$\underset{\_}{\underset{\_}{F}}=\text{d}\underset{\_}{\underset{\_}{p}}/\text{d}\tau$

where $\underset{\_}{\underset{\_}{p}}$ is four-momentum and $\tau$ is proper time

Note 1 to entry: A four-dimensional generalization of Newton’s law says: For a particle with rest mass $m_0$, $\underset{\_}{\underset{\_}{F}}=m_0\underset{\_}{\underset{\_}{a}}=\gamma (\text{j}\dot{\gamma }{m}_0{c}_0;\overrightarrow{F})$, where $\underset{\_}{\underset{\_}{a}}$ is four acceleration, $\text{j}$ is the imaginary unit, $\gamma$ is the Lorentz factor and $\dot{\gamma }$ its derivative, and $\overrightarrow{F}=\text{d}\overrightarrow{p}/\text{d}t$ is three-dimensional force.

Note 2 to entry: The coherent SI unit of four-force is newton, $\text{N}=\text{kg}\cdot \text{m}\cdot {\text{s}}^{\text{−2}}$.