Question

A train approaching a platform at a speed of 54 km/h sounds a whistle. An observer on the platform finds its frequency to be 1620 Hz. The train passes the platform keeping the whistle on and without slowing down. What frequency will the observer hear after the train has crossed the platform? The speed of sound in air = 332 m/s.

• A. 1620 Hz
• B. 1600 Hz
• C. 1480 Hz
• D. None of these

Answer 1

The correct option is C

1480 Hz

Here given, apparent frequency when the train was approaching the man = 1620 Hz .So original frequency of the train is given by

$1620=\left(\frac{332+0}{332-15}\right)f$

$\Rightarrow f=\left(\frac{1620\times 317}{332}\right)Hz$

So, apparent frequency of the train observed by the observer when the train is going away from him.

$f^{\prime }=\left(\frac{332+0}{332+15}\right)\times \left(\frac{1620\times 317}{332}\right)=\frac{317}{347}\times 1620=1480Hz$.