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An engine, blowing a whistle of frequency 200Hz moves with a velocity 16ms–1 towards a hill from which a well defined echo is heard. Calculate the frequency of the echo as heard by the driver. Velocity of sound in air is 340ms–1. 

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= 200 x 356/224 = 220 Hz.

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You copied the answer from textbook which is wrong. It comes to 318 Hz. Use calculator.

The correct answer should be calculated as :

n` = 200 X 340 / 340 - 32

Notice that velocity of echo is twice the velocity of sound i.e whistle in this case. Therefore we negate 2X16 which is 32 from velocity of sound in air. This gives the proper answer 220.77 Hz.

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