A block starts from rest at height h above the bottom of a smooth vertical loop of radius R. The minimum h required for the block to complete the loop while maintaining contact is:
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A block starts from rest at height h above the bottom of a smooth vertical loop of radius R. The minimum h required for the block to complete the loop while maintaining contact is:

G1 Physics · Accepted Answer

At the top in the limiting case, v²=gR. Energy conservation from release point to top gives mgh = mg(2R)+½mgR = (5/2)mgR. Hence h=5R/2.