A stream-lined falls through air from a height h  on the surface of a liquid. Let d  and D  denote the densities of the materials 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=2gh Let volume of the body=V Mass of the body=Vd Weight of the body=Vdg Mass of liquid displaced=VD Weight of liquid displaced=VDg Net upward force=VDg−Vdg =Vg(D−d) Retardation = net weightmass =V(D−d)gVd=(D−dd)g Acceleration a=−(D−dd)g Final velocity, v  in the liquid when the body is instantaneously at rest is zero. let the time taken be t .Now v=u+at 0=2gh−(D−dd)gt , (D−dd)gt=2gh ∴  t=[dD−d]2hg