Roller coasters provide an exhilarating ride combining emotions like fear, joy and vertigo through the application of physics principles. They are designed with safety as a top priority, using restraints, regular inspections and safety devices to prevent accidents. There are three types of wheels needed - upper wheels to control the train along most of the track, lower wheels to handle higher g-forces on hills, and lateral wheels to prevent derailing on curves. The motion of roller coasters is powered solely by gravity, gaining speed downhill and using that kinetic energy to climb hills, in a cycle that continues until friction slows the train to a stop.