A pulley of diameter 4 is free to rotate about its axis which is horizontal. A light strings over the pulley and carries a particle P of mass 2 at one end and a particle B of mass ( + 1), ( < 1) at the other end. The string is rough enough that to slip on the pulley. When the system is released from rest, a constant frictional torque acts on the pulley. Find the expression for the angular acceleration of the system in terms of , , and the moment of inertia (I) of the pulley.