Talk:Newton's laws of motion/Archive 1

message: 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)
thread_title:  Vandalism 

message: 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)
replies: text: 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)
thread_title: Energy required to apply a force

message: 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)
thread_title:  additional useful information 

message: 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)
thread_title:  "Strong form" of Newton's Third Law? 

message: 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)
thread_title:  Alternate Expression of the Third Law 

message: 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)
thread_title:  More on the third law 

message: 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)
thread_title:  What does this mean? 

message: 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)
thread_title:  Second Law ambiguity 

message: 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)
thread_title:  Second Law comments 

message: 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)
replies: text: 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)
thread_title:  Non-classical mechanics 

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

text: 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)
thread_title:  sum of Newton's Laws 

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

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

message: 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. 
</br>--Aïki 04:01, 21 January 2006 (UTC)
thread_title: English-language sources vs foreign-language sources