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

THE SLOW CONVERSATION

If you think, however, that the true speed of sound in air is a third of a kilometre per second is always sufficient rapidity, now change your mind.

Imagine that between Moscow and Leningrad instead of the electric telephone is arranged ordinary meeting trumpet like those phones, which combined previously separate areas of large stores or which was used on ships for communication with the engine room. You are standing at the Leningrad end of this 650-kilometer pipe, and that your friend is in Moscow. Ask a question and expect a response. Passes five, ten, fifteen minutes - the answer is no. You begin to worry and think with someone misfortune happened. But fears are unfounded: the question has not yet gone to Moscow and is now only half way. It will be a quarter of an hour before your friend in Moscow will hear the question and will be able to answer. But and its replica will go from Moscow to Leningrad at least a half hour, so the answer to your question you will get only an hour later.

Can check the calculation: from Leningrad to Moscow 650 km; the sound travels in the second 1/3 km; hence, the distance between the cities he will run in 2160 with extra seconds, or 35 minutes. Under such conditions, talking the whole day from morning till night, you barely have time to exchange a dozen words *.

* The author does not account for attenuation of sound waves with distance, which actually will prevent you lead the conversation, because at the other end of this pipe you will not hear. (Approx. as amended).

Entertaining physics J. Perelman

 




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