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 `10 ms^(-1)`. What are the frequency wavelength and speed of sound for an observer standing on the station's platform? Is the situation exactly identical the case when the air is still and the observer runs towards the yard at a speed of `10 ms^(-1)` ? The speed of sound in still air can be taken as `340 ms^(-1)`

Options

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 350 m/s.
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

A, B

Explanation

(a) Here source and listener both are at rest. Hence, there is not relative motion between them. Hence, `f _(L) = f _(S) = 400 Hz.` Henc,e option (A) is true,
(b) Here wind moves in the direction of velocity of sound. Hence, resultant speed of sound would be `= v + u _(W) = 340 + 10 = 350 m//s.` Hence, (C ) is false because `f _(L) = f _(S)`
(d) Option (D) is fale because `f _(L) = f _(S)`