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

Hint: Use the concept of Doppler's effect.
As the source (train) is moving towards the observer (platform), hence the apparent frequency observed is more than the natural frequency.
Frequency of whistle,                   ν= 400 Hz
Speed of train,                              vt=10 m/s 
The velocity of sound in air,                  v=330 m/s
Step 1: Find the apparent frequency of the source.
Apparent frequency when the source is moving,    
    = 330320-10400
νapp=330320×400=412.5 Hz