We've now seen quite a few examples of the moment of inertia for
a lot of different objects. But notice we have to specify the axis
of rotation. If we change the axis, in general so will the moment
of inertia. It'd be nice if we didn't have to recompute the moment
of inertia every time we chose a different axis. For example,
a disk has a moment of inertia of 
through an axis
going through the center and perpendicular to the disk. What if
the axis was still perpendicular but didn't go through the center?
What if the axis went throught the center but was in the plane of
the disk? How would you figure out these cases?
Fortunately we can do these problems without much extra effort thanks to two cool theorems, the perpendicular axis theorem and the parallel axis theorem.