- #1

Cleonis

Gold Member

- 691

- 7

## Main Question or Discussion Point

Let me propose a list of principles of classical dynamics, specifically designed for

- In the absence of any force: objects in motion move along straight lines, covering equal distances in equal intervals of time

- Composition of motion: position vectors, velocity vectors and acceleration vectors add according to the rules of vector addition in Euclidean space

- Time reversal invariance

- F=m*a

I'm not claiming that the above list hits the nail on the head, but I do think it's an improvement over Newton's formulation of the Three laws. We learned a lot over the past centuries, and we are in a position to improve on how the principles are formulated.

I believe that the third item, time reversal invariance, is an absolute must. Nöther's theorem links symmetries in physics theories to conserved quantities; in newtonian dynamics time reversal invariance is correlated with energy conservation.

In the Principia Newton asserts the Third Law as a blanket approach to statics and dynamics. I am highly skeptical about that conflation of statics and dynamics. From here on, when I refer to the third law, I mean

Perfectly elastic collision is time reversal invariant, as a matter of principle. I take it as obvious that in a Universe in which the third law doesn't exist collisions won't be time reversal invariant. Does that also work the other way round? In a Universe with time reversal invariant laws of motion, does it follow logically that the third law must hold good? I think so, but I'm interested in comments.

While newtonian physics is exuberantly alive, I believe that Newton's original formulation of the Three Laws is badly outdated. In retrospect we see that Newton's First law and Third Law are assertions of symmetry principles. Those principles are more profoundly expressed as symmetries of space and time. Novices are confused by the Third law because the Third law is so awkward.

Cleonis

*education*, for introduction to novices:- In the absence of any force: objects in motion move along straight lines, covering equal distances in equal intervals of time

- Composition of motion: position vectors, velocity vectors and acceleration vectors add according to the rules of vector addition in Euclidean space

- Time reversal invariance

- F=m*a

I'm not claiming that the above list hits the nail on the head, but I do think it's an improvement over Newton's formulation of the Three laws. We learned a lot over the past centuries, and we are in a position to improve on how the principles are formulated.

I believe that the third item, time reversal invariance, is an absolute must. Nöther's theorem links symmetries in physics theories to conserved quantities; in newtonian dynamics time reversal invariance is correlated with energy conservation.

In the Principia Newton asserts the Third Law as a blanket approach to statics and dynamics. I am highly skeptical about that conflation of statics and dynamics. From here on, when I refer to the third law, I mean

*exclusively*its application in*dynamics*.Perfectly elastic collision is time reversal invariant, as a matter of principle. I take it as obvious that in a Universe in which the third law doesn't exist collisions won't be time reversal invariant. Does that also work the other way round? In a Universe with time reversal invariant laws of motion, does it follow logically that the third law must hold good? I think so, but I'm interested in comments.

While newtonian physics is exuberantly alive, I believe that Newton's original formulation of the Three Laws is badly outdated. In retrospect we see that Newton's First law and Third Law are assertions of symmetry principles. Those principles are more profoundly expressed as symmetries of space and time. Novices are confused by the Third law because the Third law is so awkward.

Cleonis