Mechanical mass matrix ($T={\dot{\mathbf{q}}}^{\mathsf{T}}\mathsf{M}\dot{\mathbf{q}}/2$)

The symmetric coefficient matrix of kinetic energy to second order in generalized velocities. For independent horizontal point-mass coordinates it is diagonal, with the particle masses on the diagonal. Together with the mechanical stiffness matrix it gives the generalized eigenvalue problem for small oscillations.