Roller coasters is another one of my physics interests. A big question is why don't people fall out when the roller coaster goes upside down. Energy, inertia and gravity all have to do with this. Newtons first law says that an object in motion will stay in motion unless acted upon by a net force. The coaster is going too fast for a body to just fall out. The first big hill on most roller coasters is what runs the rest of it. The cart is pulled up to the top of the hill and then gravity takes over for a majority of the ending. When it goes around in the loop and you feel a tug to the side, that's the centripetal force acting on you. When the ride begins, the potential energy is converted into kinetic energy. Throughout the whole ride, there is a force looking the bodies in.
This video shows it more in depth;