# Two rods

1. Nov 29, 2011

### schaafde

I am currently trying to find the center of mass of two rods. One rod is 20 2/3 oz and is 17 inches long with a diameter of 3. The other rod is 10 1/3 oz and 17 inches long with a diameter of 1 1/2 The bigger rod is touching the smaller one if they were both standing up. It looks almost like a bottle. Would I just take the center of mass of each rod and average them together with the bigger rod accounting for more or is that too simple?

2. Nov 29, 2011

### Staff: Mentor

That's all there is to it.

3. Nov 29, 2011

### schaafde

Would that be the same for finding the moment of inertia for that system? just find the moment of inertia for each individual rod and average them together?

4. Nov 29, 2011

### Staff: Mentor

No, you wouldn't average them. You'd add them. (Make sure you have the moment of inertia of each about the same axis.)

5. Nov 29, 2011

### schaafde

So i calculated the center of mass to be 2/3 of the way up the "object" which would make it in the bigger rod? Would I be correct in assuming that? I am going off the fact that a uniform rod would have its center of mass directly in the middle.

6. Nov 30, 2011

### Staff: Mentor

Did you assume it or calculate it? Yes, assuming the rods are uniform and laid end to end, the center of mass of the system will be within the heavier rod. Calculate the exact position.
Good.