Q. A train moves towards a stationary observer with speed $34ms^{- 1}$. The train sounds a whistle and its frequency registered by the observer is $f_{1}$. If the train’s speed is reduced to $17ms^{- 1}$, the frequency registered is $f_{2}$. If the peed of sound is $340ms^{- 1}$, then the ratio $f_{1}/f_{2}$ is
A) $\frac{18}{19}$
B) $\frac{1}{2}$
C) $2$
D) $\frac{19}{18}$
Explanation
(d) Here, $v_{s_{1}} = 34ms^{- 1}$,
$v = 340ms^{- 1}$
$f_{1} = \frac{v \times n}{v - v_{s_{1}}} = \frac{340 \times n}{340 - 34} = \frac{340}{306}n$
$f_{2} = \frac{v \times n}{v - v_{s_{2}}} = \frac{340 \times n}{(340 - 17)} = \frac{340n}{323}$
$\frac{f_{1}}{f_{2}} = \frac{323}{306} = \frac{19}{18}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.