Homework Help: Moment of Inertia Questions

1. Oct 8, 2004

jjiimmyy101

**The problem and solution is attached as moment of inertia.**
**The equations for moment of inertia of known homogeneous solids is also attached**

I'm having some trouble grasping this concept. I hope someone can help clarify some things for me.

Looking at the problem I realized I would have to use the parallel axis theorem, but after that the trouble begins. (I = Ig + m*d^2)

"Deterimine the wheel's M of I about an axis perpendicular to the page and passing thru pt A."

What does this mean? What is it saying?

And a related question to the above: Why when calculating M of I for the thin ring (Homogeneous solids jpeg) do you use Izz = m*r^2 instead of Ixx or Iyy? Same question for the slender rod. How do you know which one to use?

I realize I've said a lot, but I hope someone can give me some pointers.
Thanks.

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2. Oct 8, 2004

vsage

Have you identified what the axis passing through A looks like?

The axis A appears parallel to the z axis judging from the orientation of the second picture so yes you are correct. If you look at the orienation of the rods they are perpendicular to the A axis so judging from the second picture again you should use the y M of I.

My personal opinion is that for simple objects like that it is easier to derive the M of I so perhaps just remembering that the M of I is in integral form (r^2dm)

Last edited by a moderator: Oct 8, 2004