Yes, it uses a Newtonian formulation. Another physics engine (nphysics) I wrote before rapier, had a Lagrangian formulation of joints (using reduced-coordinates

Yes, it uses a Newtonian formulation. Another physics engine (nphysics) I wrote before rapier, had a Lagrangian formulation of joints (using reduced-coordinates). This is something I intend to add to Rapier too at some points.