= Center-of-mass equation
{title2=$M\ddot{\mathbf r}_{\rm CM}=\sum\mathbf F_{\rm ext}$}
For a closed system with fixed total <mass>, summing the constituent <Newton's second law> equations makes pairwise internal forces cancel when <Newton's third law> applies. The remaining resultant external <force> is the derivative of total <momentum>, equal to $M\ddot{\mathbf r}_{\rm CM}$. This governs translation of the <center of mass>; external <torque> additionally governs changes of <angular momentum>.
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