What determines if something is rotating? For example, if we have a spaceship with a fake gravity rotation thing, how do we know if the outer ring is rotating or if it is stationary and the inner axle is spinning? Normally we could just say its all relative, etc, but weather or not you feel gravity is determined by which one is spinning. So how can you decide? The outer one might is spinning relative to the center, but that doesn't mean it's really spinning (the center axle might just be spinning) , so why should I feel gravity? And if the outer ring is (presumably) heavier than the center, than why should it spin anyway? The motor would just spin the lighter center.