Moment of Inertia for Three Point Masses

[atlantic]

Homework Statement

Three particles of mass m are placed at A=(-a, -a), B=(a, -a) and C=(0, a)

Find the moment of inertia for an axis along the z-axis through the origin

Homework Equations

I = m((r_A)² + (r_B)² + (r_C)²)

The Attempt at a Solution

I calculate that:
(r_A)² = (sqrt[(a)² + (a)²])² = 2a²
(r_B)² = (sqrt[(-a)² + (a)²])² = 2a²
(r_C)² = (a)² = a²

So that:
I = m(a² + 2a² + 2a²) = 5ma²

But the answer is supposed to be 4.12ma². What am I doing wrong?

 

[Doc Al]

So that:
I = m(a² + 2a² + 2a²) = 5ma²

Looks good to me.

But the answer is supposed to be 4.12ma². What am I doing wrong?

I don't think you're doing anything wrong. What book are you using?