For more information on energy sources, see Energy supply . Potential and kinetic energy Potential energy is often called stored energy. It represents work that has already been done. A rock lying on the top of a cliff has potential energy and so does the cartridge in a loaded rifle. When the rock topples over the cliff or the gun fires, potential energy becomes kinetic energy.
Kinetic energy is the energy of movement.
The rock on the edge of the cliff had to be placed in that position and the gun had to be loaded. If you stretch an elastic band between your fingers, it represents potential energy. Let the elastic band snap back into its normal position, and the potential energy is changed to kinetic energy as long as the band keeps moving. The word kinetic comes from a Greek word meaning to move. A moving train or a high speed electron in a TV tube exhibits kinetic energy.