Q 11-14-081JEE MainJEE Main 2022 (28 Jul, Shift 1)Medium
The frequency of echo will be ______ Hz if the train blowing a whistle of frequency $320$ Hz is moving with a velocity of $36\ \text{km h}^{-1}$ towards a hill from which an echo is heard by the train driver. Velocity of sound in air is $330\ \text{m s}^{-1}$.
Numerical value type. Enter your answer.
Answer: 340
$v_s = 36\ \text{km h}^{-1} = 10\ \text{m s}^{-1}$. The hill reflects the sound it receives, and the driver moves towards the reflected sound:
$$f' = f\,\frac{v + v_s}{v - v_s} = 320\times\frac{340}{320} = 340\ \text{Hz}$$
Solution by Sreeraj P, M.Sc Physics