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  4. A train approaching a railway platform with a speed of 20 m s-1 starts blowing the whistle. Speed of sound in air is 340 m s-1. If the frequency of the emitted sound from the whistle is 640 Hz, the frequency of sound as heard by person standing on the platform is

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Q. A train approaching a railway platform with a speed of $20\, m \,s^{-1}$ starts blowing the whistle. Speed of sound in air is $340 \,m\, s^{-1}$. If the frequency of the emitted sound from the whistle is $640\, Hz$, the frequency of sound as heard by person standing on the platform is

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Solution:

Given : $v=340\, ms ^{-1},\,u_{s}=20\, ms ^{-1}$ and
$v_{0}=640\, Hz$
From Doppler's law,
$v=\left(\frac{340}{340-20}\right) 640$
$v=680\, Hz$