# Talk:Newton's laws of motion

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## Newton's second law

Contrary to what is said in the article, Newton's second law does not state an equality of force and change in motion but rather a geometric proportionality. Note that geometric proportionality connects natural entities such as force and change in motion only insofar as they are of a different kind (see Newton, Principia (ed. 1713), Book 1 Sect. 1, Scholium after Lemma X). Consequently, the Newtonian interrelation between force and change in motion can never result in just '1' or any other mere number. Ed Dellian2003:D2:9705:8928:892A:6C7B:3FA0:A602 (talk) 15:28, 21 April 2022 (UTC)

Please phrase your edit request using particular suggested changes to the article with reference to reliable sources which support that change. This message does not make it clear what change you are suggesting or why. - Astrophobe (talk) 19:03, 21 April 2022 (UTC)
"Newton's second law does not state an equality of force and change in motion but rather a geometric proportionality." Really? The authors of hundreds of reliable published sources will disagree. Considering the nature of Ed Dellian's claim it is clearly unsatisfactory to say to readers "See Newton's Principia, Book 1, Sect 1" etc. If Ed wants to be taken seriously he needs to provide the essential text for readers to peruse, not imagine that all readers have a copy of an English translation of the Principia on their bookshelves. Dolphin (t) 12:36, 22 April 2022 (UTC)
The Principia is accessible online, eg gbooks relevant passage. The Scholium referred to seems to me to be merely Newton defining what is meant by proportionality. I'm not sure how anything deeper is to be read into that. SpinningSpark 12:53, 22 April 2022 (UTC)
The changes between Newton's original statements and what we now consider "Newtonian" mechanics are a rabbit hole of considerable depth. It's true that Newton himself didn't write the second law as ${\displaystyle F=ma}$, for example, that being due to Hermann and Euler some time later [1]. It's also true that getting deep into that in Section 1 of an article like this would be a pointless distraction. XOR'easter (talk) 17:13, 22 April 2022 (UTC)
Once again: It is not true that Newton stated an equality of force and change of motion in the second law. The law reads in Newton's original Latin: "Mutationem motus proportionalem esse vi motrici impressae". In English: The change in motion is proportional to the impressed force. "Proportional" is not "equal". Proportionality belongs to geometry, equality belongs to arithmetic. If A and B are proportional, you get A/B = C = constant. If A and B are equal, you get A = B; no constant! This is so clear that it would be up to the dissenters to prove me wrong. In any case it would be correct here to write that the formula F = ma is not Newton's but Euler's. He introduced it to the scientific world in Berlin on Sept. 3, 1750, as his "Découverte d'un nouveau principe de Mécanique" (see Mem. Acad. Roy. Sci. Berlin vol. 6 1750 (1752) pp. 185-217). 2003:D2:971D:DF15:110B:B215:C49C:BD45 (talk) 13:53, 1 January 2023 (UTC)
The standard for inclusion in Wikipedia is verifiability; not truth. Wikipedia’s mission is to present information that is provided in reliable, published sources. It is not part of Wikipedia’s mission to arbitrate on what is correct, and what is not. The information published here about Newton’s second law is taken from reliable published sources. The best place for you to raise your concerns is in a peer-reviewed journal; if your ideas are accepted by the Physics community they will quickly find their way into reliable published sources, and ultimately into encyclopaedias. Dolphin (t) 00:01, 2 January 2023 (UTC)
Thank you, Dolphin. Well, I would say the best source to inform about Newton's formulation of the second law is Newton's Principia. The correct Wikipedia information of the public would be to point to the fact that the secondary sources attribute to Newton a "second law" which however is not his but Euler's (according to the quoted primary sources). To assert that Newton (Newton! Not Euler?) put change in motion and impressed force "equal" is simply false, even though one finds this evident misinterpretation in every physics textbook around the world. 2003:D2:971D:DF15:110B:B215:C49C:BD45 (talk) 07:23, 2 January 2023 (UTC)
Read WP:PRIMARY, particularly policy #4. SpinningSpark 14:33, 2 January 2023 (UTC)
The relevant source for what an author has written is the author's work, of course, at least if there is no space for "interpretation". And there is none, since Newton's wording "proportional" does certainly not mean "equivalent" or "equal" according to the generally accepted mathematical language. Nevertheless, I can even refer to a prominent "secondary source": Max Jammer, Concepts of Mass in Contemporary Physics and Philosophy, Princeton University Press, Princeton (NJ), 2000, pp. 5, 12, 17. ("Newton's second law, in Euler's formulation..."). 2003:D2:971D:DF51:1DFA:78C4:D591:5836 (talk) 17:58, 2 January 2023 (UTC)
The issue here is not what Newton said, but what is meant by the modern conception of the law. You cannot cite Newton for that. Nor can you use Newton to interpret the law – because Newton is primary. Having said that, I tend to agree with you. When I was at school, we would always say "proprtional" because with the system of units in use at the time there really was a constant of proportionality in the 2nd law. Actually, there still is with SI units. It's just hidden because it happens to be unity. SpinningSpark 14:57, 3 January 2023 (UTC)
We agree that there is a modern conception of the law for which one cannot cite Newton (one could cite Euler). It is my point of criticism, however, that the article does just that, citing Newton, that is, by writing "When a body is acted upon by a force, the time rate of change of its momentum equals the force". This to attribute to Newton is simply wrong. I wonder why Wikipedia doesn't clear the point by informing the user that Newton's authentic second law differs (in several respects) from its modern conception?- By the way, Wikipedia is certainly not the place to discuss the question what Newton really meant with his authentic second law. I just want to point out that I have been publishing a lot on this question since 1985, in German and in English (cf. my paper "Inertia the innate force of matter a legacy from Newton to modern physics", in P. B. Scheurer and G. Debrock, Newton's Scientific and Philosophical Legacy, Kluwer Academic Publishers, 1988, pp. 227-237). So I do know that you are absolute right: One cannot cite Newton for what is erroneously called "Newton's second law" in modern textbooks! 2003:D2:971D:DF51:1DFA:78C4:D591:5836 (talk) 09:09, 4 January 2023 (UTC)
Again, you can't cite Euler as the orginator of the F=ma form. You need an independent source to verify that. However, note that the article already discusses Euler's role at Newton's laws of motion#After the Principia. SpinningSpark 10:33, 4 January 2023 (UTC)
The matter is - as you already have admitted! - that "one cannot cite Newton" for the modern conception of the "second law". This, and only this, is my point of criticism. - As to Euler, by the way, I have already cited the independent source you ask me for: It is Max Jammer, Concepts of Mass in Contemporary Physics and Philosophy. Here is another one: Giulio Maltese, La Storia di 'F = ma' ", Firenze 1992. On p. 197 he writes: "La seconda legge del moto in forma moderna fu enunciato per la prima volta nel 1750 da Eulero". In English: The second law of motion in modern form was for the first time published by Euler in 1750. 2003:D2:971D:DF51:1DFA:78C4:D591:5836 (talk) 11:21, 4 January 2023 (UTC)
I'm not disputing this can be cited, I was replying to your statement "one can cite Euler" for something alleged to be due to Euler. But I now see that does not really seem to be your point. Perhaps you didn't mean "cite" in the sense of a verifying reference. If I understand you correctly, you are objecting to F=ma being called Newton's 2nd law at all. Well, get over it, rightly or wrongly that is what it is called. Our guideline WP:COMMONNAME and the essay section WP:Righting Great Wrongs are relevant here. SpinningSpark 12:16, 4 January 2023 (UTC)
There is a difference between a formula "being called Newton's second law", or "being Newton's second law". My point of criticism is that the WP article insinuates the latter (contrary to better knowledge). - I confess it my "fault" to believe that a primary source like Newton's "Principia" would serve best to show what Newton actually has written. So I must look for secondary sources, which, however, is not really a problem. There are many many many sources that state, for instance, what can be read in the most prominent American historian of science, the late I. B. Cohen's, "Guide to Newton's Principia" (published together with his and Anne Whitman's most prominent modern English Principia-Edition, Berkeley 1999). On p. 111-117 Cohen demonstrates in all detail what he initially says, namely that "Newton's second law ... sets forth a proportionality between 'force' and the resulting 'change in motion'" (my emphasis). Therefore, to insinuate that the equality F = d(mv)/dt would be, or "is" Newton's law, is equally false as to say that, for instance, Planck's formula E = hf would read "energy equals frequency". 2003:D2:971D:DF51:1DFA:78C4:D591:5836 (talk) 14:23, 4 January 2023 (UTC)

## Third law

"For example, consider a book at rest upon a table. The Earth's gravity pulls down upon the book. The "reaction" to that "action" is not the support force from the table holding up the book, but the gravitational pull of the book acting on the Earth"

What if we remove the table? — Preceding unsigned comment added by Tireatute (talkcontribs) 09:52, 23 May 2022 (UTC)

My "opinion":

1. "at rest": the book is not "at rest" on the table regarding gravity. It is "at rest" in horizontally directions and "stationary" in vertical direction, being equally subject to gravity in one direction and to the resistance of the table in the opposite direction.

2. Actually; the "gravitational pull" allegedly opposing to the fall of the book is the negation of gravity, as it is considered to be equal to it and acting in the opposite direction: G- Gp = 0

3. If we remove the table, the book will fall to the floor, on which it will stop and stay stationary. During its fall (in the void), its resistance to it is only proportinal to its inertia / own mass. — Preceding unsigned comment added by Tireatute (talkcontribs) 10:21, 23 May 2022 (UTC)

“What if we remove the table?” No problem, the situation remains unchanged. The book accelerates towards the centre of the Earth in response to the force of gravity (weight) that the Earth exerts on the book, and the book exerts a similar force (identical in magnitude but opposite in direction) on the Earth. Newton’s third law of motion applies to all forces in all situations regardless of whether the two bodies are separated by a distance that is constant or changing at a fixed rate or accelerating. Dolphin (t) 13:02, 10 July 2022 (UTC)
Woops i think someone forgot basic structure design here ;)
I'd say whatever is under the book keeps it from falling down any further because the materials it is made of are offering a resistance equal to -or greater than Earth's gravity without breaking apart. I am not saying that the mass of the book doesn't generates a gravitational pull of totally neglectable magnitude, it does, but when you think of it, the book's own gravity pull does not prevent the book from falling down further, quite the contrary: it adds to the gravity pull of the Earth on the book (the book tries to pull the Earth upwards wich makes the total force of the attraction slightly greater than without the book). I think the book example in the article is meant to be an analogy to the Moon or something in orbit but sorry, badly chosen for that in my opinion: something in orbit has other means of resisting to Earth's gravity pull than a table !
We could take the book + table example further (out of topic you could say) and explain that the floor under the table also plays a role, then again this floor is held by the ground and eventually it comes down to that one thing: does the ground breaks under the total weight of the "structure" or is it capable of resisting it. You need to do some stress tests to determinate how much pressure (pascal or other unit) it can withstand and design the foundations accordingly. This design process should suffice without having to take into account the gravity pull the whole structure is exerting on earth. Plus you should take G=10m per sec just as a precaution, since 9.81 or 9.80xxx is actually only a matter of norms and doesn't represent the exact mesure at the structure's location (in English: depending on where you mesure gravity at ground level on Earth, you might get different values, so you take the worst possible value as a security factor). But again: totally out of topic. 80.215.157.141 (talk) 20:14, 12 July 2022 (UTC)
I'm sorry to say I'm not able to follow this discussion or how it relates to the contents of the page. What specific change is being proposed to this article, and why? - Astrophobe (talk) 22:11, 12 July 2022 (UTC)
This discussion thread begins with a quote from our article: “For example, consider a book at rest on a table.” The quoted sentences are correct and accurate. No change to the article is required or warranted. All good. Dolphin (t) 05:29, 13 July 2022 (UTC)

## The Incomplete Newton's Third Law of Motion

The weak form of the Newton's third law of motion ignores one of the independent aspect of the law. It's collinearity. The forces between the two objects must not only be equal in magnitude, opposite in direction, but also must be along the line connecting the representative points of the two particles. The Newton's third law says that the objects do not exert forces on each other in such a way that the forces are not collinear. Natha.rahul (talk) 16:44, 12 August 2022 (UTC)

The article introduces the laws in terms of pointlike or particle masses, so collinearity is guaranteed. Then it explains thinking about extended bodies as collections of particles, and what the center of mass is. I don't think we need to elaborate more. XOR'easter (talk) 17:17, 12 August 2022 (UTC)
Wikipedia acknowledges Euler's laws of motion which are applicable to rigid bodies. Euler’s laws play a role because Newton’s laws of motion are stated to apply only to particles. Collinearity is therefore an important constraint in Euler’s laws of motion. Dolphin (t) 02:35, 13 August 2022 (UTC)
That article does not mention collinearity anywhere, but it does discuss torque, which is definitely not collinear. SpinningSpark 11:35, 13 August 2022 (UTC)
And as it says, Euler's laws can be derived by integrating Newton's laws over particle distributions. XOR'easter (talk) 16:14, 13 August 2022 (UTC)

## Article needs simplification

The laws here are explained as if the reader is an undergraduate, while most people that read this article are either teachers or primary/secondary/high school students. No wonder why people hate physics. The information here should be presented in the article in a more reader-friendly manner. CactiStaccingCrane (talk) 18:56, 14 August 2022 (UTC)

Just to be clear, the technical info should NOT be removed, but that the explanation should be more gradual and accessible. See also: WP:TECHNICAL. CactiStaccingCrane (talk) 18:57, 14 August 2022 (UTC)
Why shouldn't teachers be expected to understand material taught in the first year of college? For that matter, why are we out of compliance with WP:GENERAL-ADVICE-TAKEN-AS-GOSPEL-BECAUSE-IT-HAS-A-CAPITALIZED-SHORTCUT if the intro is written at a high-school level and the article gets somewhat more advanced from there? Nothing in the discussion of derivatives, vectors, etc., would be out of place in AP Physics. It might be more terse than an AP Physics textbook, but it's an encyclopedia article, not a handholding introduction from scratch. (Indeed, policy forbids a lot of the kind of writing which that would entail.) XOR'easter (talk) 19:38, 14 August 2022 (UTC)
I agree. However, I think our explanation on the article right now isn't the best as it can be. CactiStaccingCrane (talk) 09:49, 15 August 2022 (UTC)
Simplification kills meaning - it's a wrong concept that everything should be explained in the level of 5 years old child. It's an encyclopedia article, it should be rigor in its explanation. And yes, every teacher should understand derivatives, if that's the only problem with the article. Artem.G (talk) 20:24, 14 August 2022 (UTC)
I acknowledge the principles at WP:Make technical articles understandable but we are also reminded that Wikipedia is not a textbook or guide book – see WP:NOTGUIDE. User:CactiStaccingCrane should also become familiar with the Simple English Wikipedia; it may be better suited to the primary and secondary students they have in mind. The Simple English Wikipedia has an excellent article titled "Newton's Laws of Motion". Dolphin (t) 04:41, 15 August 2022 (UTC)

## Incorrect statement of the Third Law

The statement "If two bodies exert forces on each other, these forces have the same magnitude but opposite directions" does not rule out the possibility that only one body exerts a force on another. The traditional statement is that "for every action," etc., (with no exception). The statement should begin with something like "if one body exerts a force on another...." Marty39 (talk) 17:57, 26 December 2022 (UTC)

I agree. The cited source is Thornton and Marion (2004). Can someone clarify exactly what is said in this source? Dolphin (t) 21:28, 26 December 2022 (UTC)
The direct quote from Thornton and Marion is If two bodies exert forces on each other, these forces are equal in magnitude and opposite in direction. They follow it up with a discussion of the subtleties that arise when you have fields carrying momentum. Their choice of phrasing seems rather deliberate. For a long time, the statement of the laws in the intro here was lifted from them verbatim (attributed, but with no indication that the sentences were taken word-for-word). They've been lightly edited since. XOR'easter (talk) 23:59, 9 January 2023 (UTC)