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

Question

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

TestHub · Accepted Answer

Apparent frequency of sound heard by car driver (observer) reflected from the building will be $f^{'} = f (\frac{v + v _{0}}{v - v _{s}}) = 8 (\frac{320 + 10}{320 - 10}) = 8.5 kHz$ $∴$ Correct option is (a). Analysis of Question (i) Question is simple. (ii) Driver will listen two sounds, direct and reflected. Direct sound will be of $8 kHz$ as driver has no relative motion with the car. But reflected sound is of increased frequency because driver and image of car both are approaching towards each other.

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