A train is moving with speed 72 km/h towards a hill blows a whistle of frequency 1240Hz. the frequency of echo heard by driver is(speed of sound=330m/s)

Explanation

The observer `A` is at rest with respect to the air and the source is travelling at a velocity of `72 km//h` i.e.,
`20 m//s`. As is clear from the figure, the person receives the sound of the whistle in a direction `BA` making an angle `theta` with the track where `tan theta = (1732)/(1000) = sqrt(3)`, i.e. `theta = 60^(@)`. The component of the velocity of the source is approching the person with this component, the frequency heard by the observer is
`upsilon' = (v)/(v - ucostheta) upsilon = (330)/(330 - 10) xx 640 Hz = 660 Hz`.