A whip makes a cracking sound because its tip moves faster than the speed of sound.
Whipcracking is the act of producing a cracking sound through the use of a whip. Used during livestock driving and horse riding, it has also become an art. A rhythmic whip-cracking belongs to the traditional culture among various Germanic peoples of Bavaria, various Alpine areas, Austria, and Hungary. Today it is a performing art, a part of rodeo show in the United States, a competitive sport in Australia, and increasingly popular in the United Kingdom, where it crosses boundaries of sport, hobby, and performance.
The crack a whip makes is produced when a section of the whip moves faster than the speed of sound creating a small sonic boom. The creation of the sonic boom was confirmed in 1958 by analyzing the high-speed shadow photography taken in 1927.
Recently, an additional, purely geometrical factor was recognized: the tip of the whip moves twice as fast at the loop of the whip, just like the top of a car's wheel moves twice as fast as the car itself.
A common explanation is to derive the behavior from the conservation of energy law. However, it was noted that the energy is also conserved when the crack sizzles, therefore derivations from purely conservation laws, including conservation of momentum and some others are insufficient.
Based on simulations, the high speed of the tip of the whip has been proposed to be a result of a "chain reaction of levers and blocks".
In 1997, Discover Magazine reported about the possibility of the "whip-cracking" effect millions of years ago. As part of the joint computer scientists' and paleontologists' research into the motion of dinosaurs, Nathan Myhrvold, a chief technology officer from Microsoft, carried out a computer simulation of an apatosaurus, which had a very long, tapering tail resembling a whip. Basing on the reasoning described above, Myhrvold concluded that sauropods were capable of producing a crack comparable to the sound of a cannon.