Sol. The liquid in a rotating vessel also takes part in the rotation. Since outside forces are not acting on the system of vessel and liquid, it follows that the moment of forces equals zero. Therefore the moment of momentum of the system, which is equal to the product of the moment of inertia and the angular velocity, must remain unchanged. When the cork at A is opened, the water will begin to pour out, as a result of which the system's moment of inertia will decrease and therefore the vessel's angular velocity will increase. From the moment when the water ceases to pour out of the vessel from the opening, the speed of rotation will become constant.