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Question

# An observer moves towards a stationary source of sound with a speed1/5th of speed of sound . the wavelength and frequency of the source emitted are λ and f respectively. The apparent frequency recorded by the observer

A
396.7
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B
400
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C
500
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D
600
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Solution

## The correct option is A 396.7A whistle revolves in a circle of radius R=0.5m with W=20 rad/sobserve hears the lowest frequency (shriller sound) when source approaches towards him linearly (at A) as he is stationery, due to Doppler's effect. At A, velocity of source Vs=RW =0.5×20Vs=10m/sGiven that, original frequency of source V=385Hz and speed of sound in air =340m/sThus, by Doppler's effect, apparent frequency :-V′=V(VV−Vs) =385×(340340−10)V′=396.7Hz

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