What is the axis for #21 on Moment of Inertia?

[muna580]

http://img100.imageshack.us/img100/1756/untitled2qx1.jpg

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.

 

[OlderDan]

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 theorem, 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]

Acorrding to the parallel axis theorem, 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