# Zero Moment of Inertia?

1. Dec 7, 2007

Does anyone out there know if it's possible for a 1220 kg cylindrical object of uniform mass distribution to have NO moment of inertia while in outer space? If so, when and under what conditiions? Please help!!

2. Dec 7, 2007

### Staff: Mentor

Sounds like something from an assignment dealing with satellites.** It also sounds like a typo. What's the exact wording?

** I say that because someone posted a problem recently with that some curious statement in it. I suspect it's just an error and that they meant to say that the object wasn't rotating, not that it had no moment of inertia.

Last edited: Dec 7, 2007
3. Dec 7, 2007

One of the scheduled activities during a shuttle mission was the launching of the communication satellite. This 1220 kg satellite GEO II is uniform cylinder of a diameter of 1.18 meters and length 1.72 meters. It is identical in mass, density and shape to GEO I which is already orbiting the Earth. Prior to launching, a motor inside the shuttle bay takes one minute to set the satellite spinning from rest to 1.46 rev/s about the cylinder's axis. At this instant the spinning satellite is released from the bay compartment and placed in the same orbit as GEO I. GEO I has zero moment of inertia. (a) Do the two satellites have the same total energy once GEO II is in orbit? If yes, use physics principals to explain why. If no then calculate the difference in energy.

4. Dec 7, 2007

### Staff: Mentor

Yep, same problem. See my comment in the post above.

Obviously if one satellite is "identical in mass, density and shape" to the other, then they must have the same (non-zero) moment of inertia.

5. Dec 7, 2007

In other words, all objects that have matter have non-zero moment of inertia even if they are not rotating? Sorry, I just need to be clear before I confront my professor again. He seems to think that by the equation Torque= Moment of Inertia times Angular Acceleration, he can solve for Moment of Inertia when both Torque and Angular Acceleration are zero.

Last edited: Dec 7, 2007
6. Dec 7, 2007

### Staff: Mentor

A solid cylinder does. Note that the moment of inertia depends on the axis chosen, but does not depend on whether it's actually rotating or not.

That makes no sense. You may well be able to solve for the moment of inertia by other means, but not using that approach. It's like saying: The force is zero and the acceleration is zero. So what's the mass?

Last edited: Dec 7, 2007
7. Dec 7, 2007

I agree! Can I ask you a new question or do I have to submit a new title? This is my first time on this and any forum for that matter so I don't know.

8. Dec 7, 2007

### Staff: Mentor

If it's a separate topic, start a fresh thread. If it's a follow on to this, ask it here.