The momentum of an object is equal to its mass multiplied by its velocity. When no external forces act on a system the total momentum is constant. This principle is called the conservation of momentum. 'Conservation of Momentum' takes a cue from Descartes, to show that momentum is always conserved in the mechanical universe. A clear explanation and interrogation of Descartes' legacies, Newton's Second Law, momentum, energy and right angles, and analytic geometry.