In the non-relativistic case, the momentum is , where is the particle velocity. The quantity is called the potential momentum and is the potential four-momentum .
Some additional terminology is useful.
In order to avoid confusion, we rename the total momentum . Thus, the total momentum equals the kinetic plus the potential momentum, in analogy with energy. So far, we have shown that the introduction of a potential momentum complements the potential energy so as to make the energy-momentum relationship for a particle relativistically invariant. However, we as yet have no idea what causes potential momentum and what it does to the affected particle. We shall put off answering the former question and address only the latter at this point.