Relativistic dynamics and quark confinement

First, starting from a relativistic d'Alembert's principle for a one-particle system, it is shown that whenever a classical Lagrangian formulation exists for one observer in terms of his space-time variables, a corresponding formulation exists for any Lorentz-related observer. The relationship is unique and guarantees the correct description of any physical one-particle problem for all such observers. One is then able to identify unambiguously the relativistic scalar, vector, etc. interactions classically. Second, some simple one-dimensional exactly solvable problems are discussed in order to illustrate the differences in the motions caused by relativistic scalar and vector interactions. One of the scalar interactions suggests the possibility of quark confinement classically. Finally, some generalizing comments are presented