This Eq. (6.151) of Marion and Thornton, third edition. the term as introduced in Kelvin and Tait, "Natural Philosophy" (1867). It is the origin of the Lagrange equations of motion, and also the origin of the relativistic lagrangian. The relativistic momentum as used by Einstein is derived from the conservation of momentum. Horst has found that the relativistic Newtonian force, the time derivative of the relativistic momentum, gives retrograde precession, a major discovery because EGR fails to give retrograde precession.