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internal and external forces


Mass m1 exerts a force F12 on mass m2, for example by gravity in the case of planets, or by an electric force in the case of the two nuclei of a diatomic molecule, whereas mass m2 exerts a force F21 on mass m1.
From Newton's first law (action equals minus reaction) we learn that those forces are equal and opposite:

F21 = - F12

Next to those forces the two masses may be subject to external forces, like the attraction by the sun in the case of planets. We denote by F1,ext the external force which acts on m1 and by F2,ext the external force which acts on m2.





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