node_mass index mass_x mass_y mass_z

This record adds a discrete mass to node index in $x$, $y$ and $z$ direction. In 1D only the $x$-mass needs to be specified, etc. The mass will lead to a nodal force of the size $mass\_x * \dot v$ where $\dot v\_x$ is the acceleration, and to a gravity force if force_gravity is specified. The same holds for the $y$ and $z$ direction.