How did Newton discover his third law of motion?

[Ganesh Ujwal]

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?

[elfmotat]

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.

[michel123456]

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.

[michel123456]

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, 2014 by michel123456