Question

A train moves towards a stationary observer with a speed 34 ms${}^{-1}$. The train sounds a whistle and its frequency registered by the observer is f${}_1$. If the trains speed is reduced to 17 ms${}^{-1}$, then the frequency registered is f${}_2$. If the speed of the sound is 340 ms${}^{-1}$, then the ratio f${}_1$ / f${}_2$ is:

• A. 18/19
• B. 1/2
• C. 2
• D. 19/18

Answer 1

The correct option is D 19/18

A train moves towards a stationary observer with a speed $V_{s}m/s$. The train sounds a whistle and its frequency registered by the observer is $f_1=n\frac{V}{V-V_s}$. So for two different velocity the ratio of apparent frequencies will be:

$f_1/f_2=\frac{V-V_{s2}}{V-V_{s1}}$
Putting corresponding values in above equation will get,
$f_1/f_2=\frac{340-17}{340-34}=\frac{19}{18}$.