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J. Perelman
"Entertaining physics". Book 1.
Chapter 10. Sound and hearing

THE SOUND INSTEAD OF A MEASURING TAPE

Knowledge of the propagation velocity of sound in air can sometimes be used to measure distances to inaccessible subject. Such a case is described by Jules Verne's novel “journey to the center of the Earth”. During underground wanderings two travelers Professor and his nephew is lost each other. When, finally, they managed from afar to exchange votes, they had this conversation:

“Uncle! I cried (the story leads nephew).

"What, my child? - I heard some time later.

First of all, how far are we from each other?

- It is not difficult to learn.

- Your a chronometer?

- Yes.

- Take it in hand. Say my name and just notice the moment when you begin to speak. I will repeat the name as soon as the sound comes to me, and you also notice the moment when you will get my answer.

- Well. Then half of the time elapsed between the signal and the response will show the number of seconds taken by the sound to reach you. Are you ready?

- Yes.

- Attention! I call your name.

I pressed my ear to the wall. As soon as the word “Axel” (the name of the narrator) has reached my ears, I immediately repeated it and waited.

"Forty seconds, " said uncle, " therefore, the sound came to me in 20 seconds. As the sound passes in the second one-third mile, it corresponds to the distance of nearly seven miles”.

If you understand what is told in this passage, you easily will be able to solve the following task: I heard the distant whistle of a locomotive after fifteen seconds after he noticed white smoke, which was caused by the sound; at what distance I was from a steam locomotive?

Entertaining physics J. Perelman

 




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