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. 8.25 kHz
• B. 7.75 kHz
• C. 8.50 kHz
• D. 7.50 kHz

Answer 1

The correct option is C 8.50 kHz

$f=f_0\left[\frac{v}{v-u}\right]\left[\frac{v+u}{v}\right]$

$=8\times \left[\frac{320+10}{320-10}\right]$

$=8\times \frac{33}{21}$

$=8.5kHz$