Node can have mass and inertia. In the normal case the mass matrix is
but if mass modifiers are included the mass matrix becomes
Inertia is a tensor quantity with the following terms
where the terms in the inertia tensor are defined as
where are the coordinates of the
centre of mass. If non-zero values are specified for the off-diagonal
terms, it is important that these are consistent with the diagonal
terms. If this is not done the principal inertia values can become
negative. The inertia matrix is never modified for directions.
