Fourth component of relativistic forces, Paul Stephas, Am. J. Phys. 53(7), July 1985
The Lorentz scalar interaction gives rise to a fourth component of the relativistic four-force which has a different form than that of the vector interaction, the difference arising from the variable mass for particles interacting through the vector potential. The Lorentz scalar, with its pedagogically interesting dynamics, is rarely mentioned in the intermediate-level or first year, gradate-level textbooks.