Under certain conditions usually fulfilled in classical mechanics, the principle of conservation of linear momentum and Newtons 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 Newtons second law. On the other hand, under some additional conditions the combined laws of conservation of linear and angular momentum, are equivalent to Newtons 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 Newtons second law is the clue.
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