First principle
Applied thermodynamics - First principle. Energy. Enthalpy

Historically, the first principle of thermodynamics was called the equivalence principle, because it stipulated the equivalence of two forms of energy: mechanical and thermal. More generally, this principle is still that of energy conservation. In rational mechanics, for example, this principle is used in a particular form. Thus, if a moving body is subject only to a force field derived from a potential, the sum of its potential energy and kinetic energy is constant. Overall energy is conserved.