Question

A train moves towards a stationary observer with speed 34 m/s. The train sounds a whistle and its frequency registered is f1. If the train’s speed is reduced to 17 m/s, the frequency registered is $f_2$. If the speed of sound is 340 m/s then the ratio $\frac{f_1}{f_2}$ is

• A. $\frac{18}{19}$
• B. $\frac{1}{2}$
• C. $2$
• D. $\frac{19}{18}$

Answer 1

The correct option is D

$\frac{19}{18}$

Let $f_0$ be natural frequency of source, then

$f_1=f_0\left[\frac{v}{v-34}\right],f_2=f_0\left[\frac{v}{v-17}\right]$

$\Rightarrow \frac{f_1}{f_2}=\frac{19}{18}$