What Makes the Sound When a Bullwhip Is Cracked and Where Does the Noise Come From?

The crack of a bullwhip is actually a miniature sonic boom, produced because the tip of the whip is traveling through the air faster than the speed of sound.

When the cat master snaps his whip sharply, he’s putting a great deal of energy into the handle end. That energy has no place to go except to travel down the length of the whip as a wave of motion. In Techspeak, energy of motion is called kinetic energy, and it’s a function of both weight and speed (actually, mass and velocity, but let’s not quibble).

A given amount of kinetic energy can come from a heavy object moving relatively slowly or a light object moving relatively fast. For example, in order to match the kinetic energy of a ten-ton truck moving at 50 miles per hour (80 kilometers per hour), a one-ton automobile would have to be traveling at 158 miles per hour (254 kilometers per hour).

(The mathematically unchallenged will immediately recognize that those speeds aren’t inversely proportional to the weights. That’s because kinetic energy is proportional to the square of the velocity.)

As the energy moves down the length of a bullwhip it has less and less mass to work with, because the whip is tapered. The energy has to stay within the whip because it has no place else to go, so as the thickness and weight decrease the velocity has to increase.

Have you ever played “crack the whip” on ice skates? A long line of skaters travels in unison, and when the lead skater makes a turn a wave of turning energy accelerates down the line until the last guy is yanked around so fast that he can barely hold on. In a long bullwhip snapped hard, the speed at the tip can easily exceed the speed of sound and create a small sonic boom.

What happens to the energy when it gets to the tip of a whip? If you examine a well-used one, you’ll see that many of the “guys at the end” have actually been snapped off; the tip is frayed. But much of the energy has gone directly out into the air as sound, while some of it is reflected back up the length of the whip. The reflection turnaround at the tip is incredibly fast, and that fast-reversing wave also contributes to the noise.

Now all we need to understand is why lion tamers ever decided to use chairs. You’d think they could find something more sophisticated and professional-looking in the Tamers “R” Us store.