Then there would be no motion. When the glue breaks, the forces are put out of balance with each other (one is reduced to near zero), and the centrifugal force wins, propelling the ball off the edge. Not true, motion is not dependent on force. Force is defined by an acceleration. The ball was in motion when the glue was holding, but it was being redirected into a circular motion by the glue (which would be an acceleration, and hence centripetal force exists). Once the glue breaks the ball is still in motion, but there is no longer any force on it, so it will continue in motion in the direction that it was traveling when the glue broke, which would be away from the disk. If there were a force being applied to the ball then an acceleration would have to exist, however the ball does not change velocity or direction, so no force is applied to the ball as it flies away from the disk (other than wind resistance, but we can neglect that).