1. The problem statement, all variables and given/known data Small ball moves on the inner surface of the vertical ring with radius R. Moving ball reaches maximum height equal to R/2. What minimum acceleration (in vertical direction) is required (to the system of ring and ball) to make the ball reach the top of the ring? 2. Relevant equations All are provided in my solution... there might be another solving methods I haven't tried 3. The attempt at a solution I've tried to apply energy conservation law in this situation: ball has kinetic energy Ek=m(v^2)/2 in the bottom of the ring. All kinetic energy is converted to potential energy when ball reaches the top position (height equal to R/2). I wrote down energy conservation law: m(v^2)/2=mg(R/2) ---> v^2=gR; When we give vertical acceleration a to the system, acting force is equal to m(g-a), not mg. Value of a must satisfy the condition that ball reaches top of the ring (height 2R). Then I've written down again: m(v^2)/2=2m(g-a)R ---> v^2=4(g-a)R ---> gR=4(g-a)R---> a = 3g/4; However, correct answer provided in my textbook is a=4g/5. I cant understand what's wrong with my solution... I would be very thankful for your help!