JEE PYQ: Waves And Sound Question 41
Question 41 - 2019 (10 Jan Shift 1)
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 speed of the train is reduced to 17 m/s, the frequency registered is $f_2$. If speed of sound is 340 m/s, then the ratio $f_1/f_2$ is:
(1) 18/17
(2) 19/18
(3) 20/19
(4) 21/20
Show Answer
Answer: (2)
Solution
According to Doppler’s effect, when source is moving but observer at rest
$f_{\text{app}} = f_0\left[\frac{V}{V - V_s}\right] \Rightarrow f_1 = f_0\left[\frac{340}{340 - 34}\right]$
and, $f_2 = f_0\left[\frac{340}{340 - 17}\right]$
$\therefore \frac{f_1}{f_2} = \frac{340 - 17}{340 - 34} = \frac{323}{306} = \frac{19}{18}$
BETA
