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JEE Main 2014 (12 Apr, Shift 1) · Doppler Effect

Doppler Effect question from JEE Main 2014 (12 Apr, Shift 1)

Q70JEE Main 2014 · 12 APR · SHIFT 1Difficult

Doppler Effect

A source of sound A emitting waves of frequency 1800 Hz is falling towards ground with a terminal speed $v$ . The observer B on the ground directly beneath the source receives waves of frequency 2150 Hz, The source A receives waves, reflected from ground, of frequency nearly: (Speed of sound = 343 m/s)

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Answer: (b)

At B (source approaching): $2150 = 1800 \times \frac{343}{343 - v} \Rightarrow v \approx 55.8$ m/s
Ground reflects $2150$ Hz; A is an observer moving towards it at $v$ .
$f = 2150 \times \frac{343 + 55.8}{343} \approx 2500$ Hz
So the answer is (b) 2500 Hz.

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