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

IMAGINARY EXPLOSION

Racing between the flying body and make them sound makes us sometimes involuntarily to draw wrong conclusions, sometimes completely unresponsive true picture of the phenomenon.

An interesting example is the car (gun or missile), flying high above our head. Fireballs, penetrating into the atmosphere of our planet from world space, have great speed, even when reduced by the resistance of the atmosphere, yet dozens of times greater than the speed of sound.

Cutting through the air, cars often produce noise resembling thunder. Imagine that we are at the point With (see Fig.), and above us on the line AB flying car.


Imaginary explosion of the car.

The sound produced by the car at the point Andwill reach us ( With) only when the car will have already moved to the point In; as the car goes much faster than sound, he may be able to walk to some point D and from here to send us the sound before you reach us sound from a point And. So we will hear the first sound of the point D and then the sound of the point And. And because of the point In the sound will come to us too later than the point D, somewhere over our head should be a point Tobeing in which the car takes its beep just before. Lovers of mathematics can calculate the position of this point, if you are in a certain relation to the speed of the car and sound.

Here is the result: what we hear is not at all like what we see. For the eyes, the car will primarily be located at the point And from here you will fly along the line AB. But for the ear of the car will primarily be located somewhere above our head at the point To, then we will hear at one time two sound damped opposite directions from: To to And and To to To. In other words, we will hear how the car seemed to split into two parts, which have gone in opposite directions. Meanwhile, in reality, no explosion occurred. Here is how deceptive can be auditory experience! It is possible that witnessed many "eyewitnesses" explosions of fireballs - that's the kind of deceptions hearing.

Entertaining physics J. Perelman

 




System Orphus

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