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Question

A boy blowing a whistle, is running away from a wall towards an observer with a speed of 1 ms1. The frequency of whistle is 680 Hz. The number of beats heard per second by the observer will be (given v=340 ms1)

A
zero
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B
2
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C
4
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D
8
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Solution

The correct option is A 2
Here a boy blowing a whistle, is running away from a wall towards an observer hence, the apparent frequency (of approaching) detected by observer is
n=vvun
where, u is the speed of speed of boy running towards observer, n is frequency of whistle sound, v is velocity of sound in air.
n=3403401(680) .......................(given)
n=340339(680)
n682Hz
Hence, the number of beats per second heard by the observer is 682680=2

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