A train standing at the outer signal of a railway station blows a whistle of frequency 400Hz in still air. The train begins to move with a speed of `10ms^(-1)` towards the platform. What is the frequency of the sound for an observer standing on the platform ? (sound velocity in air `=330ms^(-1))`

Explanation

v = 400 Hz, `upsilon_(s) = 10 ms^(-1)`,
`upsilon = 330 ms^(-1), v' = ?`
As the train (source of sound) is moving towards the stationary observer, the apparent frequency v' is given by
`v' = (upsilon)/(upsilon - upsilon_(s)) v = (330)/(330 - 10) xx 400`
= 412.5 Hz.