A train approaching a hill at a speed of 40 km/h sounds a whistle of frequency 580 Hz when it is at a distance of 1 km from the hill. A wind with a speed of 40 km/h is blowing in the direction of motion of the train. Find the frequency of the whitle as heard by an observer on the hill.

Explanation

According to Doppler's effect the apparent frequency when both source and observer move alond the same direction is
`n'=((v+w)-v_0)/((v+w)-v_S)n` ltbr. Where w is speed of wind.
According to problem, velocity of observer is `v_0=0`
`n'=(v+w)/(v+w-v_S)n`
Here `v=1200(km)/(h)`, `w=40(km)/(h)`, `v_S=40(km)/(h)` and `n=580Hz`
`n'=((1200+40))/(1200+40)-40xx580=599.33Hz`