# Combining Rotational Inertias

https://scontent-syd1-1.xx.fbcdn.net/hphotos-xfl1/v/t34.0-12/13015003_664975026975127_933044449_n.png?oh=0259712996d9a6dde812f6d8886f56b9&oe=5715CA95
So, it's straight forward enough to calculate the moment of inertia of basic planes with evenly distributed mass, but if I were to say glue the red and green plane together, how would I find the moment of inertia of the blue shape?

## Answers and Replies

Doc Al
Mentor
how would I find the moment of inertia of the blue shape?
About what axis?

You can find the moment of inertia of each piece about the axis of choice, then just add them. (You'll need the parallel axis theorem.)

About what axis?

You can find the moment of inertia of each piece about the axis of choice, then just add them. (You'll need the parallel axis theorem.)

probably should have specified, the z axis, or the axis perpendicular to the plane. the axis of choice is the blue COM, and I dont know how to find the moment of inertia of each shape except with central axes (I dont know much in the world of rotational inertia)

actually you know what, through more googling and researching stuffs, I've figured it out. I've been trying to find somewhere to explain this to me for a while now, but I didn't know exactly what to search for until your reply, so thanks for that 