Static friction is the force that resists the relative motion of two objects in contact with each other when the objects are not moving relative to each other. In a roller coaster, the wheels of the roller coaster cars are constantly rolling on the tracks, so there is no static friction between the wheels and the tracks.
The force that keeps a roller coaster car on the tracks is called centripetal force. Centripetal force is the force that acts on an object moving in a circular path to keep the object moving in that path. In a roller coaster, the centripetal force is provided by the force of gravity pulling the roller coaster car down towards the center of the circle that it is moving in.
