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This question has been bugging me for about three years now. It has to do with the similarities of Newton's third law (Niii)and Conservation of Momentum (CofP).

Just to re-itterate for those unclear (or unable to remember) the two laws:

Niii: For each action, there is an equal and opposite reaction.

CofP: in any action, momentum (the product of mass and velosity) is conserved within the system.

In my mind, these are just saying the same thing. For example, if an astronaut throws their shoe into space, then from Niii, the astronaut will have an opposite and equal reaction, pushing in the opposite direction of the thrown shoe. So the harder the astronaut throws the shoe, the greater the force pushing backwards.

For momentum, the faster the astronaut throws the shoe, the faster the astronaut will move backwards. And then of course, if you get into impulse, you realize they say the same thing.

My question, is which came first? I remember hearing once that Newton came up with momentum. But then why do we use two laws? I mean, really, Newton's Third Law is just the same as Conservation of Momentum (right?). So which law came first?

Thanks for anyone who takes the time to read this and answer.

Imo

~Dirac Notation: The closest most male physicists get to bras

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# What came first ?

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