Moment of Inertia
Objects that have most of their mass in the center have smaller moments of inertia than objects that have most of their mass on the outside.
Using the calculus, we can replace the summation of small mass pieces with an integral: I = S miri2 becomes I = ?r2 dm.
Each small dm has its own radius, or distance from the axis of rotation, so r is really a function of which dm we are using: r(m).