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NEET 2002 (AIPMT) · Doppler Effect

Doppler Effect question from NEET 2002 (AIPMT)

Q80NEET 2002 · AIPMTMedium

Doppler Effect

A whistle revolves in a circle with angular speed ω\omega = 20 rad/s using a string of length 50 cm. If the frequency of sound from the whistle is 385 Hz, then what is the minimum frequency heard by an observer which is far away from the centre (velocity of sound == 340 m/s)

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Answer: (b)

  1. Speed of the whistle u=ωr=20×0.5=10 m/su = \omega r = 20 \times 0.5 = 10\text{ m/s}.
  2. Minimum frequency when the whistle moves directly away: fmin=f vv+u=385×340350f_{min} = f\,\dfrac{v}{v + u} = 385 \times \dfrac{340}{350}
  3. fmin=374 Hzf_{min} = 374\text{ Hz}
  4. So the answer is (b) 374 Hz.
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