ENERGY CONSERVATION LAW
This law cannot be strictly proved, but it is the result of all human life experience. Laws of this kind are called laws of the nature. The energy conservation law is formulated like this: in any interactions of bodies energy does not disappear without leaving a trace, and also, does not appear from nothing. Energy can only be transmitted from one body to another one, or, turn from one form to another. The internal energy U of a system, which is isolated from any interactions with its environment, does not change in any interactions inside the system itself: U = const or ΔU = 0.