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

Effective height of the bird as seen by the fish, Y = y + μ y'          dYdt=dydt+μdy'dt           9=+3+43dy'dt or       dy'dt=6×34=184=92=4.5 m s-1Therefore, actual velocity of bird = 4.5 m s-1