# What happens when you rotate a rigid body

Hi,

I have a question about rigid body rotation. If I have a rod out in space in free float and I attach a battery powered fan to center of mass and then the fan is turned on to spin in a clockwise direction, then the rod should spin in a counterclockwise direction. This much I understand. But, what if, instead, the fan was attached to the end of the rod rather than to the center of mass? What would happen then?

Any help would be greatly appreciated!

As a more general question, what would happen if you attached the fan to anywhere on the rod other than the center of mass? how would the rod and the fan spin together? Is what happens similar to a clothes washer that is unbalanced? Thanks in advance for any insight into this question!

Dale
Mentor
2020 Award
If you are rotating about an axis that is not an axis of symmetry then the moment of inertia is a tensor meaning that the angular momentum does not line up with the axis of rotation. This gives you a wobble which is called gyroscopic free precession or torque-free precession.

Hi,

Thanks for your answer, would the system wobble around a certain point? Like, center of mass of the system?

Dale
Mentor
2020 Award
Yes. There is no external force so the center of mass must remain at rest (assuming it started at rest). The axis of rotation will wobble, but will always pass through the center of mass.