Question

# When a source of sound of frequency f crosses a stationary observer with a speed vS(<< speed of sound v), the apparent change in frequency â–³f is given by:

A
2fvsv
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B
2fvvs
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C
fvsv
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D
2fvvs
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Solution

## The correct option is B 2fvsvConcept: When the source approaches the observer then there will be a frequency and when the source passes by the observer then there will be another frequency.Let, the former frequency is f1 and latter frequency is f2then Δf=f1−f2or,Δf=f(vv−vs)−f(vv+vs)or,Δf=f(1−vsv)−1−f(1+vsv)−1or,Δf=f(1+vsv)−f(1−vsv) [Using binomial expansion]or,Δf=f(2vsv)or,Δf=2fvsvThe correct answer is OptionA:2fvsv

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