Q. A police car with a siren of frequency 8 kHz is moving with uniform velocity 36 km/h towards a tall building which reflects the sound waves. The speed of sound in air is 320 m/s. the frequency of the siren heard by the car driver is
A) 8.5 kHz
B) 8.25 kHz
C) 7.25 kHz
D) 7.5 kHz
Explanation
(a) $36\ $km/h$= 36 \times \frac{5}{18}$
=10m/s
[image:15_image76.emf]
Apart frequency of sound heard by car driver (observer)
$f^{''} = f\left( \frac{v + v_{o}}{v - v_{s}} \right)$
$= 8\left( \frac{320 + 10}{320 - 10} \right)$
$f^{''} = 8.5\ kHz$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.