A train standing at the outer signal of a railway station blows a whistle of frequency 400 Hz still air. The train begins to move with a speed of 10 m s-1 towards the platform. What is the frequency of the sound for an observer standing on the platform? (sound velocity in air = 330 m s-1)
Given:
Frequency of whistle = 400 Hz
Speed of train = 10 ms-1
When the train starts moving the source of the sound is also moving from the observer, thus the waves emitted by the source starts shifting as the train moves which creates a change in frequency of the sound reaching the observer. This effect is called Doppler effect. The apparent frequency of the sound for the observer is given by the relation,
Where v is the speed of sound in air, vt is the speed of the train and f is the original frequency.
Therefore the sound reaching the observer will have a frequency of 412.5 Hz.
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