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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

Answer:

Explanation:

$36 km/h^{-1}=36 \times \frac{5}{18} =10 ms ^{-1}$

Apparent frequency of sound heard by car driver (observer) reflected from the
building will be

 $f'= f\left(\frac{v+v_{0}}{v-v_{s}}\right)$

 $= 8\left(\frac{320+10}{320-10}\right)$

 =8.5 kHz

 $\therefore$  Correct option is (a) 

Analysis of question

 (i) Question is simple 

(ii) Driver will listen to two sounds, direct and reflected. Direct sound will be of 8 kHz as the driver has no relative motion with the car. But the reflected sound is of increased frequency because driver and image of car both are approaching each other