How did Newton discover his third law? Was it his original finding or was it a restatement of someone else's, like the first law coming from Galileo? What initiated the concept of what is now known as Newton's 3rd law?

Historically, I'm not really sure what prompted Newton to write down his third law. Physically, however, it is just a statement of momentum conservation. Say object 1 pushes on object 2 with force [math]F_{12}[/math]. Then by the third law object 2 pushes on object 1 with force [math]F_{21}=-F_{12}[/math]. Rearranging and using Newton's second law:

 

[math]F_{12}+F_{21} = \frac{d}{dt} \left ( p_1 + p_2 \right ) = 0[/math]

 

This just says that the total momentum of the system must be conserved. Whenever there is spatial translation symmetry present you will get some version of Newton's third law.

Now that i take a new look at it, it looks to me that Newton's 3rd law ressembles quite a bit with Archimedes principle.

Third law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

 

 

Not to say that paradoxically Archimedes should have known Newton's 3rd law in order to formulate his principle.

pay wall in front of

Action and Reaction before Newton John L. Russell Heythrop College, University of London, 11–13 Cavendish Square, London W1M oAN.

 

http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=2915316

 

or

 

http://www.jstor.org/discover/10.2307/4025704?sid=21104967019281&uid=2129&uid=4&uid=70&uid=2

Edited December 31, 201411 yr by michel123456