A train approaching a hill at a speed of `40 km//hr` sounds a whistle of frequency `580 Hz` when it is at a distance of `1km` from a hill. A wind with a speed of `40km//hr` is blowing in the direction of motion of the train Find
(i) the frequency of the whistle as heard by an observer on the hill,
(ii) the distance from the hill at which the echo from the hill is heard by the driver and its frequency.
(Velocity of sound in air `= 1, 200 km//hr`)

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`