A train standing at the outer singal of a railway station blows a whistle of frequency 400 Hz in still air. (i) What is frequency of the whistle for a platform observer when the train (a) approaches the platform with a speed of 10 m/s, (b) recedes from the platform with a speed of 10 m/s. (ii) What is the speed of sound in each case if the speed of sound in still air is 340 m/s.

Explanation

(i) Here, n= 400 Hz, `v = 340 m//s`
(a) When train approaches the platform,
`v_(s) = 10 m//s`
So, required frequency
`=v/(v-v_(s))xxn=(340xx400)/(340-10)=412.12" Hz"`
(b) When train recedes from the platfrom.
`v_(s) = 10 m//s`
Hence, required frequency
`=v/(v-v_(s))xxn=(340xx400)/(340+10)= 388.6" Hz"`
(ii) The speed of sound in both the cases is same.