A roller coaster car of mass 200 kg starts from rest at the top of a hill 20 m high and rolls down to ground level. From there it starts into a vertical loop. Exactly at the top of the loop it is 15 m high and the radius of curvature of the track is 5.0 m. Assuming negligible friction and air resistance, what is the magnitude of normal force acting on the car at the top of the loop?