Q. 16

# A small source of sound moves on a circle as shown in figure (16-Q1) and an observer is sitting at O. Let v_{1}, v_{2}, v_{3} be the frequencies heard when the source is at A, B and C respectively.

A. v_{1} > v_{2} > v_{3}

B. v_{1} = v_{2} > v_{3}

C. v_{2} > v_{3} > v_{1}

D. v_{1} > v_{3} > v_{2}

Answer :

When the source is at C the source speed is perpendicular to the line joining the observer and the source. Thus at this instant, the separation between the two is not changing and the frequency heard ν₃ is same as the source. When the source is at A the separation between them is increasing, so due to the Doppler effect ν₁ < ν₃. But when the source is at B, the separation is decreasing and due to the Doppler effect ν₃ < ν₂. So ν₂ > ν₃ > ν₁. Hence the option C is correct.

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