Q. A police car horn emits a sound at a frequency 240 Hz the frequency 240 Hz when the car is at rest. If the speed of sound is $330ms^{- 1}$, the frequency heard by an observer who is approaching the car at speed of 11 $ms^{- 1}$, is
A) 248 Hz
B) 244 Hz
C) 240 Hz
D) 230 Hz
Explanation
(a) From Doppler’s effect in sound, the perceived frequency (v’) is given by
$v^{'} = \left( \frac{v + v_{o}}{v} \right)v$
Where $v_{o}$is velocity of observer, v of sound and v the original frequency.
Give, v=240 Hz, $v = 300ms^{- 1},\ \ v_{o} = 11ms^{- 1}$
$v^{'} = 240\left( \frac{330 + 11}{330} \right)$
$v^{'} \approx 248Hz$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.