Question

A train standing at the outer signal of a railway station blows a whistle of frequency 400 Hz in still air. The train begins to move with a speed of 30 m s–1 towards the platform. The frequency of the sound heard by an observer standing on the platform is (speed of sound in air = 330 m s–1)

• A. 420 Hz
• B. 430 Hz
• C. 440 Hz
• D. 450 Hz

Answer 1

Answer: (C)

440 Hz

Answer 2

Here, Frequency of source,

$\upsilon = 400Hz$

Speed of sound, v = 330 m $s^{- 1}$

Speed of source (i.e train),

$v_{s} = 30ms^{- 1}$

As the source is moving towards stationary observer

$\therefore\upsilon` = \frac{\upsilon v}{v - v_{s}}$

$= \frac{(400Hz) \times (330ms^{- 1})}{330ms^{- 1} - 30ms^{- 1}} = 440Hz$