# Homework Help: Moment of inertia

1. Feb 2, 2008

### supercali

[SOLVED] moment of inertia

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

what is the moment of interia of these 2 rods (picture attached) about their center of mass
each rod mass is m with length L

2. Relevant equations
one rod i know is $$\frac{L^2m}{12}$$

3. The attempt at a solution

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2. Feb 2, 2008

### Staff: Mentor

First things first: Find the center of mass.

3. Feb 2, 2008

### supercali

good question

i thought the answer is $$\frac{L^2m}{12}*2+\frac{L^2m}{4}*2$$
obviously it isnt

i think center of mass is at $$\frac{L}{4}\hat{x}-\frac{L}{4}\hat{y}$$

Last edited: Feb 2, 2008
4. Feb 2, 2008

### Staff: Mentor

Good--it's right in the middle between the two rod centers. So what's the distance from the center of each rod to that center of mass? To find the rotational inertia of the rods about that center of mass, use the parallel axis theorem.

5. Feb 2, 2008

### supercali

yeah i got it it is $$\frac{L^2m}{12}*2+\frac{L^2m}{8}*2$$ and one more thing if it was a square or rods i would have to multiply by 4?

6. Feb 2, 2008

### Staff: Mentor

Good.
If it were a square of rods, the distance from each rod to the center of mass would be different. (But there would be a factor of 4 involved.)