Two trains A and B are approaching each other with `108 km h^(-1)` and 126 km `h^(-1)` , respectively . If the train A sounds a whistle of frequency 500 Hz , find the frequency of the whistle as heard by a passenger in the train B .
(a) before the trains cross each other and
(b) after the trains cross each other . (Take velocity of sound as `330 m s^(-1)`)

Explanation

(i) `n^(1) = n ((v+v_(0))/(v - v_(s))) , n^(1) n ((v- v_(0))/(v + v_(s)))`
(ii) Identify which of the trains A and B is the source and which the observer of sound is .
Recall the expression for apparent frequency in Doppler's effect .
The source and the observer approach each other before the trains cross each other .
(iii) 608 Hz
(iv) 410 Hz