A rotating flywheel has moment of inertia 12.0 kg. m^2 for an axis along the axle about which the wheel is rotating. Initially the fly-wheel has 30.0 J of kinetic energy. It is slowing down with an angular acceleration of magnitude 0.500 rev/s^2. How long does it take for the rotational kinetic energy to become half its initial value, so it is 15.0 J