# Relative Formulation of Physical Laws

## Abstract

In Chap. 7 we described the procedures that in special relativity allow us to derive the physical laws relative to an *inertial* frame starting from their 4-dimensional formulation and vice versa. Determining analogous procedures in general relativity is a more difficult task. In this chapter, we analyze reasonable criteria leading to the *relative formulation* of physical laws starting from their 4-dimensional formulation. By this approach we determine the momentum equation and the energy equation of a material point relative to an arbitrary frame of reference starting from the four-dimensional momentum–energy equation. Then we compare the obtained equation with similar equations proposed in the literature. Finally, the above procedure is applied to derive the *relative* charge conservation, Maxwell’s equations, and the balance equations of an incoherent fluid (For the content of this chapter, see the review paper [142]).