Frontiers of Physics

D’Alembert’s principle is a much more elegant and conceptually powerful formulation of Newton’s second law of motion by the 18th-century French polymath Jean le Rond d’Alembert, even though, mathematically, the reformulation amounts to a simple algebraic manipulation of Newton’s 2nd law. By defining an inertial force F = - m a , where m is the mass and  a  is the acceleration of the object, one arrives at the following: The sum of the forces on any mass always sum to zero in the reference frame of the mass. In this perspective, one always imagines oneself as being in the coordinate system of the mass, rather than in an inertial reference frame. Defining an inertial force makes discussion about inertia sensible because it is a well-defined concept. It also makes it easier to see how kinetic energy arises as the integral of the inertial force over displacement and momentum arises as the integral of the inertial force over time.  Also one can see how the centrifugal and coriolis