Q. The apparent frequency of the whistle of an engine changes in the ratio 9:8 as the engine passes a stationary observer. If the velocity of the sound is 340$ms^{- 1}$ , then the velocity of the engine is
A) $40\ ms^{- 1}$
B) $20\ ms^{- 1}$
C) $340\ ms^{- 1}$
D) $180\ ms^{- 1}$
Explanation
(a) From doppler’s effect, perceived frequency
$v^{'} = v\left( \frac{v - v_{o}}{v - v_{s}} \right)$
$\frac{9}{8} = \frac{340}{340 - v_{s}}$
$\Longrightarrow 9\left( 340 - v_{s} \right) = 8 \times 340$
$\Longrightarrow v_{s} = 37.7ms^{- 1} = 40ms^{- 1}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.