A circular disc of moment of inertia ${\mathrm{I}}_{\mathrm{i}}$ is rotating in a horizontal plane, about its geometrical axis, with a constant angular speed ${\mathrm{\omega }}_{\mathrm{i}}$. Another disc of moment of inertia ${\mathrm{I}}_f$ is dropped coaxially onto the rotating disc. Initially the second disc has zero angular speed. Eventually both the discs rotate with a constant angular speed ${\mathrm{\omega }}_{\mathrm{f}}$. The energy lost by the initially rotating disc due to friction is: