What is the moment of inertia of a hoop of mass M and radius R rotated about a cylinder axis, as shown below?
A hoop of radius R
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 R 2 dm , where r is constant. Summing over all elements, we see that I = R 2 dm = R 2 M . The sum of all the small elements of mass is simply the total mass. This value for I of MR 2agrees 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?
A cylinder being rotated about its axis
To solve this problem we split the cylinder into small hoops of mass dm , and width dr :
A cylinder being rotated about its axis, shown with a small element of mass from the cylinder
dm = ρV = ρ(2ΠrLdr)
ρ = =
dm = = 2rdr
|I||=||r 2 dm|
|=||2r 3 dr|
|=||[r 4/2]0 R|
Thus the rotational inertia of a cylinder is simply . Once again, it has the form of kMR 2 , where k is some constant less than one.