For the n particle system, let the velocity of each particle Pi be Vi, then the virtual displacement δri can also be written in the form:formula_7This means that the generalized force, Qj, can also be determined as:formula_8==D'Alembert's principle==D'Alembert formulated the dynamics of a particle as the equilibrium of the applied forces with an inertia force (apparent force), called D'Alembert's principle.