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. 3.6 m/s
B. 4.5 m/s
C. 6.0 m/s
D. 12.0 m/s

Answer

B

Explanation

(b) Fish is observer and bird is object.

Apparent distance between F and B at some instant will be,
y=(x+`mu`h)
`(-(dy)/(dt))=(-(dx)/(dt))+(mu)(-(dh)/(dt))`
`9=3+4/3(-(dh)/(dt))`
`therefore -(dh)/(dt)=4.5m/s`