Find Acceleration of Mass Hanging From String Wrapped Around Solid Disk

In this problem, worlds collide... The mass will fall in a straight line like any other hanging mass and will therefore need ΣF=ma to calculate its acceleration. The wheel is in its own world of rotations and moments of inertia, so its world is governed by ΣΤ = Iα. How can these two worlds unite to allow us to solve for acceleration?