a rotating disc of mass 'm', radius 'r', thickness 'D' can rotate in vacuum with constant angular velocity 'w' . now the disc is placed in water of density 'p', find the time in which its angular velocity becomes half.

Answer 1

Without friction, the angular velocity will not change. In real fife of course it will. Even in the vacuum there is loss of kinetic energy to the bearings. However as far as the information given here, the answer is constant angular velocity forever. or at least for a very long time like the rotation of the earth.