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 ms^(-1)`
B. `70 ms^(-1)`
C. `105 ms^(-1)`
D. `140 ms^(-1)`

Answer

B

Explanation

Let b be the speed of the engine relative to observe at rest. Velocity of sound, `v=350` m/s
Let `n'=` observed frequency, `n=` original frequency, we have
`(n')/n=5/6` or `(n')/n=v/(v+b)`
`5/6=v/(v+b)`
`5b=v=350` m/s
`b=350/5=70` m/s