Have you ever thought about the physics on a roller coaster? Well, its pretty simple. Once you are in the car of the roller coaster you build up potential energy as you go up the hill, which then is released as kinetic energy. Once you are going to the hill gravity takes over and all of the potential energy you built up is released into kinetic energy. Gravity applies a constant downward force on the cars. Since an object in motion tends to stay in motion, the coaster car will maintain a forward velocity even when it is moving up track. The potential energy and kinetic energy changes back and forth to one another.