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DAlemberts Principle

D'Alembert's Principle, in dynamics of systems of particles, is that the external forces on any system are equivalent to the mass-accelerations produced, the internal forces of the system being in that connection negligible. It is, in fact, a statement that the observed external causes are equivalent, as a whole, to the observed effects produced. By its use generalised methods for solution of innumerable problems are obtained.

“If God give thee not only earth but heaven, that thou shouldest rule over sun, moon, and stars, and have rule over the highest of the sons of men, it would not be enough to satisfy thee, except thou hadst God himself.”
–Jeremiah Burroughs, The Rare Jewel of Christian Contentment