What is the moment of inertia of a hoop of mass M and radius R rotated about a cylinder axis, as shown below?
Fortunately, we do not need to use calculus to solve this problem. Notice that all the mass is the same distance R from the axis of rotation. Thus we do not need to integrate over a range, but can calculate the total moment of inertia. Each small element dm has a rotational inertia of R2dm, where r is constant. Summing over all elements, we see that I = R2dm = R2M. The sum of all the small elements of mass is simply the total mass. This value for I of MR2 agrees with experiment, and is the accepted value for a hoop.
What is the rotational inertia of a solid cylinder with length L and radius R, rotated about its central axis, as shown below?
To solve this problem we split the cylinder into small hoops of mass dm, and width dr: This small element of mass has a volume of (2Πr)(L)(dr), where dr is the width of the hoop. Thus the mass of this element can be expressed in terms of volume and density:
Thus the rotational inertia of a cylinder is simply . Once again, it has the form of kMR2, where k is some constant less than one.