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is not available as a constant of proportionality, since it is part of the term 'change of motion'that is proportional itself to the 'force'. It is clear, then, that Newton's law cannot be represented by the F = ma of textbooks. In fact this F = ma is a formula that stems not from Newton but from
Leonhard Euler (Mechanica 1735), and can be traced back to Leibniz's Specimen Dynamicum of 1695. The true interpretation of Newton's law that for the first time takes into consideration the constant of proportionality between 'force' and 'change of motion'can be found in Ed Dellian, Die Newtonische Konstante, Philos. Nat. Vol. 22 Nr. 3 (1985) p. 400.
31:
412:
the article (of a point particle acting on a dipole) is not dealing with point particles; if the dipole is decomposed into two point particles, then the third law applies. But Marion goes on to say that the third law does not apply to moving electric charges, because the force propagates at finite speed. Same with gravity, since the effect of gravity is (very slightly) velocity dependent. Will fix the article if I have time.
298:
Using this proposed expression I have found students grasp N3L almost immediately and have almost none of the usual third law problems traditionally experienced in introductory dynamics courses. This expression also leads much more naturally into more advanced field based concepts - there is now one
129:
Hi all: I have a question. See Newton's 3rd law states that every object will apply an equal and opposite force, right? So if you are pressing your palm against the desk, the desk applies an equal and opp. force on your plam. But where does the desk get energy to create such a force? (Don't we always
744:
In the last section, I think it says something about "both classical and non-classical mechanics". What does "non-classical mechanics" mean? I looked up classical, which in the wikipedia article is contrasted with "modern mechanics", but nowhere can I find the term "non-classical." Is this a synonym
686:
When it is said that the second law implies that "objects interact by exchanging momentum, and they do this via a force," this seems to incorporate the third law. The second law implies that a force applied on an object over an interval of time changes that object's momentum; the third law, coupled
339:
I'll attempt to answer your question. Back to the A and B analogy. The ONLY case in which the equal and opposite forces would cancel out is if A and B had the same mass. Say A exerts a force of 10N on B. B then exerts a force of 10N back on A. However, these forces won't cancel out because A is 20kg
259:
Has anyone ever heard of "strong form" of Newton's third law? I've never heard that term in my physics classes, and considering that it's obeyed so little fundamentally (only by gravity--a version (er, months ago, before I changed it) had electrostatic forces as satisfying the "strong form" but that
203:
Surely the constant of proportionality depends entirely on the units involved (and so is not part of the law itself) ? If mass is in kilos, acceleration is in metres per second^2 and force is in
Newtons then the constant of proportionality is 1 (this is the definition of the Newton). OTOH if mass is
411:
Marion (Classical
Dynamics of Particles and Systems) says: "the third law is not a general law of nature. The law applies only if the force exerted by one (point) object on another (point) object is directed along the line connecting the objects." This disagrees with this article. The example in
198:
Newton's second law states a proportion, not an equivalence between the force, F, and the proportional 'change of motion'. Since Newton defines 'motion' by the product of mass and velocity, mv (Def. 2), the second law reads 'force is proportional (not equal!) to delta(mv). Obviously the mass m then
384:
Sorry I haven't been here for a while...but it's crystal clear now. Maybe you should put this (your answer to my question) in the article. I am in 12th grade and I asked a lot of children in my school (and teachers too) but nobody had a *proper scientific* answer for why the forces don't "cancel".
323:
Newton's third law should not be interpreted as a prediction that forces always cancel, or that equilibrium always exists. When objects A and B interact, the forces referred to are acting on different objects: A's force on B, and B's on A. We add forces acting on the same object, not on different
421:
I am confused by the dipole discussion. Surely the forces have to lie along the line of centres of the bodies involved. With a simple dipole, there are three charges and hence 6 forces involved - in 3 Newton 3 pairs - each pair along the line connected its two particles? Can someone supply an
293:
Since forces are either attractive (like gravity) or repulsive (like the 'normal force' between surfaces) this expression eliminates the confusion over "pairs of forces" which can be associated with two forces acting on a single object (see the student's Third Law discussion below!) which is N1L
620:
In less formal terms, thought that things stood still if you left them alone, that to be at rest was natural, and that movement needed a cause. It would be natural to think thus, as any movement (except for that of celestial objects, which were deemed perfect) that one observes eventually stops
103:
i.e. in Latin ? I can't see much point in that myself. If you mean "as Newton would have written them if he had written them in
English", then any English version is a translation, and I think we might as well show a modern translation rather than a less understandable version in 17th century
334:
2. Similarly take the skater example. It's not really clear (IMHO). Does it mean that if two skaters are standing together and one pushes the other he (the pusher) will also move back? I tried doing this with somebody it doesn't happen in practice. Am I stabilizing myself by someother means?
327:
I came to
Knowledge to clarify the same - why don't the forces just cancel out and become 0? I didn't find this explanation satisfactory. It raised more questions in my mind. N.B. :I'm not trying to debunk the law or anything, just trying to make sure everybody, including me, understands :)
371:
A block sitting on the ground has no acceleration because the forces on it cancel. Gravity is canceled by the elastic force of the floor. The block deforms the floor slightly. It oscillates a bit when you drop the block on it. That is what causes the sound you hear when you drop it.
234:
After the last adaptation of Newton's 1st law (thank to the author who tried the simplification), I have some comments: I thin, even if this is not usual, that one of the expression of the first law should explicitely make a reference to the "reference frame" as it really defines them.
241:
Lastly, I regret somehow that in this article, we rather quickly come and use derivative, some kind of calculus that cold be mentionned but that should be avoided (IMHO) as most people will anyway have forgotten most of their calculus if the ever had learnt any. Thanks.
92:
Wouldn't it be nice to include the laws as he first wrote them in addition to the way they are paraphrased in most modern textbooks? I'd put them in myself but I'm having a hard time finding them anywhere on the internet. Maybe somebody else would have better luck?
204:
in lbf, acceleration is in parsecs per fortnight^2 and force is in dynes, the the constant of proportionality will not be 1. The expression F=ma assumes that the quantities are measured in consistent units which will make the constant of proportionality equal to 1.
340:
and B is 10kg. According to Newton's Second Law, acceleration = force/mass, the more the mass, more force is needed to cause acceleration. So although A WILL be moved a bit by the force, B will be moved twice as much because it only has half the amount of mass.
156:
Energy is required to apply a force over a distance. No energy is required to sustain a force if nothing is moving. Having said that, please note that this page is not meant to be a discussion forum for Newton's laws of motion; it is for discussing the
392:
consider that your body, or at least parts of your body, are moving toward the other skater when you shove them. if your motion toward the other skater is slowed, then you have experienced an opposite force. you've been pushed back if not necessarily
637:
I think it's saying: "Before
Galileo, people agreed with Aristotle that a body's natural state was at rest, and that movement needed a cause. This is understandable, since in everyday experience, moving objects eventually stop because of
260:
isn't true), I'm not sure if it's, er, worth mentioning. Does anyone have a reference that I can look up, or simply take out the reference about "strong form"? (Er, I didn't want to take it out entirely because I wasn't sure....)
501:
Is the concept of "inertia" still used anymore, except in phrases like "moment of inertia"? My physics books don't have "inertia" or "law of inertia" in the index. Either way, we should mention it for historical interest.
385:
This may help clear misconceptions for many people. I think that the line about the action and reaction being on two different bodies is also important and IMHO, it should be in the article alongwith this explanation.
350:. Remember, although the net force on the ground would not be 0, the ground has WAY too much mass to be effected by a block. Remember, a = f/m. If f = 10 and m = 209381, you aren't going to get much acceleration.
666:
Newton's second law (more generally) is not F = m*a or a = F/m. Rather it is d/dt(m*v). That is, the time derivative of the momentum of the system. For a system of constant mass, this reduces to F = m*a.
299:
force corresponding to the one field, and this force / field is more easily visualized as the 'location' of the energy stored in the force bond (i.e. work as the integral of force over distance)....etc...!
762:
It says "classical and non-classical physics". I don't think people use the term "non-classical mechanics". I don't know if "modern physics" would be more appropriate than "non-classical physics".
724:"The second law only has meaning if we are able to assert, in advance, the value of F. Rules for calculating force include Newton's law of universal gravitation, Coulomb's law, and other principles"
779:
It seems to me that the 3 (first?) laws combine to yield a definition of momentum and the conservation of total momentum. This is not enough to make any predictions. So are there any more laws? --
868:
Because this is the
English Knowledge, English-language sources should be given whenever possible, and should always be used in preference to foreign-language sources of equal calibre.
815:
You couldnt know their positions and momentum accurately due to the uncertainty principle.Is this what Ur getting at? But apart from that, why could you not predict? In fact you
544:
Founded in 'College
Physics' by Weber, Manning & White, of 'The Pennsylvania University', McGraw-Hill Book company, at section 3-1: 'This law of inertia is usually called
375:
With the skater example, the friction between you and the ice will allow you to stabilize yourself and push someone without moving. Might work better on roller skates?
302:
The one problem I find is that every text book uses the traditional "pairs of forces" concept and it takes people a while to change their mindset.. but once you do it is
450:
576:, provided for the first time a unified quantitative explanation for a wide range of physical cool such as: the motion of spinning bodies, motion of bodies in
447:
Google "law of inertia" (exact phrase) 158,000 hits. Just glancing at the first hundred, looks like about three-fourths of them deal with Newton's first law.
99:
331:
1. If what the article says is true, then shouldn't a block kept on the floor just "sink" through? Because the net force on the ground wouldn't be 0 right?
851:
In the article, we have: 'Corpus omne perseverare in statu suo...' in latin; and: 'Unless acted upon by an unbalanced ...' as a traduction in english.
642:(except for celestial objects, which were deemed perfect)." The inclined plane part isn't relevant here, I think. And the rest is duplication.
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for "modern", or is it a category that includes all types of mechanics other than classical, which includes modern and also some other things?
727:
Do you mean "is only useful" instead of "has meaning"? Above it was asserted that F=etc. was a definition. Surely a definition "has meaning".
171:
Having said all that, think about the chemical bonds in the material the desk is made of and you'll be on the right track for an answer.
272:
As a physics teacher of 15 years I am looking for feedback on what I think is a much better expression of Newton's Third Law (N3L):
368:
Right, the forces add up to 0, but the accelerations do not. The ratio of the accelerations is the inverse ratio of the masses.
687:
with the second, implies that two objects acting upon each other via force will, over time, exchange equal amounts of momentum.
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I mean that if I tell you the position and momentum of two point particles, you couldn't _predict_ their time evolution.--
117:
The version 09:42, 1 Nov 2004 Gandalf61 is the last good version. I don't know how to revert it back in place. Thanks. --
38:
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on Newton's laws of motion. If you have comments about how the article could be improved, this is the place. --
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A force acts between two objects such that each object experiences an equal but opposite amount of force.
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objects, so it doesn't make sense physically to say that these two forces add up to zero.
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OK It was called the law of inertia in the old days when no one knew what it was.--
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T= m(g+a) -varun nehru
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If you wish to start a new discussion or revive an old one, please do so on the
362:. Yes, the pusher should move back. I suppose you must be stabalizing yourself.
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Newton's Fifth Law: "Please don't put stupid stuff in here, DJSupreme23."
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389:"...I tried doing this with somebody it doesn't happen in practice..."
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Relativistic is not classical. Quantum mechanics is not classical--
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climbs up a rope with acceleration a, the tension in the rope is
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I think also that it does not make sens to say that "dv/dt = 0"
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much easier to use, learn and teach N3L! Hope this helps!
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example where the forces do not lie along the centre line?
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as a synonym. It is very misleading. The first law is the
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Key idea: Forces act between objects = 1 force, 2 objects
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The article says the following for Newton's third law:
187:. The plural term is not standard usage in physics. --
819:
predict the complete future in
Newtonian mechanics. --
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Would you care to say what you mean by predictions?--
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Can anyone give a reference to this being called the
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English-language sources vs foreign-language sources
568:Newton's laws of gravity, together with his law of
532:Yes, I realised it was soon as I'd 'saved page'!--
139:A more interesting question is: how does the desk
830:Newton first law, original translation in english
561:Newton's law of gravity/conservation of momentum
652:OK thanks. Ive put your version back in now.--
437:. I dont think it was (but I could be wrong)--
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629:'s experiments, with a ball rolling down an
451:Webster's Third New International Dictionary
100:Philosophiae Naturalis Principia Mathematica
713:God said, Let Newton be! And all was light.
565:Moved here- doesnt fit into laws of motion
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268:Alternate Expression of the Third Law
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255:"Strong form" of Newton's Third Law?
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853:That doesn't correspond at all ! --
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491:How do you mean, No? It still is?--
143:what force to exert on your hand?--
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18:Talk:Newton's laws of motion
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286:Why I think this is better:
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459:, Mc-Graw-Hill, 1973, p. 26
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671:Agree. I changed it.--
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