Roller coasters are governed by the fundamental laws of classical mechanics, primarily involving the constant interplay between potential energy and kinetic energy. A coaster starts by gaining "gravitational potential energy" as it is pulled up the initial lift hill. Once it crests the peak, gravity converts that potential energy into "kinetic energy" (the energy of motion). Throughout the ride, several forces act on the passengers: Gravity pulls the train downward, while the Normal Force from the track pushes back up. When going through loops or tight turns, riders experience Centripetal Force, which keeps the train moving in a curved path, and Inertia, which makes the rider feel like they are being pressed into their seat (G-forces). Friction between the wheels and the track, as well as Air Resistance, gradually bleed energy from the system, which is why the second hill of a coaster must always be lower than the first. Modern coasters also use magnetism (Eddy currents) for smooth, wear-free braking systems.