A train standing at a signal of a railway station blows a whistle of frequency 256 Hz in air. Calculate the frequency of the sound as heard by a person standing on the platform when the train(i) approches the platform with a speed of 40 m s -1(ii) recedes from the platform with the same speed.

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2015-09-18T21:54:46+05:30

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Is the question properly formatted?  some mistake there ?

I suppose that the person sitting on the train is blowing the whistle.
    f₀ = 256 HZ

The change in frequency of sound heard is called the Doppler's shift.  The formulas are:

1) when the source of the sound is approaching the observer : 
f=f_0\ [\frac{c\ +\ v_{obs}}{c\ -\ v_{source}}]=256*\frac{340+0}{340-40}=290.1\ Hz

2) when observer and source are separating from each other:
f=f_0\ [\frac{c\ -\ v_{obs}}{c\ +\ v_{source}}]=256*\frac{340-0}{340+40}=229.0\ Hz
 

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