Q. A car moving with a velocity of $36\ km^{- 1}$ crosses a siren of frequency 500 Hz. The apparent frequency of siren after passing it will be
A) 520 Hz
B) 485 Hz
C) 540 Hz
D) 460 Hz
Explanation
(b) Given that, velocity of the car
$= \frac{36000m}{3600s} = 10ms^{- 1}$
$v_{emitted} = 500\ Hz\ $
$v_{sound} = 330\ ms^{- 1}$
We know that
$= v_{emitted} \times \frac{\left( v_{sound} - v_{obs} \right)}{c_{sound}}$
Hence, observed frequency $v_{obs} = 500 \times \frac{330 - 10}{330}$
$v_{obs} = 485Hz$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.