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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.

“Peace I leave with you; my peace I give you. I do not give to you as the world gives. Do not let your hearts be troubled and do not be afraid.”
– Jesus, John 14:27