A system rotating with angular velocity ω has total angular momentum equal to its moment of inertia times the angular velocity. The change of angular momentum with time gives the net torque acting on the system. If the net torque acting on the system is zero, then the system's angular momentum is conserved. The rotational kinetic energy of the system is expressed as half of the moment of inertia multiplied by the square of the angular velocity. The rotational kinetic energy can also be expressed in terms of the angular momentum by substituting the product of the moment of inertia and the angular velocity as the system's total angular momentum. Consider a merry-go-round in the park, rotating with an initial angular velocity and moment of inertia, respectively. Now, if a box is placed vertically on the merry-go-round, its moment of inertia is doubled. To keep the angular momentum conserved, its final angular velocity decreases by one-half of the initial angular velocity.