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Question

Two sources of sound S1 and S2, produce sound waves of same frequency of 660 Hz. A listener is moving from source S1 towards S2, with a constant speed u m/s and he hears 10 beats/s. The velocity of sound is 330 m/s. Then u equals

A
2.5 m/s
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B
15.0 m/s
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C
10.0 m/s
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D
5.5 m/s
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Solution

The correct option is A 2.5 m/s
Let the apparent frequencies heard by the observer are f1 , for source S1 and f2 for source S2.

f1=f(vuv) and f2=f(v+uv)

As f2>f1, the beat frequency is, f=f2f1

f=f(v+uv)f(vuv)

f2f1=f×2uv

10=660×2u330

u=2.5 m/s

Hence, (C) is the correct answer.

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