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December 22nd, 2014, 02:45 AM  #1 
Member Joined: Aug 2014 From: India Posts: 88 Thanks: 0  How did Newton discover his third law of motion?
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? 
December 23rd, 2014, 02:06 AM  #2 
Newbie Joined: Dec 2014 From: USA Posts: 1 Thanks: 0 
Newton, historically, used a pendulum experiment where he hanged two unequal masses on a string and pulled them back to estimate the impacts. Using this experimental data, he found the Forces, albeit not the acceleration, to be equal and opposite in direction.

January 12th, 2015, 03:29 AM  #3 
Newbie Joined: Jan 2015 From: uk Posts: 8 Thanks: 0 
newtons third law may not have been a law in its day, he had to make certain assumptions before proceeding to show testable results based on the initial assumptions, he is responsible for taking an assumption and proving it to be a law, I find it hard to believe that it hadn't been assumed to be correct by his predecessors when you consider keplers work amongst others, proving something to be true was a very difficult thing in his day and experimental evidence would have shown the third law to be acceptable at the then present time, don't forget that newton was intuitive, remember how he lost a calculation and wasn't able to remember his proof for some considerable amount of time, did he really have a proof ? or did he simply know it to be true, the idea of taking mass and multiplying by velocity and naming the product "momentum" or as he called it " motion" doesn't appear to be groundbreaking, I may be wrong but if newton truly discovered the third law independently of others then I would imagine it was as a young child, it is what he did with it that was monumental,

January 12th, 2015, 04:55 AM  #4 
Senior Member Joined: Apr 2014 From: Glasgow Posts: 2,068 Thanks: 692 Math Focus: Physics, mathematical modelling, numerical and computational solutions 
I think it's fair to say that many scholars at the time worked on problems such as inertia, motion and forces, but it wasn't until Newton's "Principia" that all of the literature at the time was put together into a coherent picture. There are many such review articles in the literature, on all sorts of topics, albeit that most of them do not have such historic importance or impact.

January 12th, 2015, 05:37 AM  #5 
Newbie Joined: Jan 2015 From: uk Posts: 8 Thanks: 0 
I totally agree with you, newton would have had plenty of opportunity to iceskate during his childhood considering the climate of his time, with his inquiring mind it's not hard to see how the third law would not have occurred to him early on, and possibly many others, did he discover the third law independently of others? probably, was it well known before him though? I don't know, does the law appear in any other papers before his time?


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