Take a ring with radius r and width dr as infinitesimal and infinitesimal mass dm= (m/πR? )2πrdr

Friction on infinitesimal elements f =-μ GDM =-(2 μ g/r? )rdr

Friction torque DM = fr =-(2 μ g/r? )r？ Doctor; doctor

Therefore, the friction torque on the disk m =-(2 μ g/r? )∫r？ Doctor; doctor

Substituting the upper integral limit r and the lower integral limit 0, we can get m =-2 μ gr/3.

So the angular acceleration of the disk β = m/j =-(2 μ gr/3)/(Mr? /2) =-4 μ g /3R