Question

A train approaching a railway platform with a speed of 20 ms−1, starts blowing the whistle. Speed of sound in air is 340 ms−1. If the frequency of the emitted sound from the whistle is 640 Hz, the frequency of sound to a person standing on the platform will appear to be:

A
600 Hz
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B
640 Hz
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C
680 Hz
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D
720 Hz
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Solution

The correct option is B 680 HzVelocity of source (train) vs=20 m/sSpeed of sound vsound=340 m/sOriginal frequency fo=640 HzUsing Doppler effect when a source is approaching towards the stationary observer :Apparent frequency heard f′=fo[vsoundvsound−vs]∴ f′=640[340340−20]=680 Hz

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