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. (a) `8.50 KHz`
B. (b) `8.25 KHz`
C. ( c ) `7.75 KHz`
D. (d) `7.50 KHz`

Answer

A

Explanation

(a) `f' = f[(v+v_(0))/(v-v_(s))]`
Here `v = 320m//s` (given)
`v_(0) = v_(s) = 36 xx (5)/(18) = 10m//s`
f' = 8[(320+10)/(320-10)] = 8 xx (33)/(31)~~8.5 kHz`