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Atrain approaching a railway platform wi...

Atrain 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

B

640 Hz

C

680 Hz

D

720 Hz

Text Solution

Verified by Experts

The correct Answer is:
C
Here, speed of source (i.e. Train), `v_(s) = 20 ms^(-1)` speed of sound in air, `v= 340 ms^(-1)`
frequency of the source, `v_(0) = 640 Hz`
The frequency heard by the person standing on the platform is,
`u.=u_(0) v/(v-v_(s)) = 640 xx [340/(340 -20)] = (640 xx 340)/320 = 680` Hz
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