A train moves towards a stationary observer with speed $34\text{ ms}^{-1}$. The train sounds a whistle and…
A train moves towards a stationary observer with speed $34\text{ ms}^{-1}$. The train sounds a whistle and its frequency registered by the observer is $f_1$. If the train's speed is reduced to $17\text{ ms}^{-1}$, the frequency registered is $f_2$. If the speed of sound is $340\text{ ms}^{-1}$, then ratio $\frac{f_1}{f_2}$ is
$\frac{18}{19}$
$\frac{17}{18}$
$\frac{18}{17}$
$\frac{19}{18}$
Solution
Frequency heard by observer, $f_1 = f \left(\frac{340}{340 - 34}\right)$
and $f_2 = f \left(\frac{340}{340 - 17}\right)$
$\\therefore \frac{f_1}{f_2} = \frac{323}{306} = \frac{19}{18}$