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## Homework Statement

A solid ball of mass M and radius is connected to a thin rod of mass m and length L as shown. What is the moment of inertia of this system about an axis perpendicular to the other end of the rod?

Image: http://imageshack.us/photo/my-images/35/helpfy.jpg/

## Homework Equations

I

_{total}= MD

^{2}+ I

_{cm}, where I

_{cm}= moment of inertia of the center of mass

The moment of inertia of a solid sphere is given by: 2/5 * mr

^{2}

The moment of inertia of a rod being rotated as shown is: 1/3 * mr

^{2}

## The Attempt at a Solution

The center of mass of the sphere is a distance L away from the axis of rotation and has a moment of inertia of 2/5 * MR

^{2}. What I have a problem with is the rod. I know the rod's moment of inertia is given by 1/3 * mr

^{2}(for this situation), and the distance from the center of mass of the rod to the axis of rotation is L/2.

Here is my question: our lecture slides were made goofy and it gives both answer (A) and answer (C) as the correct answer. I think answer (A) is the correct answer because we are calculating I

_{tot}by adding the moment of inertia for the solid sphere and rod. With that said, I am still confused if answer (A) is the correct answer because the parallel axis theorem is given by I

_{total}= MD

^{2}+ I

_{cm}. Why is the MD

^{2}term just ML

^{2}(mass of sphere a distance L from the axis of rotation), and not the sphere PLUS the rod? I may have worded my questions a little weird, let me know if I am being confusing. Thanks (:

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