Homework Help: Rotational Inertia Four point masses problem-diagonal

1. Nov 29, 2011

en bloc

Rotational Inertia "Four point masses problem--diagonal"

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

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

2. Relevant equations

I=mr^2

3. The attempt at a solution

I have the solution but just don't understand it! I cant 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?

2. Nov 29, 2011

grzz

Re: Rotational Inertia "Four point masses problem--diagonal"

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

3. Nov 29, 2011

grzz

Re: Rotational Inertia "Four point masses problem--diagonal"

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