Originally Posted By: Orac

->If things want to stay at there current momentum and there are no forces on things why does the water start spinning at all. Why doesn't the water just sit still and the bucket rotate ... I mean how does the water know the bucket is rotating and should start doing that itself if we have no forces?

->Now lets go beyond the start point eventually the rope will unwind and then start winding back up and will begin to slow the bucket rotation down. How does the water know the bucket is decelerating.


The first two questions are trivial. You are rotating bucket and water as a whole. Neither the bucket nor the water 'knows or decides' which one is to rotate. The force causes changes in both. The bucket being solid, and water being liquid, the changes are slightly different. As long as the centripetal force exists, the water will remain rotating; as the force subsides, water returns to its original position.