An observer is standing 500 m away from a vertically hill. Starting between the observer and the hill a police van having a siren of frequency 1000 Hz moves towards the hill with a uniform speed. If the frequency of the sound heard directly from the siren is 970 Hz, the frequency of the sound heard after reflection from the hill (in Hz) is about, (velocity of sound =$330\ ms^{- 1}$)

Question

Dr. Abubacker · Accepted Answer

(b) Sound geard directly
$v_{1} = v_{o}\left( \frac{v}{v - v_{s}} ight)$
$	herefore 970 = 1000\left( \frac{330}{330 + v_{s}} ight)$
Or $v_{s} = 10.2ms^{- 1}$
The frequency of reflected sound is given by
$v_{2} = v_{o}\left( \frac{v}{v - v_{s}} ight) = 1000\left( \frac{330}{330 - 10.2} ight)$
$= \frac{1000 	imes 330}{319.8} \approx 1032Hz$

Explanation

Reviewed by Dr. Abubacker

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