# Moments of Inertia

1. Apr 10, 2007

### Trojanof01

So I have a point A on a graph with x and y axes. From the origin, the point is 3r to the right, and 1r up. I'm to find the Moment of Inertia along the x, y, and z axes. I've found the x and y moments, but not the z and I'm not really sure on how. I'm not getting any ideas from the graph alone. How do I start?

2. Apr 10, 2007

### Trojanof01

Bump, anybody?

3. Apr 11, 2007

### andrevdh

The z-axis points up out of the page. The moment of inertia of the point mass about the z-axis will be

$$I_z = md^2$$

where d is the shortest distance of the point mass from the z-axis. Since the point mass is on the x-y plane d will be the distance that the point mass is from the origin. That is it is the hypotenuse of the right triangle formed by its x and y- components.