The Law of Conservation of Energy
The law of conservation of energy says that energy cannot be made or destroyed. In other words, energy does not just go away. Energy can change forms. For example, light energy can become heat energy, or energy from movement can become heat energy. Energy can change, but it never goes away. Learn more about this important physical law:
http://phet.colorado.edu/simulations/sims.php?sim=Energy_Skate_Park
http://www.fi.edu/guide/hughes/energyconservation.html
http://jersey.uoregon.edu/vlab/PotentialEnergy/