$\underset{\_}{\underset{\_}{f}}=\mathsf{F}\underset{\_}{\underset{\_}{J}}$

where $\mathsf{F}$ is the electromagnetic field tensor and $\underset{\_}{\underset{\_}{J}}$ is the four-current density

Note 1 to entry: The four-force density obeys the relation $\underset{\_}{\underset{\_}{f}}=Div\mathsf{T}$, where $\mathsf{T}$ is the stress-energy tensor.

Note 2 to entry: The coherent SI unit of four-force density is newton per metre cubed, $\text{N}\cdot {\text{m}}^{\text{−3}}$.