Question

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 is

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

Answer 1

Answer: (C)

680 Hz

Answer 2

Here, Speed of source (ie., train), $v_{s} = 20ms^{- 1}$

Speed of sound in air,

$v = 340ms^{- 1}$

Frequency of the source,

$\upsilon_{0} = 640Hz$

The frequency heard by the person standing on the platform is

$\upsilon' = \upsilon_{0}\left( \frac{v}{v - v_{s}} \right)$

$= 640\left\lbrack \frac{340}{340 - 20} \right\rbrack = \frac{640 \times 340}{320} = 680Hz$