# HW help

#### Mo0o

##### Lifer
I need help with my physics hw. I have a rod and a disc and I want to find the moment of intertia of the two things together. I know the moment of intertia of the disc is (1/2)MR^2 and the moment of intertia of the rod is (1/12)ML^2 . How do i find the moment of intertia of the two things stacked on top of each other?

#### Heisenberg

##### Lifer
If the axes are right on top of each other, you can just add them directly. Otherwise, if the axis are parallel but not on top of each other, use the parallel axis theorem.

#### TuxDave

##### Lifer
Originally posted by: Mo0o
I need help with my physics hw. I have a rod and a disc and I want to find the moment of intertia of the two things together. I know the moment of intertia of the disc is (1/2)MR^2 and the moment of intertia of the rod is (1/12)ML^2 . How do i find the moment of intertia of the two things stacked on top of each other?

Add them together? That's what I think, you may as well wait for second opinions.

#### esun

##### Platinum Member
That's right, you just add them.

#### chuckywang

##### Lifer
it depends on where the rod and disk are situated with respect to each other. Listen to Heisenberg (with a name like that, you can't go wrong). He knows what's up.

#### TuxDave

##### Lifer
Originally posted by: Heisenberg
If the axes are right on top of each other, you can just add them directly. Otherwise, if the axis are parallel but not on top of each other, use the parallel axis theorem.

Yeah, do that parallel axis theorem if they're any of them are rotating around a different point than the formulas are for.