Question

A train, standing in a station yard, blows a whistle of frequency $400Hz$ in still air. The wind starts blowing in the direction from the yard to the station with a speed of $10m{s}^{-1}$. Which of the following statements is correct?(Speed of sound is still air in $340m{s}^{-1}$)

• A. The frequency of sound as heard by an observer standing on the platform is $400Hz$.
• B. the speed of sound for the observer standing on platform is $330m{s}^{-1}$
• 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

The correct option is A The frequency of sound as heard by an observer standing on the platform is $400Hz$.

Here, ${\nu }_0=400Hz,v=340m{s}^{-1},v_w=10m{s}^{-1}$
as there is no relative motion 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 $={\nu }_0=400Hz$.

As the wind is blowing in the direction of sound, therefore, for an observer on platform,
Speed of the sound $=v+v_w=340+10=350m{s}^{-1}$