# Calculating the Moment of Inertia

1. Apr 3, 2010

### erok81

1. The problem statement, all variables and given/known data

The four masses shown in the attachment are connectec by massless, rigid rods.

a) find the coordinates of the center of mass.
b) find the moment of inertia about a diagonal axis that passes through mass B and D.

2. Relevant equations

I=MR^2...I think.

3. The attempt at a solution

I've solved part a which ended up being 0.0517m, 0.0517m.

Part b is where I am stumped.

I've tried $$I=M_{A}R^{2}_{A}+M_{B}R^{2}_{B}+M_{C}R^{2}_{C}+M_{D}R^{2}_{D}$$ using both the radius of the rods as well as the radius of the center of mass, but neither works.

I know I am doing this wrong and it's probably a conceptual thing, but I can't see it.

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2. Apr 3, 2010

### Filip Larsen

The R's in your equation should be the distance from the rotation axis to the mass in question.