Talk:Newton's laws of motion/Archive 1

Original format?
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?


 * Do you mean as Newton first wrote them in Philosophiae Naturalis Principia Mathematica 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 English. Gandalf61 10:40, Dec 13, 2004 (UTC)

Vandalism
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. --Nicop 16:01, 8 Nov 2004 (UTC)

Energy required to apply a force
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 need energy to apply force?)

-Shreya
 * A more interesting question is: how does the desk know what force to exert on your hand?--Light current 10:30, 26 October 2005 (UTC)


 * 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 Wikipedia article on Newton's laws of motion.  If you have comments about how the article could be improved, this is the place.  --Doradus 14:11, 26 September 2005 (UTC)

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.

Eponyms
Took out laws of inertia as a synonym. It is very misleading. The first law is the law of inertia. The plural term is not standard usage in physics. -- Decumanus

Newton's second law of motion
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 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.


 * 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 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. Gandalf61 15:11, Mar 9, 2004 (UTC)

Newton's Fifth Law: "Please don't put stupid stuff in here, DJSupreme23."

additional useful information
1.when a person of mass m climbs up a rope with acceleration a, the tension in the rope is          T= m(g+a) -varun nehru

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.

I think also that it does not make sens to say that "dv/dt = 0"

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. --Nicop 21:08, 8 Dec 2004 (UTC)

"Strong form" of Newton's Third Law?
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 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....) novakyu 00:33, 9 Dec 2004 (UTC)

Alternate Expression of the Third Law
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):


 * A force acts between two objects such that each object experiences an equal but opposite amount of force.

Why I think this is better:
 * Key idea: Forces act between objects = 1 force, 2 objects
 * 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 rather than N3L.

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 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...!

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 so much easier to use, learn and teach N3L! Hope this helps! WikiJon 18:52, 4 January 2006 (UTC)

Third Law
The article says the following for Newton's third law:

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 objects, so it doesn't make sense physically to say that these two forces add up to zero.

I came to Wikipedia 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 :)

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?

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?


 * 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 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.


 * 1a. 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.


 * 2a. Yes, the pusher should move back. I suppose you must be stabalizing yourself.

Right, the forces add up to 0, but the accelerations do not. The ratio of the accelerations is the inverse ratio of the masses.

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.

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? Pfalstad 04:04, 30 October 2005 (UTC)

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". 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.


 * "...I tried doing this with somebody it doesn't happen in practice..."
 * 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 backwards.
 * --jsnx 04:04, 12 February 2006 (UTC)

More on the third law
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 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. Pfalstad 04:04, 30 October 2005 (UTC)

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 example where the forces do not lie along the centre line? Stuart White

Newtons first law
Can anyone give a reference to this being called the Law of Inertia. I dont think it was (but I could be wrong)--Light current 23:08, 25 October 2005 (UTC)


 * 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.
 * Webster's Third New International Dictionary
 * Paul E. Tippens, Applied Physics, Mc-Graw-Hill, 1973, p. 26
 * Gene Nygaard 03:22, 26 October 2005 (UTC)

OK It was called the law of inertia in the old days when no one knew what it was.--Light current 10:00, 26 October 2005 (UTC)


 * No. Gene Nygaard 13:57, 26 October 2005 (UTC)

How do you mean, No? It still is?--Light current 17:53, 26 October 2005 (UTC)


 * 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.  Pfalstad 03:10, 30 October 2005 (UTC)

OK Paul you add it. --Light current 03:34, 30 October 2005 (UTC)


 * It's there already. Pfalstad 04:18, 30 October 2005 (UTC)

Yes, I realised it was soon as I'd 'saved page'!--Light current 04:22, 30 October 2005 (UTC)


 * 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 Newton's first law of motion.' --Aïki 01:37, 17 January 2006 (UTC)

Newton's law of gravity/conservation of momentum
Moved here- doesnt fit into laws of motion

Newton's laws of gravity, together with his law of universal gravitation and the mathematical techniques of calculus, 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 fluids; projectiles; motion on an inclined plane; motion of a pendulum; the tides; the orbits of the Moon and the planets. The law of conservation of momentum, which Newton derived as a corollary of his second and third laws, was the first conservation law to be discovered.

What does this mean?
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 because of friction. But Galileo's experiments, with a ball rolling down an inclined plane, found that "Things travel naturally at a steady speed (which may or may not be zero), if left alone".


 * 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 friction (except for celestial objects, which were deemed perfect)."  The inclined plane part isn't relevant here, I think.  And the rest is duplication.  Pfalstad 03:08, 30 October 2005 (UTC)

OK thanks. Ive put your version back in now.--Light current 04:20, 30 October 2005 (UTC)

Second Law ambiguity
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.


 * Agree. I changed it.--Light current 01:14, 1 November 2005 (UTC)

Second Law comments
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 with the second, implies that two objects acting upon each other via force will, over time, exchange equal amounts of momentum. Zeroparallax 07:39, 3 February 2006 (UTC)

Poetry
rm from page:
 * Nature and Nature's laws lay hid in night;
 * God said, Let Newton be! And all was light. &mdash;Alexander Pope

"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"

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".

A reader

Non-classical mechanics
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 for "modern", or is it a category that includes all types of mechanics other than classical, which includes modern and also some other things?


 * Relativistic is not classical. Quantum mechanics is not classical--Light current 22:50, 5 November 2005 (UTC)


 * 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".  Pfalstad 23:05, 5 November 2005 (UTC)

sum of Newton's Laws
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? --MarSch 12:30, 7 November 2005 (UTC)


 * Would you care to say what you mean by predictions?--Light current 14:42, 7 November 2005 (UTC)


 * I mean that if I tell you the position and momentum of two point particles, you couldn't _predict_ their time evolution.--MarSch 15:14, 7 November 2005 (UTC)


 * 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 can predict the complete future in Newtonian mechanics. --Light current 22:10, 7 November 2005 (UTC)

Newton first law, original translation in english
Source:  --Aïki 19:40, 15 January 2006 (UTC)

Latin and english
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. That doesn't correspond at all ! --Aïki 02:00, 17 January 2006 (UTC)

English-language sources vs foreign-language sources
Because this is the English Wikipedia, English-language sources should be given whenever possible, and should always be used in preference to foreign-language sources of equal calibre. --Aïki 04:01, 21 January 2006 (UTC)