Lorentz-invariant three-vectors and alternative formulation of relativistic dynamics

Besides the well known scalar invariants, there exist also vectorial invariants in the realm of special relativity. It is shown that the three-vector $\left(\frac{d\vec{p}}{dt}\right)_{\parallel v}+\gamma_v\left(\frac{d\vec{p}}{dt}\right)_{\perp v}$ is invariant under the Lorentz transformation. The indices $_{\parallel v}$ and $_{\perp v}$ denote the respective components established with respect to the direction of the velocity of body $\vec{v}$, and $\vec{p}$ is the relativistic momentum. We prove that this vector is equal to a force of $\vec{F}_R$ satisfying the classical Newtonian law $\vec{F}_R=m\vec{a}_R$ in the instantaneous inertial rest frame of an accelerated body. Therefore the equation $\vec{F}_R=\left(\frac{d\vec{p}}{dt}\right)_{\parallel v}+\gamma_v\left(\frac{d\vec{p}}{dt}\right)_{\perp v}$, based on the Lorentz-invariant vectors, may be used as a truly invariant (not merely a covariant) relativistic equation of motion in any inertial system of reference. An alternative approach to classical electrodynamics based on the invariant three-vectors is proposed.