Q 11-14-111JEE MainJEE Main 2019 (10 Jan, Shift 1)Easy
A train moves towards a stationary observer with speed $34\ \text{m/s}$. The train sounds a whistle and its frequency registered by the observer is $f_1$. If the speed of the train is reduced to $17\ \text{m/s}$, the frequency registered is $f_2$. If speed of sound is $340\ \text{m/s}$, then the ratio $f_1/f_2$ is:
Answer: (C) $19/18$
For a source approaching a stationary observer, $f' = f\dfrac{v}{v - v_s}$:
$$\frac{f_1}{f_2} = \frac{340-17}{340-34} = \frac{323}{306} = \frac{19}{18}$$
Solution by Sreeraj P, M.Sc Physics