A train, standing in a stationyard, blows a whistle of frequency 400Hz in still air. The wind starts blowing in the direction from the yeard to the station at a speed of `10ms^(-1)`. What are the frequency, wavelength, and speed of sound for an observer standing on the station's platform ? Is the situation exactly identical to the case when the air is still and the observer runs towards the yeard at a speed of `10ms^(-1)` ? Th speed of sound in still air can be taken as `340ms^(-1)`.

Explanation

Here, `v=400Hz , upsilon_(m)=10ms^(-1), upsilon=340ms^(-1)`
As, the wind is blowing in the direction of sound, therefore, effective speed of sound
`=upsilon+upsilon_(m)=340+10=350ms^(-1)`
As, the source and listener, both are at rest, therefore, frequency remains unchanged , i.e., `v=400Hz` wavelength, `lambda=(upsilon+upsilon_(m))/(v)=(250)/(400)=0.875m`
(b) When air is still, `upsilon_(m)=0`
Speed of observer, `upsilon_(L)=10ms^(-1),upsilon_(s)=0`
As, observer moves towards the source,
`v^(')=((upsilon+upsilon_(L)))/(upsilon)xxv=((340+10))/(340)xx400=411.76Hz`
As, source is at res, wavelength does not change, i.e., `lambda^(')=lambda=0.875m`
Also, speed of sound `=upsilon+upsilon_(m)=340+0=340m//s`
The situations in teh two cases are entirely different.