Question

A train, standing in a station yard, blows a whistle of frequency 400 Hz in still air. The wind starts blowing in the direction from the yard to the station with a speed of 10 m s–1. Which of the following statements is correct?

(Speed of sound in still air is 340 m s–1)

• A. The frequency of sound as heard by an observer standing on the platform is 400 Hz
• B. The speed of sound for the observer standing on the platform is 330 m s–1.
• C. The frequency of sound as heard by the observer standing on the platform will increase
• D. The frequency of sound as heard by the observer standing on the platform will decrease.

Answer 1

Answer: (A)

The frequency of sound as heard by an observer standing on the platform is 400 Hz

Answer 2

Here, $\upsilon_{0} = 400Hz$

$v = 340ms^{- 1},v_{w} = 10ms^{- 1}$

As there is no relative motions between the source and observer, therefore the frequency of sound remains unchanged. Hence, the frequency of sound as heard by an observer standing on the platform is $\upsilon = 400Hz.$

As the wind is blowing in the directions of sound therefore, for an observer standing on platform.

Speed of the sound

$= v + v_{w} = 340 + 10 = 350ms^{- 1}$