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J. Perelman
"Entertaining physics". Book 2.
Chapter 10. Sound. Wave motion


What would have heard you if removed from the playing of the orchestra at the speed of sound?

People traveling from Leningrad on the mail train, sees at all stations in the newspaper the same number of Newspapers, the ones that came out on the day of his departure. This is understandable, because Newspapers are traveling with a passenger, and Newspapers carried the trains behind. On this basis, we can probably conclude that, away from the orchestra at the speed of sound, we are going to hear the same note, which the orchestra took at the initial moment of our movement.

However, the conclusion is wrong; if you removed the speed of sound, sound waves, remaining relatively you alone, do not hit your eardrum, and therefore, you can't hear any sound. You will think that the orchestra stopped playing.

But why the comparison with Newspapers led to a different response? Yes, just because we haven't applied in this case, the reasoning by similarity (analogy). Passenger greeters everywhere the same number of Newspapers, imagine (i.e., could imagine, if he had forgotten his own motion)that the release of the new rooms in the capital stopped from the day of his departure. For him, the newspaper company ceased to exist, as would cease the existence of sound for a moving listener. Interestingly, this question can sometimes be confused even scientists - although, in fact, it is not so complicated. In a dispute with me - I was still in high school - an astronomer, now deceased, did not agree with this decision the previous problem and claimed to move away at the speed of sound, we should hear all the time the same tone. He proved his innocence following (here is an excerpt from his letter);

"Let sounds a note of known height. She sounded so for a long time and will sound indefinitely. Observers placed in space, hear it consistently and, for example, continued. Why you would not be able to hear if the sound speed or even thoughts shifted to any of these observers?"

Similarly proved it that the observer moving away from the zipper at the speed of light will decrease all the time continuously see this zip:

"Imagine, " he wrote me, " a continuous series of the eyes in space. Each of them will receive light impression after the previous one; imagine that you are mentally and consistently can visit the place each of these eyes, and obviously all the time you will see the lightning.

Of course, neither statement is not true: under these conditions, we will not hear sound and will not see the lightning. This is evident, among other things, from the formula on page 269; if it put v = -C, the perceived wave length l' is obtained infinite, which is equivalent to the absence of waves.


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"Entertaining physics" is over. If it is evoked in the reader the desire to get acquainted with the immense scope of the science, how gleaned this motley handful of simple details, the author's task is done, the task accomplished, and with satisfaction he puts the last point after the word "end".

Yakov Perelman Isidorovich

Entertaining physics J. Perelman


System Orphus


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