# Inertia - Moments of Inertia of a rigid body (different axes)

1. Nov 5, 2013

### frownifdown

Inertia -- Moments of Inertia of a rigid body (different axes)

Here is the problem http://imgur.com/pL6Bdgw [Broken]

So I missed class today because I was studying for a genetics test. I don't need the answer or anything but I was wondering what the general rule for inertia that I would use for solving this problem. I looked up some inertia rules but don't really see how they apply. Thanks in advance

Last edited by a moderator: May 6, 2017
2. Nov 5, 2013

### Staff: Mentor

The general concept is the "moment of inertia". Look up that formula, and the problem should be straightforward.

Last edited by a moderator: May 6, 2017
3. Nov 5, 2013

### frownifdown

Alright so I looked it up and saw the equation for it (I=mr^2 correct?). Now the problem says to ignore the mass so do I just count the distance from each ball to each axis and square and add them? That seems very tedious. Can you get it from just looking at them?

4. Nov 5, 2013

### Staff: Mentor

Right. That's the moment of inertia for a point mass, which is what you need.

Yes. (You are ignoring the mass of the rods, not the balls.)

Get busy! (No shortcuts.)

5. Nov 5, 2013

### Staff: Mentor

It says to ignore the masses of the interconnecting rods. So yes, you do the sum of the mr^2 number about each axis to get the total I for each axis. Ignore the masses that are on-axis for this problem. It should go pretty fast...

EDIT -- Doc is quicker on the draw than I am, again!

6. Nov 6, 2013

### frownifdown

Alright, so I went through and thought I did it right but apparently not. I got G>D>A>F>E>B=C

Edit: I just went back and realized that my value for C was wrong, and it should be 16 instead of 7. I'm unsure on how to add and if there should be negatives though. For instance, would B be 7 or 1? Would I add the 4 on top and subtract 3 from the bottom or would they all contribute 1 to it.

Nevermind, Solved! Thanks everyone

Last edited: Nov 6, 2013
7. Nov 6, 2013

### Staff: Mentor

There are no subtractions in MOI calculations...

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted