A police car with a siren of frequency 8 kHz is moving with uniform velocity 36 km/hr 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

Question

A police car with a siren of frequency 8 kHz is moving with uniform velocity 36 km/hr 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) 8.50 kHz (B) 9.50 kHz (C) 7.75 kHz (D) 7.50 kHz

Tardigrade Admin · Accepted Answer

36 km/h = 36 × (5/18) = 10 m/s Apparent frequency of sound heard by car driver (observer) reflected from the building will be f'=f((v+v0/v-vs))=8((320+10/320-10))=8.5 kHz ∴ Correct option is (a).

Q. A police car with a siren of frequency $8 k Hz$ is moving with uniform velocity $36 km / h r$ 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

8.50 kHz

9.50 kHz

7.75 kHz

7.50 kHz

$36 km / h = 36 \times \frac{5}{18} = 10 m / s$
Apparent frequency of sound heard by car driver (observer) reflected from the building will be
$f^{'} = f (\frac{v + v _{0}}{v - v _{s}}) = 8 (\frac{320 + 10}{320 - 10}) = 8.5 k Hz$
$∴$ Correct option is (a).

Q. A police car with a siren of frequency 8kHz is moving with uniform velocity 36km/hr towards a tall building which reflects the sound waves. The speed of sound in air is 320m/s. The frequency of the siren heard by the car driver is