A police car with a siren of frequency `8 KHz` is moving with uniform velocity `36 Km//hr` towards a ball 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

Options

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

Answer

A

Explanation

Let the speed of sound is `upsilon` and car is approaching the wall with speed u.
`upsilon` = 320 m/s
u = 36 km/hr = 10 m/s
For reflected sound it will be an equivalent case of source and observer moving towards each other.
`f = f_(0) ((upsilon + u)/(upsilon - u)) = (8) ((320 + 10)/(320 - 10)) kHz = 8.52` kHz
Hence option (a) is correct.