State the principal of conservation of energy?

The law of conservation of energy states that the total amount of energy in an isolated system remains constant. A consequence of this law is that energy cannot be created nor destroyed. The only thing that can happen with energy in an isolated system is that it can change form, i.e. kinetic energy can become thermal energy.

Albert Einstein's theory of relativity shows that energy can be converted to mass (rest mass) and mass converted to energy. Therefore, neither mass nor pure energy are conserved separately, as it was understood in pre-relativistic physics. Today, conservation of "energy" refers to the conservation of the total mass-energy, which includes energy of the rest mass. Therefore, in an isolated system, mass and "pure energy" can be converted to one another, but the total amount of energy (which includes the energy of the mass of the system) remains constant.

Another consequence of this law is that perpetual motion machines can only work perpetually if they deliver no energy to their surroundings. If such machines produce more energy than is put into them, they must lose mass and thus eventually disappear over perpetual time, and are therefore impossible.