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  4. A man standing on a platform observes that the frequency of the sound of a whistle emitted by a train drops by 140 Hz. If the velocity of sound in air is 330 m / s and the speed of the train is 70 m / s, the frequency of the whistle is

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Q. A man standing on a platform observes that the frequency of the sound of a whistle emitted by a train drops by $140\, Hz$. If the velocity of sound in air is $330\, m / s$ and the speed of the train is $70 \,m / s$, the frequency of the whistle is

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Solution:

Velocity of sound in air $=330\, m / s$.
Speed of train $=70\, m / s$
Let initial frequency be $f_{0}$
$f=f_{0} \times \frac{v}{v+v_{s}} $
$f_{0}-f=140 $
$f_{0}\left(1-\frac{330}{400}\right)=140$
$f_{0}=800\, Hz$