An observer moves towards a stationary source of sound, with a velocity one-fifth of the velocity of sound. What is the percentage increase in the apparent frequency?
Zero
0.5%
5%
20%
A
0.5%
B
20%
C
Zero
D
5%
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Solution
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Let velocity of sound in air be v.
⇒ Velocity of observer, vobserver=v5
Let ν be the original frequency of the sound source and ν′ be the apparent frequency heard by the moving observer.
Using Doppler effect when observer is moving towards the stationary source:
ν′=ν[vsound+vobservervsound]=ν[v+v5v]=65ν
Percentage increase in frequency =ν′−νν×100=65ν−νν×100=20 %
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