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Question

One train is approaching an observer at rest and another train is receding from him with the same velocity 4 m/s. Both the trains blow whistles of same frequency of 243 Hz. The beat frequency in Hz as heard by the observer is (speed of sound in air is 320 m/s)

A
10
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B
6
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C
4
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D
1
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Solution

The correct option is B 6
When the train is approaching,
n1=vvvs×n=3203204×243=8079×243
When the train is receding,
n2=vv+vs×n=320324×243=8081×243
Beat frequency is
n=n1n2=80×243(179181)=6Hz

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