In physics , the conservation of energy means that the total amount of energy in an isolated system remains constant , although it may change forms , e.g. friction turns kinetic energy into thermal energy . So conservation of energy refers to the conservation of the total energy of an isolated system over time ( including rest mass energy mcÂ associated with the rest mass of particles ) and all other forms of energy ( kinetic , potential , nuclear , chemical , thermal , etc. ) in the system together .