When running, why does the ponytail swing left and right instead of back and forth? This question interests not only boys who like to stare at girls, but also mathematicians who like to study all kinds of interesting problems-Joseph Keller, who won the 20 12 Ig Nobel Prize in physics for this research.

Keller said that to understand this problem, you need to know something about astrophysics. Because the equation deduced by astronomer george hill from the law of moon motion is the key to solve this problem.

For the sake of understanding, we can think of ponytail as a pendulum, and the head and body connected by ponytail are equivalent to the supporting frame of the pendulum. We know that every object has its own natural frequency, and ponytails are no exception. The natural frequency of ponytail is about 1.4 cycles per second. Ordinary people jog at a speed of about 140- 160 steps per minute, and the body swinging frequency is about 2.5 times per second.

Goldstein of Cambridge University studies complex physical systems. He and two other physicists have been trying to determine whether the shape of ponytail can be obtained from the properties of hair. After all, a head with 654.38 million hairs is a complex physical system, as anyone with thick hair can prove.

Their conclusions are simple, and two of them are more critical. The key parameters of hair include elasticity, density and curl. These characteristics determine the overall elasticity of hair; Another key parameter is the length of the ponytail. These scientists have come up with a simple formula to describe the shape of these hairs when they are tied together in a ponytail. They call this formula Rapunzel Index. The short ponytail made of elastic hair with low Rapunzel index will spread out. A long ponytail with a high Rapunzel index will droop gently, because gravity completely masks the elasticity of hair. This explanation shocked everyone.