Question

# A source emitting a sound of frequency f is placed at a large distance from an observer. The source starts moving towards the observer with a uniform acceleration ‘a’. Find frequency heard by the observer corresponding to the wave emitted just after the source starts. The speed of sound in medium is v.

A

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B

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C
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D
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Solution

## The correct option is B Suppose at t = 0, distance between source and observer is l. First, wave pulse (say p1) is emitted at this instant. This pulse will reach the observer after a time t1=lv Source will emit the next pulse (say p2) after a time T(=1f). During this time the source will move a distance (12) aT2 towards the observer. This pulse p2 will reach the observer in a time t2=T+1−12aT2v The changed time period as observed by the observer is T1=t2−t1=T+1v−12aT2v−1v Substituting T1=1f′ and t=1f in the above equation, we get f′=2vf22vf−a

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