Question

A police car with a siren of frequency $8kHz$ is moving with a uniform velocity $36kmh^{- 1}$ towards a tall building which reflects the sound waves. The speed of sound in air is $320ms^{- 1}$ . The frequency of the siren heard by the car driver is

• A. $8.5kHz$
• B. $8.25kHz$
• C. $7.25kHz$
• D. $7.5kHz$

Answer 1

Answer: (A)

$36 \, kmh^{- 1}$ $=36\times \frac{5}{18}$
$=10ms^{- 1}$

Apart frequency of sound heard by car driver (observer)
$f^{′ ′}=f\left(\frac{v + v_{o}}{v - v_{s}}\right)$
$=8\left(\frac{320 + 10}{320 - 10}\right)$
$f^{′ ′}=8.5 \, kHz$