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Question

Two factories are sounding sirens at 800 Hz. A man goes from one factory to the other at a speed of 2 m/s. The velocity of sound is 320 m/s. The number of beats heard by the person in 1 s will be

A
10 Hz
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B
20 Hz
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C
8 Hz
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D
5 Hz
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Solution

The correct option is A 10 Hz
Let the frequency of sound by the man heard be n , When he is approaching the factory.n=(v+v0v)n n=(320+2320)=322320×800

Let the frequency of sound by the man heard be n′′ , when he is receding from the factory.n′′=(vv0v)n n′′=(3202320)×800=318320×800

Thus,

Beat frequency n=n′′n

n=(322318320)×800=10 Hz

Hence, option (a) is the correct answer.

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