Consider two disks (not friction-less) with some moment of inertia ($I_1$ and $I_2$). Both of them are given angular velocities ($\omega_1$ and $\omega_2$) both in same sense.
Now if we bring both disks in contact after some time they will have common angular velocity. Now my text says that the new angular velocity ($\omega$) is given by the equation $I_1\omega_1+I_2\omega_2=I\omega$
But how can angular momentum be conserved in this case? Isn't friction applying torque?
And if the explanation contains that friction is applying internal torque then please explain.