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. 8.25 $kHz$
• C. 7.75 $kHz$
• D. 7.50 $kHz$

Answer 1

The correct option is A 8.50 $kHz$

Frequency of siren is given by,
$f=\frac{320}{320-10}\times 8\times {10}^3\times \frac{320+10}{320}$

$f=8.5kHz$