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When a train is approaching the observer...

When a train is approaching the observer, the frequency of the whistle is 100 cps. When it has passed the observer, it is 50 cps. The frequency when the observer moves with the train, is :

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In case of approaching of source `100=(fV)/(V-V_(S))`
while in case of recession of source. `50=(fV)/(V+V_(s))`
Which on simplification gives `f=(200)/(3)=66.67Hz`
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