Talk:Force

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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 Truesdell and Nolls work 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.

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)

Shouldn't F=MA be F=mã or F=ma?

And shouldn't

F = Limit as T goes to zero of (mv - mv_o)/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.

L^AT_EΧed 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)

Combining Forces

"The more powerful force cancels out the less powerful; a resultant force is produced."

It's not like the "less powerful" force is completely ignored - both forces will have an influence on the net force. That sentence is misleading.

Brianjd 06:36, 2004 Jun 17 (UTC)

Why Forces ARE fundamental in Physics

The view (expressed at the beginning of the article) that energy and momentum are more fundamental physical quantities than forces is simply wrong. Energy and momentum are merely quantities associated with the path and time integrals over the force field respectively and are in principle not required in physics at all (one can happily integrate any equation of motion without ever mentioning energy and momentum).
The mathematical Hamilton and Lagrange formalisms in theoretical physics suggest the opposite, but they implicitly use forces as well because they can not be defined without potential functions which in turn depend on force fields.
The school approach of emphasizing the role of forces is therefore not only didactically but also factually correct. (for related aspects see my website http://www.physicsmyths.org.uk ).

OK, could you precisly define force? I don't know any exact definition. Already Newton knew that force had problem with a correct definition. 130.230.12.39 08:23, 24 Sep 2004 (UTC)

"Imperial newton"

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)

In other words, it would have made a lot more sense to never call the newton by its own name, instead only calling it the "metric poundal," than it did to call the English unit an "imperial newton" without even giving its own name. Gene Nygaard 08:22, 11 Dec 2004 (UTC)

Usage examples

Icairns has changed my examples of

Torque wrenches in units such as "meter kilograms"
Pressure gauges in "kg/cm²"

to the following:

Torque wrenches in units such as "kilogram-force metres"
Pressure gauges in "kgf/cm²"

What I had in mind was quoting the actual units used on my torque wrench, and the actual units as we see them written on many of those pressure gauges. I guess I could be more explicit about my intentions, but I don't know if it would add anything of value to the article to do so. I don't have any big problem with the revised versions, but I'm wondering if anyone else has any ideas on what should be done here.

Furthermore, while the proper order of units in the SI units is "newton metres", when the obsolete units "metre kilograms" or "centimeter kilograms" (no matter how any of those words were spelled) were used it was more common to put the metres first than it was to put them last. So switching the word order for torque bothers me more than the other changes. Gene Nygaard 22:49, 15 Dec 2004 (UTC)

Thanks for that. I take your point about US usage being as you state. However, without further explanation, it may well be confusing to non-US readers. Your meter is the SI metre; your kilogram is the kilogram-force. I have attempted to reword the para to reinsert your words. Is this any better? My work colleagues have marked up 'newton meters' instead of 'spring balances'. This is using 'meter' in the sense of a measuring device, but its appearance clashes with 'newton metre', an SI unit. Ian Cairns 23:50, 15 Dec 2004 (UTC)

I'm not very concerned about the spelling.

Your changes in these examples are probably an improvement. I'd question your charactarization of those torque wrenches as peculiarly U.S. usage. Here, for example, is a U.K. .com site, http://www.agriemach.com/products/tools_-_general_serv_+_specialit

BEAM TYPE TORQUE WRENCH This is an easy to use and low cost wrench which measures from 0-140 ft lbs (0-20 metre kilograms). It has a ½” square drive and is ideal both for D.I.Y. and workshop use

Another U.K. site http://www.elsham.pwp.blueyonder.co.uk/cx500/tw.html

A torque wrench is a tool which is adjustable to a certain level of force, measured in either foot pounds (lbf / ft) or kilogram metres (kgf / m).

A Dutch site http://www.xs4all.nl/~sotty/car/Heavy_duty_____amp_quot__Drive_Torque_Wrench.html

HEAVY-DUTY 1/2" DRIVE TORQUE WRENCH Range: 25-250 ft.-lbs. and 3.5-34.5 m.-kgs.

Note also that in the U.S. today, as it likely is most other places, it is easier to find a torque wrench in "newton meters" than in "meter kilograms". That probably wasn't true anywhere in the fairly recent past, maybe only 20 or 30 years ago, or even less in many places.

Furthermore, as far as the more general question of the use of "kilogram-metres" or "metre-kilograms" to measure torque, that most certainly is not something peculiar to the U.S.:

U.K. http://www.istonline.org.uk/Handbook/09-10.pdf

Energy or Work or Torque (cont’d)

. . .

gf cm 9.80665 x10^-5

kgf m 9.80665

U.K. http://www.pumaracing.co.uk/power1.htm

1 PS is 75 kilogram metres per second. The correct measure of torque when power is stated in PS is kilogram metres.

. . .

Kilogram metres don't even translate nicely into Newton metres because the conversion is the value of g which is 9.81. Copyright David Baker and Puma Race Engines

U.K. http://www.vatech.co.uk/pdf03/ds203.pdf

[columns omitted]

WRIST    JT4 100 kg.m 100 kg.m 
RATED    JT5 100 kg.m 100 kg.m
TORQUE   JT6  50 kg.m  50 kg.m

U.K. http://www.symonsnet.fsnet.co.uk/sv650.html

Front brake calipers mounting bolt tightening torque - 39 N.m (3.9 Kg-m, 28.0 lb-ft)

New Zealand http://www.pdu.co.nz/resources/show.php?mode=1&id=0035

Maximum torque (kg-m/rpm) 15.6/4,000 19.0/5,000

New Zealand http://www.murraycostello.co.nz/ssangyong/specspage3.htm

Max torque kg-m/rpm 25.5/2250 31.8/4600 21.8/2700 26.1/2250

Japan http://www.daiwakiko.co.jp/hp/english/kensetukikai/atomic%20150.htm

Maximum torque 32 kg-m/1600 rpm

Austria http://www.edu.uni-klu.ac.at/~fvogl/motorcycle.htm

Max. Torque: 6.2 kg-m / 9,500 rpm

Canada http://www.mgb.bc.ca/reference/specification.html

Torque : High C 110 lb./ft. (15.2 kg.m.) at 3,000 r.p.m.

Norway http://home.online.no/~odd-gro/Specifications.htm

Max.torque 3.2 kg-m (31.6 Nm) at 7,500 rpms

France http://www.le-moteur-moderne.fr/Pages/fichesfr/PIV/XI1A-GT.html

Maximum torque 18.3 kg-m à 5000 rpm 21.3 kg-m @ 4500 rpm 21.2 kg-m @ 5300 rpm

.com http://www.calibrationsales.com/index.html?torque_manual.htm~main

Read in lb-ft, lb-in, oz-in, Nm and M Kg

Now, when it comes to your editing of the "colloquial usage" paragraph, that's a different story—utter nonsense. I'll deal with that separately below, soon. Gene Nygaard 02:11, 16 Dec 2004 (UTC)

"Colloquial" usage

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)

Kilogram-force in scientific usage

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)

Defining force

It is very important to mention that it is impossible to define force. All attempts in history failed because of definitions in circles. This is a reason why modern physics theories don't operate with the forces as the source or symptom of interaction. General relativity uses a conception of curved spacetime and Quantum field theory talks about exchanging of intermediate particles like photons, W and Z bosons or gluons. Both theories don't need force. However, because it is easy to imagine forces, one can compute them from these theories. But we must not forget, that correct definition of this concept does not exist.

That is a silly paragraph, so I removed it. Force is defined as the derivative of momentum, and the article gives several alternative definitions.
—Herbee 14:37, 3 Mar 2005 (UTC)

I agree this is a silly paragraph but what you say is quite silly also. If force were defined as the derivative of momentum, then Newton's second law of motion would be a simple definition and not a relation deduced from logical thinking and observation. For example if you study tho movement of the moon, the force acting on the moon is defined by F=GmM/r2 and not by F=ma. If you apply the definition of F to the second law you obtain GmM/r2=ma which is a differential equation and not at all a definition of anything. Newton was maybe not aware of Quantum Mechanics and Special Relativity but he perfectly new what is a definition and what is a principle (axiom). So I agree with you It is very important to mention that it is impossible to define force. is silly but Force is defined as the derivative of momentum is not that better.--Vb 15:31, 29 August 2005 (UTC)[reply]

Editorial Change

In Reference to: Although not a fundamental 'quantity' in physics, force is an important basic mathematical concept from which other concepts, such work and pressure (measured in pascals), are derived. Force is sometimes confused with stress.

The article could incorporate this sentence into an introductory paragraph about "Force" - elaborate on aspects that set it apart from the fundamentals of physics. What is 'quantity'?

From a basic, introductory viewpoint, when analyzing force, there are two stereotypes: "static" force (when objects are at rest) and "dynamic" force (when objects are in motion). "Stress" is a term used on occassion during analysis of static forces. That may, just may, explain why Force is sometimes confused with stress (e.g. how much stress can a bolt holding two steel beams together withstand when an elephant does its balancing act?)

Definition of force

I don't agree with User:Crazynas. A force is not defined as an acceleration applied to a mass. If it were, Newton's second law would be a definition of force and not a law of nature linking different quantities. A force is a concept which must be defined without referring to Newton's second law. Look at the causality article. I think it is explained there what is a force. There you find example of causes like :

The Moon's gravity causes the Earth's tides.
A good blow to the arm causes a bruise.
My pushing of the accelerator caused the car to go faster.

Which is exactly what is meant by force. If you want to make this quantitative and provide a numerical value to forces then you need to postulate Newton's second law. Then you use Newton's second law as a quantitative definition of forces but this does not change the original definition which must be independent of Newton's second law. 131.220.44.10 12:11, 12 September 2005 (UTC)[reply]

F=ma that, in physics is the fundemental (non calclus) definition of a force. Gravity and magnatism are both good examples of the inverse square law which states that the attratction (force) between two objects is inversely proportional to the product of the two masses times the distance squared. acceleration is distance per time (squared). Acceleration IS fundemental to the definition of a force in physics, why does it have to be independent of the secound law? Crazynas 10:02, 13 September 2005 (UTC)[reply]

NO! Read Einstein and Infeld, The evolution of ideas in physics. More seriously look simply at Inertial mass. You will discover that F=ma is a definition of m but not at all of F. You can also have a look at Causality. If F=ma were a definition of force, then it could not have been tested because a definition is always true independent of any observation. The theory of relativity and the MOND model would make then no sense. This is just a question of logic. On one point you are right F=GMm/r2 is a definition of the gravitational force. It is difficult for me to more clear than that. 131.220.68.177 12:13, 13 September 2005 (UTC)[reply]

I don't like authoritative arguments but look at [[1]]. Their definition is force: any action or influence that causes an acceleration. They are right. The definition here is a bit more general because WP is an encyclopedia and not a glossary. 131.220.68.177 12:37, 13 September 2005 (UTC)[reply]

Once again from [2] force : Any external agent that causes a change in the motion of a free body, or that causes stress in a fixed body. I think this one is better. 131.220.68.177 12:52, 13 September 2005 (UTC)[reply]

This one is the best [3]: Force: An agency or influence that if applied to a free body results chiefly in an acceleration of the body and sometimes in elastic deformation and other effects. 131.220.68.177 13:19, 13 September 2005 (UTC)[reply]

... all of your definitions use the word 'aceleration' in them... what do you have aginst it being in the tag line for the physics article on force??

The current tagline

"In physics, a force is an external cause responsible for any change of a physical system. For instance, a person holding a dog by a rope is experiencing the force applied by the rope on his hand, and the cause for its pulling forward is the force exercised by the rope."

makes no sense, how can force be a definition of mass??? mass is an independent quality existant in ALL matter, just because a definition is always true, doesn't mean it can't be tested have a look at scientific theory. By your own definitions aceleration IS fundemental to the definition of force, and regardless, F=ma IS the fundemental physics equation for this idea. Crazynas 23:37, 13 September 2005 (UTC)[reply]

Now I agree with the current definition in the article: a force is a cause (or an agent) but not an acceleration -- even times a mass. But I still don't agree with you. Though the term acceleration can be used in the definition of force, it must not be because both concepts are independent. Even Aristotle knew what a force was. What he didn't know was the relationship between force and acceleration. Though F=ma may not be seen as the definition of force, it can be seen as the definition of inertial mass. This is the linear coefficient linking force with acceleration (if Newton's second law is valid). A given force induce different acceleration on different systems depending on a linear parameter which is called inertial mass. This inertial mass is not to be confused with gravitational mass which is the coefficient appearing in F=mg (|g|=9.81m/s2). The experimental fact that both parameters are equal has been seen as fortuitous till Einstein recognized this as a basic principle (Mach's principle). For example in modified Newtonian dynamics model one replaces Newton's second law by F=mμ(a)a. When one does this one does not change the definition of F (F is still GMm/r2) but Newton's second law. It becomes a non linear relation between F and a defining both the factor m (the inertial mass) and μ a non linear function. Of course one could see this model also as new definition of the inertial mass with m:=mμ(a) in order to save formally Newton's law. If F=ma were a definition of force one would not need to change it. It would remains the same independent of any observation a bit like

\mathbf {L} =m\times \mathbf {v}

which is the definition of angular momentum and not a law (axiom) of physics and which validity has not to be checked.131.220.68.177 07:42, 14 September 2005 (UTC)[reply]

This page is of no help to the aspiring highschool physics student or non technical engineer. Clyde frogg 08:13, 18 October 2005 (UTC)[reply]

Indeed, that's what I was trying to change. Crazynas 23:10, 19 October 2005 (UTC)[reply]

Internal and external forces

This article doesn't talk about internal and external forces and the article's definition makes it seem like all forces are external. Below is a definition from here.

Internal force: Forces acting between body parts
External force: Forces acting between the body and environment. It can be distant forces (gravity) or contact forces.

Here is a page with diagrams. -- Kjkolb 11:43, 26 November 2005 (UTC)[reply]