A fish rising up vertically toward the surface of water with speed `3ms^(-1)` observes a bird diving down vertically towards it with speed `9ms^(-1)` . The actual velocity of bird is

Options

A. `4.5ms^(-1)`
B. `5.4ms^(-1)`
C. `3.0ms^(-1)`
D. `3.4ms^(-1)`

Answer

A

Explanation

Effective height of the bird as seen by the fish,
`Y=y+muy^(')`
`(dY)/(dt)=(dy)/(dt)+mu(dy^('))/(dt)`
`9=3+(4)/(3)(dy^('))/(dt)`
or `(dy^('))/(dt)=(6xx3)/(4)=(18)/(4)=(9)/(2)=4.5ms^(-1)`
Therefore, actual velocity of bird `=4.5ms^(-1)`