initial speed [of the whip]."

Subscribe to Support Independent Journalism

Great science journalism requires human expertise, time, effort and creativity. And it costs money. That’s why I and the journalists here at Scientific American hope you’ll join our community.

When you subscribe, you are supporting staff and freelance journalists who are passionate about telling science stories that are true, important and compelling. Our editors and reporters are often experts in their fields, which means they understand the nuances of big discoveries and can untangle the breakthroughs from the hype. With a subscription, you are also supporting rigorous fact-checking to ensure the words we publish are precise and accurate. And you’re supporting original illustrations, graphics and photos that bring you closer to an advanced laboratory, an ice sheet in Antarctica or a space mission in orbit. You’re helping us craft other types of high-quality journalism as well: Our newsletters are carefully written, edited and curated by staffers you have or will come to know and love. Our Science Quickly podcast is based on original reporting, collaboration with editors and scientists and exacting production.

Subscriptions keep this engine running so we can continue to deliver thoughtful, rigorous and independent science journalism to you. In an era of viral misinformation, this work is crucial. If you value what we do, I hope you’ll consider joining us as a subscriber

Thank you,

Jeanna Bryner, Editor in Chief, Scientific American