Suppose you have a ball on a string, and you make the ball to move around in a circle. The force on the ball is caused by the tension of the string, and is called the centripetal force F = mv^2/r. If you were to let go at any moment, the ball would stop rotating and move with linear velocity. Now suppose you had a disk on a pole, and you make the disk rotate at constant angular velocity. According to the conservation of angular momentum, the disk would keep on rotating at the same speed (assuming no external torque). My question is, if you focus on only one point on the disk, is it correct to say that the point is in circular/centripetal motion, and therefore is no different than a ball on a string? If that is so, then there must be a centripetal force making that point move in a circle. For the case of the ball on the string, the human (and by extension the string) is the ultimate "source" of the centripetal force. What is the "source" for the disk, and why doesn't it exist for the ball on the string when the human lets their grip off the string? I've been a bit rusty with mechanics, so it's possible I'm just misunderstanding.