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A train approaching a railway platform w...

A train approaching a railway platform with a speed of 20 m `s^(-1)` starts blowing the whistle. Speed of sound in air is 340 `m s^(-1)`. If the frequency of the emitted sound from the whistle is 640 Hz, the frequency of sound as heard by person standing on the platform

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 (ie. train), `v_(s)= 20 m^(s-1)`
Speed of sound in air, `v=340 ms^(-1)`
The frequency heard by the person standing on the platform is
`upsilon'=upsilon_(0)[v/(v-v_(s))]=640[340/(340-20)]=(640xx340)/320=680 Hz`
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