Question

A train moving at a speed of 72kmph towards a station is sounding a whistle of frequency 600Hz. What are the apparent frequencies of the whistle as heard by a man on the platform when the train (1) approaches him (2) recedes from him.....[Given speed of sound in air=340m/s

Answer 1

Given :

Speed of train=vs=±72kmph= 72x5/18 =±20 m/s

frequency of source =f0=600Hz

Apparent frequency= f.

v = velocity of sound = 340 m/s

velocity of the observer = Vo = 0

f=fo [v±vo]/v±vs

1) Approaches him :

=600x 340+0/ 340-20=637.5Hz

ii) Recedes from him

f= 600 x 340+0/340+20 = 566.6Hz

When the train approaches the frequency increases and it will be less as the train goes away.

This is the Doppler's shift.