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Question

A source of sound S moving with constant speed approaches a junction A. If the original frequency produced by the source is 640 Hz then for the situation shown in the figure, the apparent frequency heard by the observer at rest at O will be
[speed of sound =340 m/s]

A
720 Hz
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B
660 Hz
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C
700 Hz
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D
680 Hz
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Solution

The correct option is D 680 Hz
From Doppler's effect, the apparent frequency heard by the observer is,
f=f0(v±v0v±vs), here,
f0 is the original frequency produced by the source, v is the speed of sound, v0 is the speed of observer and vs is the speed of source.
The frequency heard by observer,
f=f0(v+v0vvs)


f=f0(v+v0vvscosθ)
[we take speed of source and observer along line joining the source and observer]
f=640×⎜ ⎜ ⎜340+03401003×35⎟ ⎟ ⎟
f=680 Hz

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