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NEET 2012 (AIPMT, Mains) · Doppler Effect

Doppler Effect question from NEET 2012 (AIPMT, Mains)

Q73NEET 2012 · AIPMT · MAINSMedium

Doppler Effect

A train moving at a speed of 220 m s ${}^{-1}$ towards a stationary object, emits a sound of frequency 1000 Hz. Some of the sound reaching the object gets reflected back to the train as echo. The frequency of the echo as detected by the driver of the train is

(Speed of sound in air is 330 m s ${}^{-1}$ )

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Answer: (c)

Echo heard by a driver moving towards the reflector: $f' = f\,\dfrac{v + u}{v - u}$ .
$f' = 1000 \times \dfrac{330 + 220}{330 - 220} = 1000 \times \dfrac{550}{110}$
So the answer is (c) 5000 Hz.

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