# Moment of Intertia

muna580
http://img100.imageshack.us/img100/1756/untitled2qx1.jpg [Broken]

I am having trouble with #21. I already did #20, but I have NO idea of how to do #21. First of all, I am not really sure where is the axis suppose to be.

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## Answers and Replies

OlderDan
Science Advisor
Homework Helper
I am having trouble with #21. I already did #20, but I have NO idea of how to do #21. First of all, I am not really sure where is the axis suppose to be.

Use the parallel axis theorem. Put the axis through the tip of the arrow in the diagram

muna580
Acorrding to the parallel axis theorm, you get the inertia by dong this

I = Icm + D^2M

Where Icm is the inertia with respect to the center, and D is the distnance, and M is the mass.

Well, Icm is the answer I got for #20 right?

Also, what do I use for the D? Is D the radius?

Also, what is M? Is M the mass for the hoop plus the mass of all the 4 spokes?

OlderDan
Science Advisor
Homework Helper
Acorrding to the parallel axis theorm, you get the inertia by dong this

I = Icm + D^2M

Where Icm is the inertia with respect to the center, and D is the distnance, and M is the mass.

Well, Icm is the answer I got for #20 right?

Also, what do I use for the D? Is D the radius?

Also, what is M? Is M the mass for the hoop plus the mass of all the 4 spokes?

Yes. Icm is what you found in #20.

M is the total mass and D is the distance from the center of mass to the point of rotation. In this case, D = radius