The apparent frequency of the whistle of an engine changes in the ratio 9 : 8 as the engine passes a stationary observer. If the velocity of the sound is 340 `ms^(-1)`, then the velocity of the engine is :

Options

A. 40 `ms^(-1)`
B. 20 `ms^(-1)`
C. 340 `ms^(-1)`
D. 180 `ms^(-1)`

Answer

A

Explanation

From Doppler.s effect, perceived frequency
`" "f.=f((v-v_(0))/(v-v_(5)))`
`" "9/8=340/(340-v_(5))`
`rArr 9(340-v_(s))=8 times 340`
`rArr" "v_(s)=37.7ms^(-1) approx 40ms^(-1)`