A rotating flywheel has moment of inertia $12\mathrm{.}0kg{m}^2$ for an axis along the axle about which the wheel is rotating. Initially the flywheel has $30\mathrm{.}0J$ of kinetic energy. It is slowing down with an angular acceleration of magnitude $0\mathrm{.}500rev{s}^{-2}.$ Find how long it takes for the rotational kinetic energy to become half its initial value.