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

Options

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

Answer

C

Explanation

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`