What Makes a Bull Whip Crack?

When you think of someone cracking a long, tapered bull whip, you think of the old west cowboys herding cattle across the barren desert. But it's not the cowboy or the old west that tends to stick with most people; it's the sharp, loud crack of that whip. So, what exactly is that crack and how does a whip make the sound? The answer might surprise you.

First of all, the construction of the whip is important for it to crack correctly. The handle of the whip is the non-flexible part that is gripped as if you are shaking hands with someone. The next portion of the whip is the thong, or flexible section. The thin, tapered end section is the popper or cracker. This is the part that makes the sound. And that sound is actually a small sonic boom. Yes, that's right, a sonic boom created by a hand-held device.

Whips are composed of braided leather, whether cow hide or kangaroo, around a core of leather. The length and taper can vary, be if the whip isn't constructed well, it won't crack right. Since the cracker has less mass than the handle, the energy created by the movement of the handle travels along the tapered whip to the cracker. This means that as the mass of the tapering whip decreases, the velocity of the whip increases, which can be measured by the formula for kinetic energy. Recent research has shown that the increase in velocity is created by the initial loop in the thong as it travels the length of the whip. This loop is the trick to creating the crack. Without the correct loop, the crack won't work or will be poor. Whip crackers first perfect this loop technique before even attempting to make the crack. Apparently a bull whip can reach Mach 1 and beyond. Mach 1 is the critical speed that creates a sonic boom, but bull whips have been measured at more than Mach 1, which is about 761 miles per hour.

The crack, or sonic boom, occurs when the cracker slices through the air, making pressure waves behind and in front. The waves are moving at the speed of sound, but when the cracker's velocity increases, the waves compress together. As they compress, they get closer and closer together, forming a single wave at the speed of sound. This compressed wave causes the sonic boom, which is actually the sound of the air going into the vacuum that was formed around the cracker. It's the end of the cracker that achieves a speed above the speed of sound, breaking through the sound barrier.

For this reason, whip makers know from experience how to exactly taper the whip to make the mass of the handle more than that of the thong. Then the thong's mass must be less than the tapered part of the whip, all in proportion, so that the energy transfer from the handle to the cracker is not lost from friction with the air around it. This is the principal of the conservation of energy. This principal says that the total energy in a system stays constant, even if there are changes to the system. The cracking of the whip is a change to the system, yet it does not alter the energy, the energy simply moves down the whip as it increases in velocity.

But, regardless of the physics of why a bull whip cracks, it's that ear-splitting sound that sticks in one's memory. No wonder cowboys were able to get the attention of their cattle.