# Prediction of Newton's third law

1. Jun 12, 2015

How did newton predict his third law? If we take the system as the whole universe Newton's third law leads that the net force of the system is zero.Did newton have an insight about this?

2. Jun 12, 2015

### axmls

Newton's third law is simply a postulate. It must simply be accepted, and then the conclusions drawn from it are put to the test. If the test confirms the postulate, then you've got a good postulate.

It's a fairly intuitive one at that. Certainly if I punch a wall, I might do some damage to the wall, but the wall also does damage to me.

3. Jun 12, 2015

### Noctisdark

By observation, like all physical laws, you can try out this expirement by yourself, try you push the wall, you'll notice that it's pushing you too. How do you jump ? You simply apply some force on the ground downward and it pushes you upward ! By repeating the same expirement, one can suggest that for any given force there is some reaction.
Good luck, hope I've helped !

4. Jun 12, 2015

OK.
THANKS YOU BOTH HELPED!

5. Jun 12, 2015

### DrStupid

I think he started from conservation of momentum and the third law is a simple method to keep momentum constant. Of course the next question is: How did Newton predict conservation of momentum? Here I agree with Noctisdark that it results from experimental observations. As nobody ever observed that something changes its momentum without external interactions it is obvious to use this as an universal principle.

6. Jun 12, 2015

### brainpushups

Indeed, I believe this is correct. Newton respected, and was aware of, both Descartes and Huygens work. Both gentlemen (Descartes incorrectly, and Huygens correctly) formulated versions of the principle of conservation of momentum for collisions. It is not a stretch to cast this in terms of 'action' and 'reaction.'

7. Jun 12, 2015

### andresB

Although technically correct, there are infinite postulates possible from where to choose. Generally speaking, one postulate things after some observations and experiments.