Question

A train, standing at the outer signal of a railway station blows a whistle of frequency 400 Hz 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 10 m s^-1, (b) recedes from the platform with a speed of 10 ms^-1? (ii) What is the speed of sound in each case? The speed of sound in still air can be taken as 340 m s^-1.

Answer 1

Here, the frequency of whistle

v = 400 Hz

Speed of sound v = 340 ms^-1

Speed of train, v_s = 10 ms^-1

(i) (a) When the train approaches the platform, i.e., the observer at rest.

Then,

(b) When the train approaches the platform i.e., from the observer at rest;

(ii) Speed of the sound does not change, therefore, it is again 340 ms^-1.