The mechanical energy in a moving object is the combination of potential (puh-TEN-shul) energy and kinetic (kih-NEH-tik) energy. Potential energy—often referred to as stored energy—is energy of position, and kinetic energy is energy of motion.

When engineers design a roller coaster, they must create the greatest amount of potential energy at the beginning of the ride. This is because after the start, the cars will move under their own power to the finish. Engineers have a motor pull the cars to the highest peak—where potential energy is greatest—very early in the ride and then allow the force of gravity to keep the cars moving.

As the cars start to descend (fall) down the track, the potential energy converts (changes) into kinetic energy. At the bottom of the first hill, kinetic energy is greatest, and this drives the cars up the next hill. The roller coaster cars, the track, the motor that drives the chain that pulls the cars uphill, and Earth’s gravity all work together as a system in which energy changes from one form to another.