Solution:
To identify the change in gravitational energy, think of the height through which the centre of mass falls. From the parallel-axis theorem, the moment of inertia of the disk about the pivot point on the circumference is
The pivot point is fixed, so the kinetic energy is entirely rotational around the pivot. The equation for the isolated system (energy) model
for the disk-Earth system becomes
Solving for
At the lowest point on the rim,