Talk:Newton's laws of motion

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Figure skaters and 3rd law[edit]

hi, I wonder if the figure skater diagram is optimal? It doesn't indicate any forces: are they holding hands while spinning? are they (about) to push apart? Are there any forces at all involved?

Also, the key 3rd law concept is that of opposite laws: An ideal diagram would show one person pushing the other, and experiencing a force back into their body (the opposite reaction, and also the correct image for a free body diagram... ojryosryun9zeryjzu9tja9*


I removed the following as the conservation of energy article disproves it: Conservation of energy was discovered nearly two centuries after Newton's lifetime, the long delay occurring because of the difficulty in understanding the role of microscopic and invisible forms of energy such as heat and infra-red light.

Newton's 3rd law actually discovered by Leonardo da Vinci[edit]

I can't seem to edit the article... can someone add that Newton's 3rd law was actually discovered by Leonardo da Vinci? Just google 'da vinci 3rd law' for a bunch of sources

The role of the 3rd law in self-consistency with respect to scaling up from subsystem to system[edit]

The question of what Newton's laws actually apply to (fundamental bodies or extended systems) should be addressed. Definitions I and II in the Principia characterize bodies as extended objects whose mass and momentum are (what we in the current era would call) the volume integrals respective of density and density multiplied by velocity; and Definition IV described the force applied to the body as the total external force (or "impressed" force) applied to its parts.

It needs to be noted that this raises the issue of self-consistency: does a system, whose parts each satisfy Newton's laws, also satisfy those laws, and if so, what mass, momentum, position and velocity is to be ascribed to the system that the laws are to be taken with respect to.

In this context, (1) the first law can also be characterized as a "no self-force" law; (2) the underlying additivity assumption for mass, mass moment, momentum and force needs to be noted (with references to relevant parts of the Principia), (3) the role should also be noted that the third law plays for self-consistency with respect to scaling up from subsystems to systems, by ensuring that the internal forces within a system do not produce a net self-force (so that the only force that acts on the system is the sum of the "impressed" forces), (4) that the requirement of self-consistency also implies the third law and (5) the first law becomes a special case of the third law, when the latter is generalized to also include the case where the two bodies in question are one and the same. (It's not entirely clear whether the original wording of the third law already does so or not.)

Finally, there is the ambiguity inherent in Third Law's statement: is it meant to also assert that the forces acting between two subsystems act along the line separating the two. The wording in the Principia (particularly with the examples raised), tends to suggest this but leaves the issue open. It is required for self-consistency with respect to the matter of angular momentum and self-torques (which in turn has a bearing on Kepler's law of Area, which Newton used as one of the motivations for his work).

The question of ambiguity of the Third Law has been noted in the literature. References to the different interpretations should be provided, and at the very least, a distinction should be noted between a "strong" form of the Third Law (equal, opposite and directed toward or away from each other) versus the "weak" form (equal, opposite, but not necessarily directed toward or away from each other). It hinges partly on whether the extra clause "et in partes contrarias dirigi" ("directed to contrary parts") in the Third Law is meant as "directed toward or away from each other" or not. A possible reference on this may be V. F. Lenzen, Isis, Vol. 27, No. 2 (Aug., 1937), pp. 258-260; though I don't have access to it.

Action and reaction with velocity[edit]

Newton's original Latin reads:

Hisce volui tantum ostendere quam late pateat, quamq; certa sit Lex tertia motus. Nam si aestimetur Agentis actio ex ejus vi et velocitate conjunctim; et Resistentis reactio ex ejus partium singularum velocitatibus et viribus resistendi ab earum attritione, cohaesione, pondere et acceleratione oriundis; erunt actio et reactio, in omni instrumentorum usu, sibi invicem semper aequales. Et quatenus actio propagatur per instrumentum et ultimo imprimitur in corpus omne resistens, ejus ultima determinatio determinationi reactionis semper erit contraria.

from Philosophiae Naturalis Principia Mathematica, Axiomata, sive Leges Motus, P.24,_sive_Leges_Motus

Translated to English, this reads:

I was only willing to show by those examples the great extent and certainty of the third Law of motion. For if we estimate the action of the agent from its force and velocity conjunctly, and likewise the reaction of the impediment conjunctly from the velocities of its several parts, and from the forces of resistance arising from the attrition, cohesion, weight, and acceleration of those parts, the action and reaction in the use of all sorts of machines will be found always equal to one another. And so far as the action is propagated by the intervening instruments, and at last impressed upon the resisting body, the ultimate determination of the action will be always contrary to the determination of the reaction.

Semi-protected edit request on 12 October 2021[edit]

Newton’s first law: the law of inertia Newton’s first law states that if a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at constant speed unless it is acted upon by a force. In fact, in classical Newtonian mechanics, there is no important distinction between rest and uniform motion in a straight line; they may be regarded as the same state of motion seen by different observers, one moving at the same velocity as the particle and the other moving at constant velocity with respect to the particle. This postulate is known as the law of inertia. (talk) 11:19, 12 October 2021 (UTC)

 Not done: it's not clear what changes you want to be made. Please mention the specific changes in a "change X to Y" format and provide a reliable source if appropriate. ScottishFinnishRadish (talk) 11:28, 12 October 2021 (UTC)
  1. ^ Smith, Julius. "Newton's Three Laws". Newton's Three Law Of Motions.