What is the moment of inertia of the object

1. The problem statement, all variables and given/known data
An object is formed by attaching a uniform, thin rod with a mass of mr = 6.94 kg and length L = 5.56 m to a uniform sphere with mass ms = 34.7 kg and radius R = 1.39 m. Note ms = 5mr and L = 4R.

What is the moment of inertia of the object about an axis at the right edge of the sphere?

2. Relevant equations
I = mr^2
I(rod-end)=1/3 mr^2
I(spherical shell)=2/3 mr^2
I(sphere) = 2/5 mr^2

3. The attempt at a solution

I figured that it would be a spherical shell going around the axis, because the whole sphere is rotating, rather that it rotating at it center of the sphere, and then the rod going around as well. I'm obviously wrong, seeing as I'm reaching out for help, but here's what I had put, I am not sure why it's wrong though, and I don't know what to do.

1/3 (m rod)(L + 2R)^2 + (2/3) (m sphere)(2R)^2
(1/3)(6.94)(5.56+2.78)^2 + (2/3)(34.7)(2.78)^2


Staff Emeritus
Science Advisor
Homework Helper
Welcome to Physics Forums.

Looks like you need to use the parallel axis theorem for this problem. It should be in your textbook or class notes, or you can find it in the PF library https://www.physicsforums.com/library.php?do=view_item&itemid=31".

Are you using the m.o.i. of a hollow sphere? The problem seems to describe a solid sphere.

Also, is the rod attached to the right, left, top, or bottom (or other) edge of the sphere?
Last edited by a moderator:
oh, i didn't notice that.. you're right, i was using the moment of inertia for a solid sphere. the rod is connected to the left end of the sphere, and the axis is on the right side of the sphere.. like...



Staff Emeritus
Science Advisor
Homework Helper
I meant that I think it is a solid sphere. I saw the "2/3" factor in your calculation, which should be "2/5" if it is a solid sphere.
Last edited:

The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Members online