# Homework Help: Moment of Inertia through center of mass

1. Jul 10, 2013

1. The problem statement, all variables and given/known data
A rod is 10 ft long and has a density that goes from 4 to 24 lb/ft.

a) Find the mass.
b) Find the center of mass.
c) Find the moment of inertia through the center of mass.

2. Relevant equations
I = Ml^2
I = I(cm) + Md^2 (parallel axis theorem)

3. The attempt at a solution
So I got figured out the solution to this problem.

a) M = 140
b) x(cm) = 6.19
c) I(cm) = 6.92M

My question revolves around part c. The way I found it was to use the parallel axis theorem. I found the moment of inertia at one end of the rod. Then I subtracted M*d^2 where d= 6.19 (the distance to the center of mass. This gave me the correct answer.

However, I was trying for some time to just get the moment of inertia about the center of mass directly from the definition of moment of inertia, but I couldn't get the integration limits right.

If I have my axis at the center of mass, I don't know how to setup the integral properly. Or is it that you have to use the parallel axis theorem?

Any help would be greatly appreciated.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jul 10, 2013

### SteamKing

Staff Emeritus
1. You are given units in the OP. Your answers should also reflect the proper units.

2. Your description of the calculation of the moment of inertia is not clear.

3. One way to calculate the moment of inertia is to set up the inertia calculation using one end of the rod as the initial reference point for both the first and second moments of mass. Once these moments are determined about the endpoint, then the PAT can be used to find the moment of inertia about the COM. There should be no confusion about the limits of integration in this case.

3. Jul 10, 2013

### Staff: Mentor

You can still integrate from 0 to 10, just take the squared distance to the center of mass instead of the position as integrand (multiplied by the density of course).
You don't have to.

4. Jul 10, 2013

1. In regards to the units, that's my bad. In haste, I wrote the answers without units.

M = 140 lb
x(cm) = 6.19 ft.

2. I found my error when calculating the moment of inertia about the center of mass. Just as mfb stated, the l^2 term will have to be the distance from the center of mass point, which is (6.19 - l)^2. This would be, of course, multiplied by the density and then integrated from 0 to 10 for the correct answer.

Thanks so much!