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 platfrom ? The speed of sound in air = 332 m/s.

Answer 1

Here given, Apparent frequency

=1620 Hz

So original frequency of the train is given by,

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

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

(where p =original frequency)

So, apparent frequency of the train observed by the observer in f

$=(\frac{332+0+0}{332+15}\times \frac{1620\times 317}{332})$

$=\frac{317}{347}\times 1620=1480Hz$