# Newton's 2nd - assumption or observation?

1. Jul 3, 2010

### AJ Bentley

For some reason, I've seen several recent questions that can be answered by simply pointing to Newton's 3rd law (Action and Reaction).

Then I stopped to think. The first law is observational. We've all seen enough (sort of) practical verifications of things not stopping or refusing to start. So I think we can say it's an observed fact about the universe.

But the 3rd seems to me to be fundamentally unverifiable. It's a sort-of err... fact.

Does that mean it has to be downgraded to an assumption?

Newton's first assumption.

Last edited: Jul 3, 2010
2. Jul 3, 2010

Fundamentally unverifiable? Well, take two objects, one heavier than the other. Push them with the same force. Do you recognize a law here?

3. Jul 3, 2010

Then, change the force. Does it affect the motion somehow?

4. Jul 3, 2010

### Staff: Mentor

Action and reaction is Newton's 3rd. But I would say that Newton's laws are the postulates of Newtonian mechanics. As such, they are all assumptions whose logical conclusions are to be checked against experiment.

5. Jul 3, 2010

### AJ Bentley

Ooops I meant 3rd of course.
(Always have trouble with names)

What I mean is, I can't see a way to check this one against experiment.

The nearest I can get is to say - put something between the jaws of a vice and squeeze it hard, then look to see if it moves.

But then you have to show that the two jaws of the vice are squeezing with equal force.

You see the dilemma?

6. Jul 3, 2010

### Unit

Appealing to ignorance you could argue, when has Newton's 3rd law failed?

7. Jul 3, 2010

### Staff: Mentor

Newton's 3rd law is the conservation of momentum. It is pretty easy to check experimentally. Here is a typical Freshman-level physics lab experiment:
http://sdsu-physics.org/physics_lab/p182A_labs/indi_labs/Momentum.pdf [Broken]

Last edited by a moderator: May 4, 2017
8. Jul 3, 2010

### The riddler

I don't know what you mean when you say Newtons 3rd law is unverifiable and unobservable, i can think of many examples in physics where two bodies are causing an equal and opposite force on each other. I shall show one of them below.

The Gravitational force between any two bodies (of anysize) is always equal. This means that the force the Earth has on an apple (causing it to fall) is equal to the force the apple has on the earth. This means that not only does the apple fall towards the Earth but the Earth also (very slightly) falls towards the apple.

Many various examples of Newtons 3rd law (like the one above) have been observed in space and fit perfectly into the laws of physics, this law is as verified and observable as any other of Newtons laws.

9. Jul 3, 2010

### AJ Bentley

Re: Newton's 3rd - assumption or observation?

I would argue that c-of-m is a consequence of Newtons' law applied to movement.

Obviously, it's perfectly correct, and you can check any one of it's consequences 'till the cows come home' . But the form in which Newton stated it interests me. He was apparently very good at spotting the blindingly obvious that no-one had seen before.

The observation that gravity acts downwards and not up, or sideways was another one.

Here, he's pointing out that when you push something it pushes back. (which ain't obvious if you haven't thought of it - remember all those ladder-against-a-wall problems?).

But as to directly proving it... He obviously left that small task to lesser minds.

10. Jul 3, 2010

### Staff: Mentor

Re: Newton's 3rd - assumption or observation?

Obviously. Isn't that what I just said? If you understand that then I don't get your previous comments about it being experimentally untestable.

11. Jul 3, 2010

### AJ Bentley

I'm talking about an absolute and direct test of the actual statement of the law.

The other two laws can be verified directly - they are almost statements of an experimental procedure in themselves.

1/ Push something with a known force and measure the acceleration.
2/ Get something moving then keep observing the velocity.

But the 3rd - you can't directly measure the reaction independently of the action. You can measure all sorts of things that tell you it must be the same but the direct measure is impossible. You can't measure it because the 'acting' object is in the way and if you move it to get at the reaction, it vanishes!

12. Jul 3, 2010

### Staff: Mentor

You have the order backwards, but how do you know the force in your proposed experiments?

13. Jul 3, 2010

### AJ Bentley

Ah well, that was the next question.

You have to invoke the 3rd law to measure it!

PS I really don't give a *** for the names and numbers, you'll have to forgive me on that.

14. Jul 3, 2010

### Staff: Mentor

Exactly. So you cannot make an "absolute and direct" test of the "actual statement" of the first two laws either.

Understood, but it makes communication a little difficult.

15. Jul 3, 2010

### AJ Bentley

The whole shaky edifice comes tumbling down...

16. Jul 3, 2010

### Staff: Mentor

No. You just have to understand what science does. First, you start with a theory which is a mathematical framework and a set of rules for relating the mathematical framework to the results of experiments. Then, you use the theory to make predictions about the results of experiments, run the experiments, and compare the results to the predictions. If the results of the experiments agree with the predicted then the theory is verified and if the results of the experiments disagree then the theory is falsified.

Often, a theory is inspired by some loosely stated physical principles, often called laws or postulates. The laws or postulates rarely if ever lead unambiguously to a theory. The laws or postulates cannot themselves be tested, only the experimental predictions of the theory. However, if the theory is verified then you have good reason to accept the loosely stated physical principles that inspired the theory (or are at least compatible with the theory).

The edifice is solid and well constructed, but many people misunderstand the architecture.

17. Jul 3, 2010

### AJ Bentley

It was a joke .

Last edited: Jul 3, 2010
18. Jul 3, 2010

### novop

19. Jul 3, 2010

### Staff: Mentor

That is not very polite. But at least novop is amused.

20. Jul 3, 2010

### AJ Bentley

No, I apologise. but at least I deleted it!

I know it's hard to tell who you are talking to on this internet thingie and there's an awful lot of genuine idiots and schoolkids online but I kept hoping you would realise I'm not one of 'em.