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The cars on a typical roller coaster are not self-powered. The coasters are powered by being pulled up with a chain or cable along the lift hill to the first peak of the coaster track. Then potential energy becomes kinetic energy as the cars race down the first downward slope. Kinetic energy is converted back into potential energy as the train moves up again to the second peak. This is necessarily lower as some mechanical energy is lost due to friction. Then the train goes down again, and up, and on and on.

However, not all coasters run this way. The train may be set into motion by a launch mechanism. Some coasters move back and forth along the same section of track; these roller coasters are called shuttles because of this motion and usually run the circuit once with riders moving forwards and then backwards through the same course. Some roller coasters are powered by a kind of locomotive. A properly designed roller coaster under good conditions will have enough kinetic, or moving, energy to complete the entire course, at the end of which brakes bring the train to a complete stop and it is pushed into the station. A brake run at the end of the circuit is the most common method of bringing the roller coaster ride to a stop.

One notable exception is a powered coaster. A powered coaster is a roller coaster type ride that instead of being powered by gravity, uses one or more motors in the cars to propel the trains along the course.

Safety

Statistically, roller coasters are very safe. The U.S. Consumer Product Safety Commission estimates that 134 park guests required hospitalization in 2001 and that fatalities related to amusement rides average two per year. According to a study commissioned by Six Flags, 319 million people visited parks in 2001. The study concluded that a visitor has a one in one-and-a-half billion chance of being fatally injured, and that the injury rates for children's wagons, golf, and folding lawn chairs are higher than for amusement rides. In fact, driving to the amusement park has a higher risk of injury than riding the rides at the amusement park. It is not unusual for park management to pay higher insurance premiums for carousels than they do for roller coasters.

Many safety systems are put in rollercoaster systems. The key to the mechanical fail safes is the control of the coasters operating computers: programmable logic controllers. Most roller coasters run with three separate PLCs. However, only one of the PLCs is required to find a fault for the ride's fail-safes to work. This is often the reason that the ride trains may stop on the lift or the brake runs, yet after a short time the ride starts again without any obvious maintenance by staff. It is likely in such a case that one of the PLCs detected a fault by mistake, and the ride's operator only needed to restart the ride.

In recent years, arguments have come up about the safety of the increasingly extreme rides. There have been suggestions that these may be subjecting passengers to translational and rotational accelerations that may be able of causing brain injuries. In 2003 the Brain Injury Association of America said in a report that. There is evidence that roller coaster rides pose a health risk to some people some of the time. Equally evident is that the overwhelming majority of riders will suffer no ill effects.