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

G1 Physics · Accepted Answer

At the top in the limiting case N=0, so v_T²=gR. Energy gives mgh=2mgR+½mgR=(5/2)mgR. Thus h=5R/2.