Two discs of moment of inertia \( I_1 = 4 \,\mathrm{kg\,m^2} \) and \( I_2 = 2 \,\mathrm{kg\,m^2} \), about their central axes & normal to their planes, rotating with angular speeds \( 10 \,\mathrm{rad/s} \) & \( 4 \,\mathrm{rad/s} \) respectively are brought into contact face to face with their axes of rotation coincident. The loss in kinetic energy of the system in the process is ______ J.