State principle of conservation of energy.

Principle of conservation of energy states that "Energy can neither be created nor be destroyed but can be changed from onee form to another form" 

Conservation of energy, principle of physics according to which the energy of interacting bodies or particles in a closed system remains constant. ... When the pendulum swings back down, the potential energy is converted back into kinetic energy. At all times, the sum of potential and kinetic energy is constant. The common forms of energy include thermal, electrical, chemical, mechanical, kinetic, and potential. It may also be stated that the sum of all kinds of energy is constant.

This law, first proposed and tested by Émilie du Châtelet, means that energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another. For instance, chemical energy is converted to kinetic energy when a stick of dynamite explodes.