A train, standing in a station -yard, blows a whistle of frequency 400 Hzin 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 platforms ? Is the situation exactly identical to the case when the air is still and the observer runs towards the yard at a speedof `10 ms^(-1)` ?The speed of sound in still air can be taken as `340 ms^(-1)`.

Explanation

Here `v = 400 Hz, v_(m) = 10 ms^(-1) , v = 340 m//s`
As the wind is blowing in the direction of sound, therefore effective speed of sound
`=v +v_(m) =340 + 10 =350 m //s `
As the source & listner both are at rest, therefore, frequency remains unchanged
i.e., `v = 400 Hz`
Wavelength ,`lambda = (v+ v_(m))/( v) = ( 350 )/( 400)`
`= 0.875M `
When air is still , `v _(m) =0 `
Speed of observer `v_(1) = 10 m//s , v_(s) = 0`
As observer moves toward the source
`:. v^(1) = ((v+v_(l)))/(v) xx v = ( (340 + 10))/( 340) xx 340 `
`= 411.76Hz`
As source is at rest , wavelength does not change
i.e., `lambda^(1)= lambda = 0.875 M `
Also,speed of sound `=v+ v_(m) =340 +0 `
`= 340 m //s `
The situation in the two cases are entirely different.