Is Newton's 3rd law incorrect?

[jbriggs444]

And why do you think that conservation of angular momentum is not related to Newton's 3d law?

Edit. I explain.
For a system of mass points, the fact the resultant moment of internal forces is zero comes from Newton's 3d law. Conservation of the system's angular momentum can be proved using this fact.

If you assume that all interactions are local then this follows -- the two third law partner forces have equal and opposite moments because their moment arms are necessarily identical.

If you allow for force at a distance then it does not follow immediately. It requires the additional condition that forces at a distance must operate in a direction on the axis between the point particles upon which they act. That additional condition must be met if we demand that the laws of nature be isotropic with respect to direction.

 

[lightarrow]

If you assume that all interactions are local then this follows -- the two third law partner forces have equal and opposite moments because their moment arms are necessarily identical.

If you allow for force at a distance then it does not follow immediately. It requires the additional condition that forces at a distance must operate in a direction on the axis between the point particles upon which they act. That additional condition must be met if we demand that the laws of nature be isotropic with respect to direction.

Certainly. I should have specified that the law of conservation of angular momentum requires that additional condition, thank you to have noticed.

--
lightarrow

 

[DrStupid]

And why do you think that conservation of angular momentum is not related to Newton's 3d law?

The 3rd law conserves momentum but not angular momentum. That would be the case if the forces between two bodies always act parallel to their displacement vector (as jbriggs444 already told you) but Newton's laws of motion do not include such a condition. In fact they do not say anything about angular momentum. Momentum and angular momentum are completely different things.

For a system of mass points, the fact the resultant moment of internal forces is zero comes from Newton's 3d law. Conservation of the system's angular momentum can be proved using this fact.

The fact that the 3rd law can be used to prove something else does not mean that something else is the same topic.

 

[DrStupid]

If you assume that all interactions are local then this follows -- the two third law partner forces have equal and opposite moments because their moment arms are necessarily identical.

This still needs the additional assumption that changes of angular momentum always require forces and moment arms. Does that apply to spin-transfer?

 

[ogg]

Elementary blunder #1: No definition of what is MEANT by "Newton's 3rd Law". It is generally assumed it refers to rigid particles and deals with their dynamics. Newton's 3rd Law is an expression of an underlying conservation law and as Noether proved, a conservation law requires an "equivalent" symmetry (which is an abstract concept rather than a physical "thing"). Elementary blunder #2: failure to maintain strict logical separation between an object (or set of objects) of interest and the "system". What we know: Energy and momentum are conserved locally. (by locally, I mean at distances less than those in which cosmological expansion becomes significant ~ mega-lightyear.) Information is conserved locally. (by locally, I mean within our observable universe, which 'ends' at any and all event horizons).

 

[lightarrow]

Newton's 3rd Law is an expression of an underlying conservation law...

Then it's not a "mathematical trick" as stated in post #23.

--
lightarrow

 

[lightarrow]

The 3rd law conserves momentum but not angular momentum. That would be the case if the forces between two bodies always act parallel to their displacement vector (as jbriggs444 already told you) but Newton's laws of motion do not include such a condition.

I know, infact I wrote that is an additional condition required. Nonetheless, the fact: F_A = -F_B is required for the law of conservation of angular momentum.

In fact they do not say anything about angular momentum. Momentum and angular momentum are completely different things.

You insist on "momentum" (and/or momentum conservation law) but that is not the topic, according to your reasoning: it's "Newton's 3d law".

The fact that the 3rd law can be used to prove something else does not mean that something else is the same topic.

That's wrong. Everything related to Newton's 3d law, especially what you can prove using that law is the same topic.
Anyway I see we can't go anywhere keeping on our discussion, so it's terminated for me and I won't reply to you again here.
Regards.

--
lightarrow

 

[DrStupid]

Nonetheless, the fact: F_A = -F_B is required for the law of conservation of angular momentum.

Could you please provide a prove for this fact?

You insist on "momentum" (and/or momentum conservation law) but that is not the topic, according to your reasoning: it's "Newton's 3d law".

What do you think Newton's 3d law is about if not conservation of momentum?

Everything related to Newton's 3d law, especially what you can prove using that law is the same topic.

I would not even agree if you would show me such a prove because that would open the floodgates to thread-highjacking.