# Homework Help: Moment of inertia tensor

1. Nov 30, 2006

### Logarythmic

1. The problem statement, all variables and given/known data
Compute the moment of inertia tensor I with respect to the origin for a cuboid of constant mass density whose edges (of lengths a, b, c) are along the x,y,z-axes, with one corner at the origin.

3. The attempt at a solution
I get

$$I = M \left( \begin{array}{ccc} \frac{1}{3} (b^2+c^2) & -\frac{ab}{4} & -\frac{ac}{4}\\ -\frac{ab}{4} & \frac{1}{3} (a^2 + c^2) & -\frac{bc}{4}\\ -\frac{ac}{4} & -\frac{bc}{4} & \frac{1}{3} (a^2 + b^2) \end{array} \right)$$

Can this be right?

2. Nov 30, 2006

I think it may be right, since the matrix is supposed to be symmetric.

3. Nov 30, 2006

### Logarythmic

Yeah, but it can be symmetric in many ways. ;)

Can someone please explain the equality

$$\int_V \rho(\vec{r}) (r^2 \delta_{jk} - x_jx_k) dV = \int_V \rho(x,y,z) \left( \begin{array}{ccc} y^2+z^2 & -xy & -xz\\ -xy & z^2+x^2 & -yz\\ -xz & -yz & x^2+y^2 \end{array} \right)dxdydz$$

for me? I think this is the most important step in my understanding for this problem.