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]."