These equations define equilibrium and govern statics.
$\Sigma \vec{F}=0$ $\Sigma \vec{M}=0$
These equations articulate how integrating loading functions gives internal force functions.
Axial force and axial load
$N'_x=-q_x$
Torsional force and torsional load
$T_x'=-t_x$
Internal beam moments, shears, and lateral loads
$M_z''=V_y'=q_y$
$M_y''=V_z'=q_z$