Physics problem with a ring?

Question

There is a ring which radius is R. A little ball moves inside this ring. Ring's flat is normal to the surface of ground. When a ball is moving inside a ring (ring is in quiet), the ball reaches a height which is equal to R/2.
The ring starts to move plumb with a fixed acceleration. What is the value of the fixed ring acceleration, if the ball inside the ring reaches the top of the ring?

Well, now I know that the acceleration is equal to 4g/5. How to achieve this answer?

Answer 1

The acceleration only has to be greater than gravity sustained long enough for the ball to reach the top.

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