The serious-sounding crack generated when a bullwhip is sharply flicked is, in fact, a small sonic boom. The tip of the whip, known as the “cracker,” was thought to be the source of the menacing crack -- historically used as a means of getting the attention of livestock, and, in some cruel applications, unfortunate humans. However, researchers have since determined that it is the uncoiling of the loop during the whipping action that makes the snapping noise and gets your attention.
"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," wrote University of Arizona researcher Alain Goriely in the scholarly journal Physical Review Letters. Though some parts of the whip, including the tip, travel at faster speeds, "it is the loop itself that generates the sonic boom."
Cracking the whip, then and now: