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 .......... $Hz$
Medium
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(b)
Velocity of sound in air $=330 \,m / s$.
Speed of train $=70 \,m / s$
Let initial frequency be $f_0$.
$f=f_0 \times \frac{v}{v+v_s}$
$f_0-f=140$
$f_0\left(1-\frac{330}{400}\right)=140$
$f_0=800 \,Hz$
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