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Question

# A boy is walking away from a wall at a speed of 1m/s in a direction at right angle to the wall. As he walks, he blows a whistle steadily. An observer towards whom the boy is walking hears 4 beats per second. If the speed of sound is 340m/s, what is the frequency of the whistle?

A
680 Hz
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B
720 Hz
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C
760 Hz
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D
800 Hz
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Solution

## The correct option is A 680 Hz The situation is equivalent to two sources giving beats. First is the whistle of the boy and the second is wall. fb=fs−fs′ 4=f(340340−1)−f(340340+1) Or 4=f(1−1340)−f(1+1340) on applying binomial approximation, 4=f(1+1340)−f(1−1340)=f170 or f=680 Hz

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