Rolling Without Slipping

[physicsss]

A thin hoop of mass m and radius R is rolling without slipping along a horizontal track. It then encounters a loop of radius r. If the hoop travels fast enough along the horizontal portion of the track it is able to go around the loop without losing contact with it at the top. Find the minimum speed the hoop needs to have on the horizontal portion of the track to accomplish this. Express in terms of m, R, r, and g.

[Tide]

What exactly have you tried so far?

Meanwhile, here's a couple of hints:

(1) Energy is conserved.

(2) Centripetal acceleration is speed squared divided by radius of circular path.