Rotational Inertia Four point masses problem-diagonal

[en bloc]

Rotational Inertia "Four point masses problem--diagonal"

Homework Statement

Four point masses of 3.0 kg each are arranged in a square on mass-less rod. The length of a side of the square is .50 m. What is the rotational Inertia for rotation about an axis passing through masses A and C?

A B


D C

Homework Equations

I=mr^2

The Attempt at a Solution

I have the solution but just don't understand it! I can't on my own find the radius if it's in a diagonal problem like this one. For B and D, the radius is .5m * $\sqrt{}2$ / 2.

Where does the $\sqrt{}2$ / 2 come from?

 

[grzz]

Points B and D are each 0.5sin45m away from the axis.OK?

 

[grzz]

and sin45 = $\frac{\sqrt{2}}{2}$