Rope model: the rope binds the ball to do uniform circular motion on the vertical plane. At this time, if the ball is at the highest point, if the speed does not reach √gr, it will fall forward and the rope will relax. Which means we can't reach the highest point.
The ball moves on the inner wall of the cylinder (the ball is not supported by the lower wall), which also belongs to rope ink.
Rod mold: the polished rod is connected with the sphere and moves in a uniform circular motion in the vertical plane. At this time, the speed of the highest point is 0, and the ball will not fall under the support of the polished rod. That is, the speed is zero.
The small ring is surrounded by numerical ring (big ring), and the small ball is in a vertical closed annular orbit (supported by upper and lower walls), which belongs to rod mold.
To sum up, the speed at which a particle falls from the circular orbit to the center of the circle is at least v=√rg, and the speed at which it is fixed on the circular orbit can be 0.