A train, standing at the outer signal of a railway station blows a whistle of frequency 400Hz in still air. (i) What is the frequency of the whistle for a platform observer when the train (a) approaches the platform with a speed of `10ms^(-1)` , (b) receds fromk the platform with a speed of `10ms^(-1)`? (ii) What is the speed of sound in each case ? The speed of sound in still air can be taken as `340ms^(-1)`

Explanation

Here, `v=400Hz, upsilon=340ms^(-1)`
(a)Train (source) approaches the platform, `upsilon_(s)=10m//s :. v^(')=(upsilon)/(upsilon-upsilon_(s))xxv=(340xx400)/(340-10)=412.12Hz`
(b) Train recedes from the platform , `upsilon_(s)=10m//s :.v^(')=(upsilonxxupsilon)/(upsilon+upsilon_(s))=(340xx400)/(340+10)=388.6Hz`
(ii) The sped of sound in each case in the same `=340ms^(-1)`