Roller coasters are propelled by the conversion of potential energy to kinetic energy. Roller coasters do not have an engine. Instead, the roller coaster cars are pulled up the first hill mechanically. Once the roller coaster is released to descend the first hill, the ride creates all of the energy that it needs to run through the track.
Three different types of wheels work to keep the roller coaster cars on the track and moving smoothly. The running wheels guide the cars along the track. Friction wheels control the motion on either side of the track, which is called lateral movement. A third set of wheels provides traction, working to keep the roller coaster cars on the track, even through inversions and rolls. Compressed air brakes help to slow the roller coaster cars down when necessary for safety throughout the track and are also used to stop the movement of the roller coaster cars at the end of the ride. The design of the roller coaster cars and tracks takes into account the gravitational, centripetal and inertial forces that propel the coaster cars, keep them safely on the track and able to come to a complete stop at the end of the ride. Designers must also take into account the weight and height minimums and maximums required by the roller coaster design for riders to safely enjoy the coaster.