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

Explanation

(i) Here, `v = 400 Hz, v = 340 m//s `
(a) Train approaches the platform
`v _(s) 10 m//s `
`v^(1)= ( v)/( v-v_(s)) = ( 340 xx 400)/( 340- 10) = 412.12 Hz`
(b) Train receds from the platform
`v_(s) = 10 m //s `
`v^(1)= ( v)/( v-v_(s)) = ( 340 xx 400)/( 340+ 10) `
`= 388.6 Hz`
(ii) The speed of sound in each case is the same =` 340 m //s `