A clown balances a small, spherical grape at the top of his bald head, which also has the shape of a sphere. After drawing sufficient applause, the grape starts from rest and rolls down without slipping. It will leave contact with the clown's head when the radial line joining it to the curvature makes what angle with the vertical? This is the solution; normal force must be zero: What I dont understand is, they say that mechanical energy is conserved. But shouldn't there be a friction force doing work on the grape as it rolls downwards?