What high-tech technology can achieve supersonic speed? An old man in the park can do it with one hand!

When we talk about supersonic speed, what do you think of? Fighter jets or bullets? Anyway, they must be a lot of powerful guys, right? However, there is also a little-known hall-of-fame figure in the Supersonic Hall of Fame, Uncle Park!

Yes, you heard it right. This old man playing with the whip can also reach supersonic speed, but he doesn't make himself fly up like Superman and exceed the speed of sound. Instead, he makes the whip in his hand reach supersonic speed. In fact, you must have seen this before. The crackling sound when they swing the whip is the sonic boom produced when the end of the whip exceeds the speed of sound.

Then the question is, the uncle is still the same uncle, and the whip is also an ordinary whip, so how come the whip becomes supersonic when he waves his hand?

In fact, in principle, it is not difficult to go supersonic, as long as there is enough energy. If you were good at physics in high school, you should still remember the kinetic energy formula (Ek=1/2mv2). If the speed is constant, the greater the mass of the object, the more energy is required. As for the whip that the old man swung, it is not that the entire whip exceeds the speed of sound, but only the tail of the whip exceeds the speed of sound. The mass of the tail of the whip is very small, so if you want it to exceed the speed of sound, the energy required is not particularly large. Just let the old man shake his hand to provide this energy.

Since the energy required to make the little tail reach supersonic speed is not particularly much, why can't we throw it at supersonic speed if we just throw it out with the little tail, but must we put it on a whip and swing it?

This involves the issue of energy transfer. If we just throw the tail with it, then according to the kinetic energy formula (Ek=1/2mv2), the energy that the tail can get can only be its mass multiplied by the square of the speed of our arm swinging, and then multiplied by one-half. The tail has a small mass, and we can't swing it very fast, so the energy is naturally insufficient. In fact, if you think about it, you will know that if you want to throw it at supersonic speed, first of all, our hands must be able to swing at supersonic speed, which is obviously impossible.

The reason why the whip can make the little tail go supersonic is that it is not just "thrown" once, but is "thrown" all the time, accelerating itself.

First of all, the mass of the entire whip is much larger than that of the small tail, which ensures that it can obtain more energy at the same speed. As the whip swings, the speed of the end of the whip will decrease successively, and then transfer all its energy to the front. If we imagine the whip as a large piece of whip connected by many small whips, then this process is like each small whip trying its best to throw the front part forward again, and every time it is thrown together, the speed of the front will be a little faster, just like you throw me and I throw him. In the end, the whip swings faster and faster, and the kinetic energy obtained by the entire whip is finally concentrated on the small tail. Now for the small tail, with such a small mass, but with the kinetic energy collected by the entire whip for it, the speed is naturally super fast.

As for why a whip will make a cracking sound when it goes supersonic, it has to do with the propagation of sound. Before the whip goes supersonic, it actually emits sound waves, just like water waves slowly spread on the surface of a lake, these sound waves will also spread around in the air, and the speed at which these sound waves spread is the speed of sound.

As the whip moves, new sound waves are generated. If the whip is swung to one side, the later sound waves will be squeezed together with the previous ones, becoming denser and denser, just like when you row a boat slowly, there are layers of water waves in front of the boat. From the perspective of the whip, these sound waves are blocked in front of it. Each sound wave has a certain amount of energy. So many sound waves gather together to form a sound wave wall full of energy.

When the whip exceeded the speed of sound, it was equivalent to breaking through the wall at once, and the accumulated energy burst out, causing a huge noise.

Finally, let’s summarize from a scientific perspective. Next time you see an old man who can run supersonic speed with one hand, you must be in awe. The problem of respecting the elderly has been solved scientifically!

Author: Problem Squad

Reviewer: Luo Huiqian, researcher at the Institute of Physics, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.