Question

A train moves towards a stationary observer with speed 34 m/s. The train sounds a whistle and its frequency registered by the observer is $f_1$. 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

$\frac{19}{18}$

$f_1=f(\frac{v}{v-v_s})$

$f_1=f(\frac{340}{340-34})=f(\frac{340}{306})$

and $f_2=f(\frac{340}{340-17})=f(\frac{340}{323})$

$\therefore \frac{f_1}{f_2}=(\frac{323}{306})=\frac{19}{18}$