When she takes both her arms in, her moment of inertia decreases because the particles of her arms that were away from the axis of rotation have come closer to the axis. Now when she brings only one arm close to herself, obviously the same thing happens since again though half of the previous time but particles do come close to the axis of rotation.

Added after edit: why would internal forces cause torque, had there been a torque, net angular momentum would change but it does not, so no torque is applied.

Conservation of angular momentum experiment
I recently bought a UPS for my iMac and it runs just fine.

However, sometimes I also run a MacBook Pro nearby and when I have my hand in contact with the MacBook and reach across to touch the aluminium keyboard attached to my iMac I can feel electricity running through my finger!

It doesn't happen when the MacBook is running on battery power, only when it's connected to the mains. It also doesn't happen when the UPS isn't connected.

The MacBook Pro is plugged into a standard wall outlet, the iMac is plugged into a UPS and that's in a similar wall outlet (not the same one).

I'm not very electrically minded, so am puzzled as I'd like to plug the MacBook into the mains again! Does anyone have any ideas as to why it's happening and how I can fix the problem?
