A stream-lined body falls through air from a height h on the surface of a liquid. Let d and D denote the densities of the material of the body and the liquid respectively. If D > d, then the time after which the body will be instantaneously at rest is:

Velocity u of the body when it enters the liquid is given by mgh = 12mu2  or u = 2ghLet  Volume of the body = V        Mass of the body = Vd        Weight of the body = Vdg       Mass of liquid displaced = VD        Net upward force = VDg-VDg                                      = Vg (D-d)         Retardation = net weightmass                           = V(D-d)gVd = (D-dd)gAcceleration a = -(D-dd)gFinal velocity, v in the liquid when the body is instantaneosuly  at rest is zero. Let the time taken be t.         v = u+at         0 = 2gh-(D-dd)gt.(D-dd)gt = 2gh          t = [dD-d]2hg