A pulley consists of a large disk of radius R and a small disk of radius r that are welded together and mounted on a horizontal frictionless axle through their common centers. The moment of inertia of this pulley is I. A box containing a turkey of mass M is suspended from a rope wound around the large disk; a box containing a pumpkin of mass m is suspended from a rope wound around the small disk. The ropes do not slip on the disks. The system is released from rest and the turkey begins to descend, while the pumpkin is being lifted up.

Required:
Derive an expression for the magnitude of the angular acceleration a of the pulley, in terms of system parameters.