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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