Physics: Loop the Loop?

A roller-coaster car may be represented by a block of mass 50.0 kg. The car is released from rest at a height = 54.0 m above the ground and slides along a frictionless track. The car encounters a loop of radius = 18.0 m at ground level, as shown. As you will learn in the course of this problem, the initial height 54.0 m is great enough so that the car never loses contact with the track.