Talk:Force/Archive 1

message: Ok, so the force page is going down the same path as the tensor page.  Which is not good.  I propose the following structure:
* brief remarks indicating the force is not an easy thing to define or understand, and that Real Smart People have thought about for a long time.
* elementary definitions useful for mechanics, such as F = ma, with examples,
* an explanation of rational mechanics and the difficulty of defining force rationally.  This would take into account Truesdell and Noll works specifically, with a bow to Hilbert for granting mathematical legitimacy.  The Hamel's work should be dealt with here also.
* discussion on the new configurational forces schemes of Morton Gurtin and Gerard Maugin, and why they are different, and why it matters.
Whew.  
I can take on the Gurtin stuff, I have his monograph.  
I think you miss the point that F=ma is the definition of F, and definitions by themselves have no physical content.
The related physical content is the reification of this defined concept of force.  Please be sure you undestand this distinction before you edit the page further.
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:In most expositions of mechanics, force is usually taken as a primitive, without an explicit definition.  Rather it is taken to be defined implicitly by the (often vague) presentation of the theory within which it is contained.  Various physicists, philosophers and mathematicians, such as Ernst Mach, Clifford Truesdell and Walter Noll have contributed to the intellectual effort of obtaining a more rational, non-circular, and explicit definition of force.
I toned this down a good deal. While I haven't read these authors, there are many contexts in which an implicit definition is both useful and logically rigorous. In math, for example, there are many "undefined terms". (In modern formulations of Geometry, terms like "like" and "point" are such terms; they're "defined" only implicitly, by how they're used.) Thus there's no need to link implicitly defined terms to circularity or irrationality. --Ryguasu 21:39 Nov 19, 2002 (UTC)
-----------------------------------
I have disambuated Force into Force (physics) and Force (law) and started going back and sorting out the links, but there are so many of them that I need some help.  Will anyone who's willing to help please use the 'what links here' on Force to find articles linking here and change those links to the right article; if the topic is one of the many that are a definition of something in physics that don't start out with that qualifier, please add "In physics, " at the beginning, and don't forget to lowercase the next letter, which used to be the initial capital.  (If you run across a link to a different meaning of "force" than in physics or in the law, please either leave it alone, so it comes to this disambiguation page, or define the new one with a parenthetical title and make it go there.) Thanks for any and all assistance. -- isis 09:53 Oct 26, 2002 (UTC)
I'm not sure we should relegate physical force to a disam-type title. I suspect it it by large the most commonly-used meaning of "force" (in terms of links on wikipedia); maybe it should get priority like Newton does. -- Tarquin 10:19 Oct 26, 2002 (UTC)
Please remember that Wikipedia:Wikipedia is not a dictionary. If there really isn't very much to be said about the alternative definitions of "force", then there's no point making a disambiguating page! -- CYD
I think there's a possible encyclopedia article about the concept of force in law, but I agree that by far the overwhelming number of links will want force (physics), so I suggest that force (physics) redirects to force and that that article in its first sentence points out that force (law) exists as well. This is what we typically have been doing if one meaning is much more common than another. AxelBoldt 18:47 Oct 26, 2002 (UTC)
: Moving it back to "Force". -- Tarquin 16:50 Jan 15, 2003 (UTC)
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Shouldn't F=MA be F=mã or F=ma?
And shouldn't
F = Limit as T goes to zero of (mv - mvo)/T
be something like
\vec{F}=\lim_{\Delta t\to 0} {m\vec{v}_{t+\Delta t} -m\vec{v}_{t} \over\Delta t}
Haven't changed that part, in case everyone happens to disagree with me.
LATEΧed 3 lines, using bold for vectors like the articles here seem to do, instead of vector arrows (or underline for typewriters and arrowless document formats), as I thought was normal.
-- Cyp 20:53, 28 Jan 2003 (when I wrote this, but the wikipedia seemed to be broken when I tried to post this.) (Now 0:24, 29 Jan 2003)
Arrows / underlines tend to be used in handwritten documents. Convention for print is bold, AFAIK; older books use arrows -- Tarquin 00:21 Jan 29, 2003 (UTC)
:I note that at least one major physics text book has returned to arrow notation for vectors. Namely, the sixth (and seventh?) edition of Fundamentals of Physics, by Halliday, Resnick and Walker. -- Jason Le Vaillant 08:58 Feb 9, 2004 (UTC)
::Whether they use boldface or arrows to indicate that they are vectors, that F and a should still always be italic as well, should they not? I haven't changed this yet, but likely will when I make some other changes, unless anyone disagrees on this point in which case we can discuss it.  Gene Nygaard 12:41, 12 Dec 2004 (UTC)

message: The stuff that was on this page about an "imperial newton" was about the biggest piece of nonsense I've ever seen in quite some time.  Not because there is no such unit--but nobody calls it by that name.
The unit that was called an "imperial newton" here is, in fact, the poundal.  It already had its own Wikipedia entry.  It can be found in any decent dictionary.  It is part of the very first coherent subset of English mechanical units, the absolute foot-pound-second system introduced back in 1879.
Note that newtons did not exist in 1879.  That unit was only invented 25 years later, and it wasn't officially accepted by the CGPM as the name for the mks unit of force until 1948--nearly 70 years after the poundal was invented.  (However, dynes are older than poundals.  They are the units of force in coherent cgs systems.)
Just go look at any physics or engineering textbook from the first half of the 20th century--there's a pretty good chance you will find it using poundals.  For example, the slug was invented in 1902 by the English physicist A.M. Worthington.  Yet 18 years later, in a later edition of the little book in which he introduced the slug, he was still telling us in A.M. Worthington, Dynamics of Rotation: An Elementary Introduction of Rigid Dynamics, London, New York, Bombay, Calcutta, and Madras: Longmans, Green, and Co., 1920, p. 9:
	"British Absolute Unit of Torque.–Since in the British absolute system, in which the lb. is chosen as the unit of mass, the foot as unit of length, and the second as unit of time, the unit of force is the poundal, it is reasonable and is agreed that the British absolute unit of torque shall be that of a poundal acting at a distance of 1 foot, or (what is the same thing, as regards turning) a couple of which the force is one poundal and the arm one foot.  This we shall call a poundal-foot, thereby distinguishing it from the foot-poundal, which is the British absolute unit of work."
Note also that slugs did not exist in 1879.  They are also a 20th century invention.
There are lots of other problems in the same neighborhood of the article; I'll work on them, too.
Gene Nygaard 08:12, 11 Dec 2004 (UTC)
thread_title:  "Imperial newton" 

message: Icairns first changed my statement as follows, 
In colloquial, non-scientific usage, the "kilograms" used for to measure "weight" are almost always the proper SI units for this purpose. They are units of mass force, not units of force. mass, as might be expected.
Then he edited his own change to read:
In colloquial, non-scientific usage, the "kilogram" used to measure "weight" is almost always the kilogram-force. 
This is totally incorrect.  Here are lots of references in support of my point.
Another relevant factor is the fact that the pound (mass)s which are still used for the same "colloquial, non-scientific" purposes in the United States, and which used to be common in many other parts of the world (and are still used for things such as cattle sales in Canada, for example) are legally defined as units of mass exactly equal to 0.45359237 kg.
Just put your thinking cap on for a minute.  When we buy and sell goods "by weight," as we often do, would it make any sense whatsoever to measure some quantity which varies with the variations in the strenght of local gravity?  No.  Certainly not.  We should not do so, and we do not.  In fact, 
*Nowhere in the world are newtons legal units for the sale of goods.
*Nowhere in the world are pounds-force legal units for the sale of goods.
*Nowhere in the world are kilograms-force legal units for the sale of goods.
Now, let's get into what the experts in the fiels have to say about this:
NIST Special Publication 811 (1995 ed.),Guide for the Use of the International System of Units (SI) by Dr. Barry N. Taylor: [emphasis added]]
:   In commercial and everyday use, and especially in common parlance, weight is usually used as a synonym for mass.  Thus the SI unit of the quantity weight used in this sense is the kilogram (kg) and the verb "to weigh" means "to determine the mass of" or "to have a mass of".
:Examples:  the child's weight is 23 kg
::the briefcase weighs 6 kg 
::Net wt. 227 g
The current National Standard of Canada, CAN/CSA-Z234.1-89 Canadian Metric Practice Guide, January 1989:
:5.7.3  Considerable confusion exists in the use of the term "weight." In commercial and everyday use, the term "weight" nearly always means mass.  In science and technology, "weight" has primarily meant a force due to gravity.  In scientific and technical work, the term "weight" should be replaced by the term "mass" or "force," depending on the application.
:5.7.4  The use of the verb "to weigh" meaning "to determine the mass of," e.g., "I weighed this object and determined its mass to be 5 kg," is correct.
Note a couple of things about these statements:
*That "nearly always" is much stronger than "primarily"—they even got that part right.
*This difference in usage between the noun forms and the verb forms, with the application-specific meanings for the former, and the unqualified "is correct" for the latter.  This is because using to mass with this meaning remains substandard usage which grates on the ears of most people, including many who would not use the noun form for the results of this process.
National Physical Laboratory (the national standards laboratory of the U.K.), NPL FAQ [emphasis added]
:Weight
:In the trading of goods, weight is taken to mean the same as mass, and is measured in kilograms. Scientifically however, it is normal to state that the weight of a body is the gravitational force acting on it and hence it should be measured in newtons, and this force depends on the local acceleration due to gravity. To add to the confusion, a weight (or weightpiece) is a calibrated mass normally made from a dense metal, and weighing is generally defined as a process for determining the mass of an object.
:So, unfortunately, weight has three meanings and care should always be taken to appreciate which one is meant in a particular context.
U.S. Federal Standard 376B, Preferred Metric Units for General Use by the Federal Government, January 27, 1993 [emphasis added]
:In commercial and everyday use, and in many technical fields, the term "weiqht" is usually used as a synonym for mass. This is how 'weight" is used in most United States laws and regulations.  See the note at 5.2.1 for further explanation. 
:... 
:[note at 5.2.1]
:NOTE: There is ambiguity in the use of the term weight to mean either force or mass. In general usage, the term weighr nearly always means mass and this is the meaning given the term in U.S. laws and regulations.   Where the term is so used, weight is expressed in kilograms in SI. In many fields of science and technology the term weight is defined as the force of gravity acting on an object, i.e., as the product of the mass of the object and the local acceleration of gravity. Where weight is so defined, it is expressed in newtons in SI. 
American Society for Testing and Materials, Standard for Metric Practice, E 380-79, ASTM 1979. (This is a standard which together with a similar ANSI/IEEe Standard has now been replaced by the joint standard SI 10; I don't know if the current standard includes similar wording, but I am reasonably certain it includes nothing contradictory.): [emphasis added]
:       3.4.1.2 Considerable confusion exists in the use of the term weight as a quantity to mean either force or mass.  In commercial and everyday use, the term weight nearly always means mass; thus, when one speaks of a person's weight, the quantity referred to is mass.   This nontechnical use of the term weight in everyday life will probably persist.  In science and technology, the term weight of a body has usually meant the force that, if applied to the body, would give it an acceleration equal to the local acceleration of free fall.  The adjective "local" in the phrase "local acceleration of free fall" has usually meant a location on the surface of the earth; in this context the "local acceleration of free fall" has the symbol g (commonly referred to as "acceleration of gravity") with observed values of g differing by over 0.5 % at various points on the earth's surface.  The use of force of gravity (mass times acceleration of gravity) instead of weight with this meaning is recommended.  Because of the dual use of the term weight as a quantity, this term should be avoided in technical practice except under circumstances in which its meaning is completely clear.  When the term is used, it is important to know whether mass or force is intended and to use SI units properly as described in 3.4.1.1, by using kilograms for mass or newtons for force.
:   3.4.1.3 Gravity is involved in determining mass with a balance or scale.  When a standard mass is used to balance the measured mass, the effects of gravity on the two masses are equalized, but the effects of the buoyancy of air or other fluid on the two masses are generally not equalized.  When a spring scale is used, the scale reading is directly related to the force of gravity.  Spring scales graduated in mass units may be properly used if both the variation in acceleration of gravity and the buoyancy corrections are not significant in their use.
:    3.4.1.4 The use of the same name for units of force and mass causes confusion.  When the non-SI units are used, a distinction should be made between force and mass, for example, lbf to denote force in gravimetric engineering units and lb for mass.
American National Metric Council, Metric Editorial Guide, 3d ed. 1978 [emphasis added]
:7.1  In commercial and everyday use, the term "weight" nearly always means mass; the use of the word "weight" to mean "mass" and the word "weigh" to mean "determine the mass of" or "have a mass of" is acceptable.
:Examples:  My weight is 60 kilograms.
::           Weigh the envelope carefully.
::           The suitcase weighs 12 kilograms.
Society of Automotive Engineers, Technical Standards Board Standard TSB003, Rules for SAE Use of SI (Metric) Units, Rev. May 1999 (first issued Jun 1966 as SAE J916 until 1992)  [emphasis added]
:3.12 Weight— The weight of a body in a particular reference frame is defined as the force that provides the body an acceleration equal to the local acceleration of free fall in that reference frame. Thus the SI unit of weight is the newton (N).  In commercial and everyday use, the term "weight" is often used as a synonym for mass, for which the SI unit is the kilogram. The verb "to weigh" means "to determine the mass of" or "to have a mass of." Nevertheless, in scientific and technical practice, the term "weight" should not be used to mean mass.
Enough yet?
Gene Nygaard 03:22, 16 Dec 2004 (UTC)
thread_title:  "Colloquial" usage 

message: In conjunction with the changes in the "colloquial usage" section, Icairns also added this separate paragraph:
:In scientific usage, the kilogram is a unit of mass.  
The implicit but intended contrast with the following paragraph on colloquial usage, of course, builds in part on the shaky foundation of Icairns' mistaken beliefs about that colloquial usage (discussed in detail in the previous comment).  In fact, this is even worse.
The kilogram-force has always been a unit used primarily in science and technology, and very little used in our everyday lives.
That remains true today.  The kilogram-force is now much more common in science and technology than it is in everyday use, just as it always has been.  Fortunately, the use of this obsolete unit is diminishing in all areas, though more slowly than some of us would like.
The examples I gave were largely from science and technology.  Rocket thrust is not some colloquial usage by the people hanging around in a beauty shop.  The discussions of torque might be common among racing fans, but this is from the technical aspects of the sport of racing.  Similarly, the measurement of draw weight of bows deals with the technical aspects of the sport of archery.  The pressure gauges might an oil pressure gauge on a farm tractor, something read by the farmer who is using it—but it was put there by the engineers who designed the tractor.  There is, of course, no clear dichotomy between "in science" and the rest of the activities of humans in any case.
Those kilograms-force were the primary units used for thrust in the Russian space program into the late 1980s or early 1990s, and even today the part-numerical names of some of their rockets are based on their thrust in megagrams-force.  I've seen evidence (but not totally convincing proof) that in the Chinese space program today, kilograms-force remain the primary units for thrust.  They are the units used, together with pounds force, for the thrust of jet engines in Tom Clancy's nonfiction Airborne (1997). Even in Wikipedia, the article on the X prize winning craft, SpaceShipOne had kilograms-force as the primary, original units in which the thrust was expressed, along with an incorrect conversion.  (This was likely somebody's unattributed estimate, since as far as I know the builders hold secret the actual thrust.)
Throw out the totally mistaken, illogical belief that "2 kg" on a bag of sugar means two kilograms-force, and you'll have a hard time finding any "colloquial, non-scientific" usage of kilograms force.  Contrast that to the abundant examples which are found within science and technology.
Of course, those kilograms used for human body weight in the medical sciences and sports—the primary reasons we weigh ourselves—are every bit as much units of mass as those other units the British still like to use for this purpose, when they say they "weigh twelve stone three".  Unlike the pounds unidentified with the spoken "three" there, and unlike the kilogram we have been discussing, the stone has never spawned a unit of force, at least not one that has seen any significant use. Gene Nygaard 13:33, 16 Dec 2004 (UTC)
thread_title: Kilogram-force in scientific usage

message: The name of the author of "Classical Mechanics" is not Corbell but Corben. See http://www.amazon.com/Classical-Mechanics-Edition-Dover-Physics/dp/0486680630
141.35.45.79 (talk) 06:24, 20 June 2013 (UTC)
replies: text: Fixed, thanks. Vsmith (talk) 10:17, 20 June 2013 (UTC)
thread_title:  Edit request on 20 June 2013 

message: In Force#Friction, Feynman is cited for the statement "For most surface interfaces, the coefficient of kinetic friction is less than the coefficient of static friction." Actually, he says nothing of the kind. Not only does he never define kinetic friction, but he says (on page 12-5) "Many people believe that the friction to be overcome to get something started (static friction) exceeds the force required to keep it sliding (sliding friction), but with dry metals it is very hard to show any difference. The opinion probably arises from experiences where small bits of oil or lubricant are present ..." RockMagnetist (talk) 05:26, 25 October 2013 (UTC)
replies: text: This is a "less than or equal to" scenario. For dry metals, the two may very well be close to equal. Perhaps a rewording is in order, but the idea is fairly clear. jps (talk) 11:40, 25 March 2014 (UTC)
thread_title:  Feynman didn't say that 

message: (Disclaimer: I work in condensed matter theory and much of what I'm about to say may not be as applicable in other areas of physics that I'm less familiar with, like astrophysics and plasma physics, so if someone with actual training in one of those fields wishes to dispute anything I say here based on his or her own experience, I'll probably concede the point) I'd just like to point out, as a physicist, that Newton's first law is really pretty useless for most problems. First, it is only applicable at particular scales of velocity and mass: too high a mass or velocity and relativistic corrections become important; too low a momentum and quantum theory becomes relevant (sometimes you even get systems that are, gosh, both relativistic and quantum). Second, even for cases within which it is applicable, it's almost impossible to get any useful information out of anything but a system of small particles of number less than three. So after elementary mechanics, Lagrangian and Hamiltonian Mechanics become unavoidable, even when you're trying to derive the force, and you're rarely deriving the force for its own sake - force itself is only really relevant in cases in which it is convenient to use it to calculate something else, and because Newtonian mechanics is so restrictive, it almost never is the most convenient tool. So really this article is about a pedagogical tool that was historically very important in calculations, but now is not as useful, which is why I think this article is incomplete without going through the history of the concept, how it was used, and why it isn't as important today. This information should be added. The first volume of Landau and Lifshitz, Mechanics, would be a good place to start for anyone looking for some inspiration on how to do so. BTW, don't listen to anyone who says that you can't treat non-conservative forces with Lagrangians and Hamiltonians - they clearly either haven't been exposed to Noether's Theorem or aren't comfortable with working directly from the symmetries of the system to derive conservation laws. Sometimes even asymmetries can be useful if they are of a certain type. Quodfui (talk) 19:35, 7 November 2013 (UTC)
replies: text: This is all true, for essentially the entire subject of forces. Elementary mechanics is still taught to give a conceptual framework for physicists, but it is entirely abandoned after the first year of a good undergraduate education in favor of more useful methods. I don't know that this history is relevant to the topic of force per se. It's more of a caveat emptor.jps (talk) 11:47, 25 March 2014 (UTC)
thread_title:  First law - relationship with least action principle and Euler-Lagrange equations 

message: The force diagram is misleading. There should be a major x and y component supporting the top of the upper pulley. Otherwise the two forces shown would cause the pulley system to accelerate to the left and downward. Ward20 (talk) 01:39, 6 August 2014 (UTC)
thread_title:  pulley system in info box 

message: I would have liked to have seen more discussion of this notion "non-fundamental."  The very idea force seems to me non-fundamental, in that it is defined by  Newton's First Law as mass times acceleration, both of which have quite objective existence.
 
Looking over at Wiki: Mass I see that mass, acceleration and force are all said to be fundamental in the menu at the side of the page.  This seems perhaps, well, problematic.  If that is indeed the convention of classical physics, what did Newton do?    — Preceding unsigned comment added by David Lloyd-Jones (talk • contribs) 03:24, 23 October 2014 (UTC)
replies: text: The fundamental forces are explained at the top of Force#Fundamental forces. They are the four forces - strong, weak, electromagnetic and gravitational - that, as far as we know, cannot be reduced to other forces. The first three are mediated by elementary particles and the fourth might be as well. All other forces are non-fundamental; if you look at them closely, they turn out to be disguised versions of the fundamental forces. In the sidebar you're talking about, Template:Classical mechanics, "Fundamentals" just refers to basic concepts that are used a lot in physics; they're not fundamental in the sense of being irreducible. RockMagnetist(talk) 17:12, 23 October 2014 (UTC)
thread_title:  Non-fundamental forces 

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