Four-Force

In the LCF \(\Sigma \) two mass points 1, 2 are in contact exerting forces of action–reaction \(\mathbf {f_{12}}\) and \(\mathbf {f_{21}}\) respectively to one another. Prove the third Newton’s Law \(\mathbf {f_{12}}=-\mathbf {f_{21}}\) (Law of action–reaction) assuming that the masses of the two particles remain constant.