Think about the last time you rode a roller coaster.
First, the car is pulled to the highest part of a track. At the top of the track, the car has potential energy. As the car rolls down the first steep slope, some of the potential energy changes into kinetic energy. At the bottom of the hill, all the potential energy has changed into kinetic energy.
When the car climbs to the top of the next hill, the kinetic energy changes back into potential energy. This process repeats itself throughout the ride. Normally, after the first hill the car moves without outside help. Each time it coasts down a hill it gains enough kinetic energy to climb the next one. However, some of the energy turns into heat in overcoming friction. For this reason, each hill in the ride is a little smaller than the one before it.
When scientists measure energy changes in a system such as a roller coaster, they find that when energy disappears in one form, an equal amount appears in another form. In other words, energy is neither created nor destroyed. It only changes form. This basic law of nature is called the law of conservation of energy.