  Under certain conditions usually fulfilled in classical mechanics, the
principle of conservation of linear momentum and Newton's third law are
equivalent. However, the demonstration of this fact is usually incomplete in
textbooks. We shall show here that to demonstrate the equivalence, we require
the explicit use of the principle of superposition contained in Newton's second
law. On the other hand, under some additional conditions the combined laws of
conservation of linear and angular momentum, are equivalent to Newton's third
law with central forces. The conditions for such equivalence apply in many
scenarios of classical mechanics; once again the principle of superposition
contained in Newton's second law is the clue.
