Two discs of moment of inertia $I_1=4 \mathrm{~kg} \mathrm{~m}^2$ and $\mathrm{I}_2=2 \mathrm{~kg} \mathrm{~m}^2$ about their central axes \& normal to their planes, rotating with angular speeds $10 \mathrm{rad} / \mathrm{s} \& 4 \mathrm{rad} / \mathrm{s}$ respectively are brought into contact face to face with their axe of rotation coincident. The loss in kinetic energy of the system in the process is J .