The Pitch of the whistle of an engine appears to drop to `5/6` th of original value when it passes a stationary observer if the speed of sound in air is 350 m/s then the speed of engine is

Options

A. 35 m/s
B. 70 m/s
C. 105 m/s
D. 140 m/s

Answer

B

Explanation

In this case, the observer is at rest and since the frequency is decreased, the source is moving away from the observer
We use, `n_(a) = ((v)/(v + v_(s)))n`
In this case, `n_(a) = (5)/(6)n and v = 350 m//s`
`:. (5)/(6) = (350)/(350 + v_(s))`
`:. 350 xx 6 = 350 xx 5 + 5v_(s)`
`:. 350 = 5v_(s) " " :. v_(s) = (350)/(5) = 70 m//s`