[Physics] Why does a “speed of sound” exist?


I've recently read that wind cannot be faster than the speed of sound (german source).

But why is there a speed of sound? I understand (well, mostly accept to be honest) that the speed of light in vacuum is a maximal speed for all matter. And I understand that you need more energy the more you accelerate particles. But why can't you make wind faster?

Best Answer

There's a few things going on here. First of all, wind can move faster than the speed of sound.

As to your second question "Why is there a speed of sound"? Sound is caused by a change in position of molecules relative to a collection of other molecules. Since molecules have electrons on the outside, they are repulsed from one other (like charges repulse). Thus if you change the position of certain molecules, they will repulse the neighboring molecules. These neighboring molecules will then repulse their neighboring molecules, and thus you have this "shift in position" (usually measured as a shift in pressure) that propagates through space. This is what we hear as sound. The speed of propagation depends on how close the molecules are together when they are in equilibrium, the bonds the molecules have, etc.

For a simple understanding of sound, think of a bunch of train cars attached to one another through springs. Now push one end of the train a small amount. This push will compress the spring between train cars 1 and 2. The spring will then uncompress and push train 2 forward. This compresses the spring between trains 2 and 3. And so on... Until the last train car is moved forward. The total length of the train divided by the time it takes from the moment you push train car 1 till the last train car moves is the speed of sound (for train cars, note that the speed of sound is different depending on the material through which it moves - most often we refer to speed of sound as the speed of sound of air).

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