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Sound Wave:Displacement Equation

a siren is pr...

Question

a siren is producing sound of frequency 930 Hz. This siren is moving away from an observer towards a wall with, velocity of 20 $m s^{- 1}$ . The frequency of sound directly coming from the siren will be: (Velocity of sound is 330 m/s)

A

882 Hz

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B

1000 Hz

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C

930 Hz

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D

977 Hz

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Solution

The correct option is A 882 Hz
Doppler effect when source moves away from the stationary observer:

$n^{'} = n [\frac{v_{s o u n d}}{v_{s o u n d} + v_{s o u r c e}}]$ $= 930 [\frac{330}{330 + 20}]$

$⟹ n^{'} \approx 877 H z$

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