Talk:Force/Archive 7

Unnecessary specialization
Discussion of special-relativity and feynman diagrams seems very inappropriate for an introductory article on basic newtonian mechanics. Most readers have not studied calculus, let alone modern physics. —Preceding unsigned comment added by 68.232.115.6 (talk) 17:08, 20 February 2011 (UTC)

Torque c.f. Moment
I don't want to be changing what is generally such a well written document on the topic of 'Force'; for your good grace I shall leave that to the moderators (or whomever). For this reason I am bringing this to your attention, but leaving it up to your good judgement what you do with it.

I wish to point out that in the section 'Rotations and torque' it is incorrectly described that a force acting at a distance to a point is a 'torque', when in fact it is a 'moment'. Although very similar they are different in that a 'moment' (in simple terms) is the action of a force acting upon a point, whereas a 'torque' is the effect of a 'couple' of moments acting upon a point.

I understand (and being involved in teaching physics myself have to endure) that it is very common (at high school level) that 'moments' and 'torques' are often confused and incorrectly labelled. But for the benefit of trying to make Wikipedia as professional as possible I believe that it is important that we make an effort to use the correct nomenclature, and to label things correctly. —Preceding unsigned comment added by 118.93.161.106 (talk) 06:35, 27 July 2009 (UTC)


 * The technical ambiguity of a moment and a torque is featured on our Moment (physics) page. I have yet to see an authoritative source which disambiguates torques and moments as you suggest. As our page there mentions, in mechanical engineering often torques and moments are used in different senses (torques applied only to rotations while moments are what you roughly describe them to be), but trying to disambiguate as such leads to other problems in theoretical treatments. It's very much like saying a "torque is an unbalanced moment". While this statement is true in certain circumstances, it is not universally accepted in all literature on the subject. ScienceApologist (talk) 14:13, 21 August 2009 (UTC)

F = ma
I heard somewhere that Newton's equation of F = ma (force = mass x acceleration) was actually proven wrong by Einstein. But I can't remember what the right equation was. Does anyone know? —Preceding unsigned comment added by 222.153.21.181 (talk) 10:21, 3 July 2008 (UTC)
 * Newton's law F = ma was proven to be wrong by experimentalists; its predictions were different from high precision measurements for fast moving objects. Einstein created Special relativity and General relativity theories that proved to be consistent with all experimental data until now. In special relativity, the Newton's law is updated by introducing relativistic mass that increases with velocity. In general relativity, the notion of force does not apply at all; the effect of gravity is described with curvature of space-time, this curvature affecting the motion of objects. Vitalikk (talk) 13:53, 11 September 2008 (UTC)

Changes
I have made several changes; some are minor, but some are important.
 * In the lead, the statement that the acceleration is inversely proportional to the mass is true only for constant mass. This applies in classical (non-relativistic) mechanics also. Consider rockets.
 * In the section on Feynman diagrams, I have removed any implications that the diagrams for attractive and repulsive forces differ. (For instance, electron-electron scattering has the same diagram as electron-positron scattering.)
 * In the section on Gravity, the phrasing garbled the distinction between the weight of an object and the opposing force of its support that keeps it from falling. The diagrams have it right.

PKKloeppel (talk) 02:18, 17 July 2008 (UTC)

I have also removed the statement in the section on the normal force that states that in an inelastic collision it is proportional to the square of the initial velocity. All that can be said is that the work done in deformation must be equal to the initial kinetic energy; the magnitude of the force (as a function of time) will depend in detail on how the body is deformed.PKKloeppel (talk) 13:51, 18 July 2008 (UTC)

Kinematic integrals: Power
The derivation of power from work is apparently not very clear. Probably Gamesguru2 is right, see e.g. Feynman lectures, vol. 1, p 13-2, so P=Fv. The confusion seems to stem from:
 * In the integral for work W, time is not an independent variable, but it is a dependent variable. The time depends now on the position where the particle was during its trajectory.
 * The integration limits are missing. Power is related to the extra work done when the trajectory is extended. So when the upper integration limit changes. The path, and the moments in time the particle was on these positions in the past (so the time-dependence of the force) are not to be varied. -- Crowsnest (talk) 06:28, 3 September 2008 (UTC)


 * An appropriate treatment of this requires the calculus of variations. Suffice to say that for most instances of practical use, velocity can be thought of as an independent variable and then the integral form is appropriate. However, there are instances where the dW of interest is associated with a dF (in particular when you are interested in potentials). In such cases you may end up at the end of the day taking the integral of vdF. These situations are very weird and not well-defined for most physical systems (I cannot think of a natural problem where anyone would ever use such a formulation). However, from a strictly theoretical standpoint, the integral is properly of d(Fv) which by the chain-rule gives both values.


 * This problem is akin to state variable calculations in thermodynamics. As far as I know, there is not a very good resolution to it and it is made even more complicated here by the fact that we are not talking about state variables but instead talking about vectors. Probably the right thing to do is appeal to the time derivative of the appropriate energy tensor. In this case, our work integral is wrong as well.


 * ScienceApologist (talk) 22:26, 11 September 2008 (UTC)

Newton's 2nd Law
I revamped the section on Newton's Second Law, simplifying the math a bit and cleaning up the flow. I also added a reference over to the Newton's Laws of Motion page that states the law only describes systems of constant mass, which is important. Some people try to claim that F=d(mv)/dt is generally valid for systems of varying mass, but that is false. Because it only describes constant mass systems, it always collapses to F=ma for any system in the realm of classical mechanics (no relativistic speeds or large gravity wells). MarcusMaximus (talk) 01:13, 13 September 2008 (UTC)

You are wrong. If one wants to describe the motion of a rocket, for example, then the mass of the rocket changes, yet its motion can be easily described using F=dp/dt. —Preceding unsigned comment added by 193.40.244.203 (talk) 19:50, 30 September 2010 (UTC)

Well, you're both kind of wrong. F = dp/dt (always) but p is not generally mv just as F is not generally ma. — Preceding unsigned comment added by 24.143.59.148 (talk) 22:10, 20 May 2012 (UTC)


 * A rocket cannot be properly modeled by treating mass as a variable in F=dp/dt. Newton's laws of motion apply to systems of constant mass.  If you are dealing with varying mass systems, you have to add extra terms to the equation. This is well established.  See the three references in the Newton's Second Law section.
 * Can you please elaborate on this statement: "F = dp/dt (always) but p is not generally mv just as F is not generally ma."? MarcusMaximus (talk) 03:39, 22 May 2012 (UTC)

force does not causes anything
Force is relation btw things. Expresses how is movement (change in positinos - velocity) affected / changed. Nobody know why anything changes, we know how it changes. And we know how to use it for our own good (building stable bridges).

This meme, force causing something, is very widely spread. Unfortunately even in textbooks.

Generally it is swaping cause and effect meme.

I believe Feynmann talked about it somewhere ... 84.16.123.194 (talk) 16:57, 10 February 2009 (UTC)
 * Tell me the difference btw "God caused rock to fall" and "Gravity caused rock to fall" ;) ? I just want to know since when people started to say "force causes". 84.16.123.194 (talk) 12:27, 19 March 2009 (UTC)
 * To me it is just like in math when You want something to exist You just create it. Like in topology neighborhood of object is set (list, enumeration) of everything what is close to him. 84.16.123.194 (talk) 12:29, 19 March 2009 (UTC)


 * Does it have something to do with total rejection of what was previously called "God", which was something causing things to happen. This "God" was previously falsely personified (in christian religion). 2nd commandment believed (!) to be from "God" clearly forbids that. Intuitively there has to be something which causes because things happens (causality ...). And they were able to make sure those 'constants' (called forces) are "apples and apples" so they can be summed together (I presume he wanted force to be like natural(!) number, e.g. create such algebra).


 * By experimentation Newton and the others found out that certain things (measured mass, percieved&measured velocity(acceleration)...) happen to be constant under certain circumstances (enviroment : measured pressure, ...) when they are related somehow (F=m*a ...).


 * PAnd that it clear description 'how' and 'what' happens not 'why' it happens. All the math is based upon certain axioms. That is like saying "it is so because it comes handy and we can create amazing things if we presume that". Or is it an axiom to say force is a cause? 84.16.123.194 (talk) 12:54, 19 March 2009 (UTC)
 * No. Force is a word we use to talk about perfectly normal things that we'd otherwise have to use some other term for. When you put out your hand and push a small cart, what do you MEAN by "push." You can push it with varying degrees of vigor. What do you MEAN by "push HARDER"? Are you going to argue that your push does not cause the cart to move? S  B Harris 01:43, 9 June 2009 (UTC)

Einstein never proved Newton wrong
Einstein used Newton's equations all his life. Newton's laws of gravity are perfectly correct, Einstein used them all his life. So called general relativity allows simply for relativistic effects, which enter in certain regimes. The article should be reworded. —Preceding unsigned comment added by 173.169.90.98 (talk) 17:21, 8 June 2009 (UTC)

Relativity applies in all cases, not just for gamma approaching 1. And in fact relativity is used in quantum electrodynamics, which is decidely not dealing with speeds approaching c. Collect (talk) 21:44, 8 June 2009 (UTC)


 * Collect is right, it's just that relativistic effects are hardly noticeable at human speeds, so we don't bother with the extra calculus because it really gunks up your calculations. MarcusMaximus (talk) 00:52, 9 June 2009 (UTC)

Diagram of body on incline should have correct lengths of force arrows
I would like to point out that the vector m*g*sin theta is much too long. In my humble opinion, this is misleading - we try to draw vectors using a constant scale, so that we can conclude the (at least relative) sizes of vectors from the diagram. It would probably be good idea to show the parallelogram of forces for the forces m*g, m*g*sin theta and m*g*cos theta explicitly. I cannot draw such nice diagrams, therefore I cannot "just change it". 91.12.64.214 (talk) 19:37, 16 October 2009 (UTC)


 * Indeed, quite silly :-)
 * I have created a new version of the file, but I have no idea how to upload it and replace it here. Can someone pick it up? - DVdm (talk) 20:59, 16 October 2009 (UTC)
 * ✅ ScienceApologist (talk) 20:41, 19 October 2009 (UTC)
 * There was a problem because the Friction force vector was too long (it looked like the block was going to be forced uphill by friction!). I fixed this and uploaded it to the commons version of the file.ScienceApologist (talk) 20:45, 19 October 2009 (UTC)


 * OOPS! - Ok, uploaded new version to same location. Can you send it to the commons again please? - That seems not to work for me :-(
 * Thanks and cheers, DVdm (talk) 21:00, 19 October 2009 (UTC)


 * ✅ ScienceApologist (talk) 21:45, 19 October 2009 (UTC)

Question
In the sentence, "Deformation and pressure are the result of stress forces within an object." is "stress forces" correct? I would think it should be:

Deformation and pressure are the result of forces that produce stress within an object.

or,

Deformation and pressure are the result of stress (force/unit area) within an object.

or,

Deformation and pressure are the result of stress (force intensity) within an object. Ward20 (talk) 04:35, 23 October 2009 (UTC)

Agent?
Regarding the opening sentence "In physics, a force is any agent that causes a change in the motion of a free body, or that causes stress in a fixed body." What does "agent" mean in this sentence? —Preceding unsigned comment added by 76.175.72.51 (talk) 21:49, 28 October 2009 (UTC)


 * A "thing", "object", "field", etc.'' Kmarinas86 (6sin8karma) 17:01, 29 October 2009 (UTC)


 * So I agree that forces can cause a change in motion or a stress, but they only do this if they are external forces. Internal forces actually do not cause change in motion and, arguably, prevent stress. I changed the definition accordingly, though we might want to find a better definitional source since the NASA one is a bit ambiguous on this point. ScienceApologist (talk) 17:25, 29 October 2009 (UTC)
 * In the book "Engineering Mechanics" by Timoshenko he uses the word "action" instead of "agent": A force is the action that tends to change the state of rest or constant velocity, of a free body to which it is applied'. Perhaps the word "action" could be used. sanpaz (talk) 17:42, 29 October 2009 (UTC)
 * Action is not a force. ScienceApologist (talk) 17:55, 29 October 2009 (UTC)
 * I went to check in britannica and they use the term action for defining force within classical mechanics. However, for weak force, strong force etc, the use the term "interaction". Let me know what you think. sanpaz (talk) 18:28, 29 October 2009 (UTC)
 * A fundamental interaction is very particularly defined and often these are colloquially known as "forces", though they are in fact mediated in the standard model by gauge bosons. I'm afraid that sticklers may take issue with us using "interaction" to define forces for this reason, though it is definitely better than simply "action", IMHO. ScienceApologist (talk) 19:22, 29 October 2009 (UTC)

IIRC, at the age of 12 we learned it like this: Agere: ago, agis, egi, actum. DVdm (talk) 18:38, 29 October 2009 (UTC)

How?
What is the mechanism behind force? How does force cause some thing to accelerate or move places?

Maybe someone could explain it in easy terms? —Preceding unsigned comment added by 59.148.242.98 (talk) 05:40, 3 September 2010 (UTC)


 * Force is a concept, not a thing. It is a property of any activity which results in an acceleration. As such, the "mechanism" of a force is entirely circular just as the "mechanism" of any concept. To ask an an analogous question, "What is the mechanism of your name? How does it cause your identity?" Your name only "causes" your identity by being the thing we normally associate with your identity. ScienceApologist (talk) 11:13, 7 September 2010 (UTC)

The very odd first sentence
Said something about force being "used to describe how a massive body is affected by acceleration or mechanical stress." What?? Force is not an "effect" of acceleration, but a cause. It is causal if you see it as a push or a pull, which it is. So I've replaced this with the straighforward:

'''In physics, the concept of force is used to describe an influence which causes a free massive body to undergo an acceleration. Forces which do not act uniformly on all parts of a body will also cause mechanical stresss.[1]'''

I don't know if that is still supported by the reference.

I qualified the idea of forces always producing stresses, since if we are talking about coordinate accelerations and not proper accelerations, this isn't always true. Gravitational acceleration produces no stresses (in the limit of no divergence of field, such as tides and micrograv). There is little or no stress in diamagnetic levitation, either. This can be explained away by positing gravity as not a real force, or redefining acceleration as proper acceleration, but this gets into relativity and still leaves us with the problem of magnetic levitation. So let's just leave it as is. Stresses are produced sometimes, but not always. S B Harris 02:33, 21 January 2010 (UTC)


 * I wrote the original sentence, but I support this change because it preserves its meaning exactly while making it clearer.'' Kmarinas86 (6sin8karma) 11:24, 21 January 2010 (UTC)

Too long introduction
The introduction is too long and contains elements which are repeated at the section Pre-Newtonian concepts.MagnInd (talk) 17:48, 12 February 2010 (UTC)
 * I think I (mostly) disagree. A 5 paragraph intro is not inappropriate to such a complex and mysterious and important subject. Perhaps a bit of the deep history could be offloaded, but that wouldn't shorten the LEDE here by much. [Later] Okay, I did it. We now have a 4-paragraph lede, which is missing only some early historical detail. Some stuff about drag, thrust, and stresses in the second paragraph would go, or be shortened, next, if anybody wants to shorten even more. S  B Harris 20:36, 12 February 2010 (UTC)

Misuse of sources
A request for comments has been filed concerning the conduct of. Jagged 85 is one of the main contributors to Wikipedia (over 67,000 edits, he's ranked 198 in the number of edits), and practically all of his edits have to do with Islamic science, technology and philosophy. This editor has persistently misused sources here over several years. This editor's contributions are always well provided with citations, but examination of these sources often reveals either a blatant misrepresentation of those sources or a selective interpretation, going beyond any reasonable interpretation of the authors' intent. I searched the page history, and found 2 edits by Jagged 85 in March 2010 and 4 more edits in April 2010. Tobby72 (talk) 17:11, 8 June 2010 (UTC)
 * That's an old and archived RfC. The point is still valid though, and his contribs need to be doublechecked. Tobby72 (talk) 21:06, 10 June 2010 (UTC)
 * I deleted a great deal of original research please see J8079s (talk) 23:46, 13 August 2010 (UTC)

Dumb question about F=MA
If I brake hard, but run into the tree anyway...

a is negative (relative to the direction of travel) F is positive m is scalar

So, how can F=ma? Noloop (talk) 18:29, 3 November 2010 (UTC)


 * The force that results in the acceleration of the car is negative (relative to the motion of travel).PhySusie (talk) —Preceding undated comment added 18:09, 6 November 2010 (UTC).

Forget the tree. If you're braking, F is negative also. It's velocity that is positive. When you hit the tree, a and F go WAY up in absolute value (though they are both still negative), till v becomes zero. S B Harris 19:33, 6 November 2010 (UTC)

is it still a good article?
is it still a good article? 190.51.151.74 (talk) 23:46, 14 December 2010 (UTC)

Newton's Second Law
These two sentences from near the end of the 2nd law section seem a bit silly: ''The equality between the abstract idea of a "force" and the abstract idea of a "changing momentum vector" ultimately has no observational significance because one cannot be defined without simultaneously defining the other. What a "force" or "changing momentum" is must either be referred to an intuitive understanding of our direct perception, or be defined implicitly through a set of self-consistent mathematical formulas''. The silliness is in the claim that changes in the momentum vector have no observational significance. To simplify a bit, if you can measure the mass and velocity of an object you can find its momentum; if you do this repeatedly, and the momentum you find changes over time, you have observed a "changing momentum vector". Maybe someone was trying to make a real point that here but, if so, what is it? Cardamon (talk) 07:06, 31 December 2010 (UTC)


 * You can add a citation needed tag, but I think it is very unlikely that a source will be found, so if you tag it, it could be sitting here for years. Changing the wording would be wp:OR anyway, so I think you can safely remove it per wp:OR and wp:UNSOURCED. If the maker of the point has a source for this, (s)he can restore it —with the source. DVdm (talk) 11:32, 31 December 2010 (UTC)

Kuhn on Aristotle
I'm a little concerned that the paragraph in the lede that mentions Aristotle misstates the relationship between classical physics and Newtonian physics, as it is currently understood by academic studies of the history and philosophy of science. If I remember correctly, Thomas Kuhn discussed Aristotelian physics in The Structure of Scientific Revolutions. As I recall, it was a major inspiration for his ideas about paradigm shift in the history of science. In Kuhn's view, these older approaches were not simply "errors", as the article states. They are part of a different "paradigm", that is to say, they are about a different subject; they are not "physics" as we know it. They are a technique for classifying substances based on how they typically move, which is different than "physics" (a way of explaining the precise mathematical curve of a body's motion against time).

Forgive me if I have made a hash of this. The last time I read Kuhn was around 1983.

I don't think that Aristotle should be mentioned here at all, because the concept of force was not invented until the 17th century. I'm pretty sure it was Descartes (and possibly Hobbes) who first began talking in terms of measurable, directional "force", and the idea of force isn't complete until you have Newtonian force-at-a-distance. This is the modern idea and it is different than any predecessor --- abstract, quantified, directional force. Aristotle is talking about "causation" and "purpose", which is not the same thing at all. Aristotle does not discuss "force" as we define it.

If the section must be included, I could we find a better source, preferably a history of science source for the sections that are about the history of science? I notice that the sentences in question use only one source (the complete works of Archimedes? with no page reference?) CharlesGillingham (talk) 08:04, 22 February 2011 (UTC)


 * The Aristotle WP:OR later on should all be deleted too. Looking through the edit history, one cite was just copied as an afterthought and fails WP:V. The other was a Saganesque history of science... no doubt supporting the genesis of science in the 17th century. Paradigms are incommensurate, so barring counter-revolutionary cherries... take your pick. By Kuhn's demarcation criterion, loser Aristotelian physics is still a physics paradigm, as we knew it in 1983.—Machine Elf 1735  22:14, 17 July 2012 (UTC)

The article poorly adresses rotational effects of forces
even after the limited corrections I undertook. Would it be worthwhile to include an illustration like this one, in order to describe the issue? In that case, it should be noted that the illustration (in the section "Descriptions") is rather typical for many texbooks, but that it is both counter-intuitive (the friction and normal reaction acting at the center of the body) and misleading for calculation of torque and angular acceleration.--Ilevanat (talk) 02:25, 27 November 2011 (UTC)

Primitive concept
links:
 * --86.125.185.88 (talk) 12:11, 12 December 2011 (UTC)
 * --86.125.185.88 (talk) 12:23, 12 December 2011 (UTC)
 * --86.125.185.88 (talk) 12:29, 12 December 2011 (UTC)

The article poorly adresses most common practical applications of the concept of force
I have no clue as to what the above contribution was intended for. In my oppinion, the article in its present form is a reasonably smooth compendium of various aspects and illustrations of the concept of force, which are generally explained in greater detail elsewhere in Wikipedia. The article already seems to be more concerned with the gnoseological aspects of the concept than with its everyday applications upon which much our civilisation (e.g. buildings and machines) was built. If the above note was aimed at strengthening this gnoseological aspect, the links offered do not seem to be intelectually most compelling sources.

My concern, however, is that the article "Force (physics)" provides very little (if any) information on most common ("everyday", "practical") applications of the concept of force. The question I would like to propose is: do we need another article, such as e.g. ‘’Force (mechanics)‘’, or should that be included in this one?

I have already tried to start a discussion on proper graphical representation of forces in order to facitilate description of their rotational effects (the second section above) - no response yet. I have written the article "Net force" anew, of which at least some parts acually belong to some reasonable article on force. In the work article discussion I proposed:


 * ...I cannot agree with any argument that the concept of tangential force component is too difficult to grasp, simply because without it there is no understanding of the concept of work done by the force. Nor can there be any understanding of how the force changes the speed of an object (nor how it changes direction of its velocity). Your phrase "introducing a reference frame aligned with the velocity so the force can be decomposed into tangential and normal components" does sound discouraging (and pompous); however, it is my experience that even the least capable students can easily understand a simple explanation such as:


 * "If a force acts generally forward (not necessarily exactly in the direction of velocity, there may be an angle of say 30 or 60 degrees), it increases the speed of the object. If it acts generally backward, it decreases the speed. But if it is perpendicular to velocity, than it is neither forward nor backward, so it cannot affect the speed: therefore such force only changes direction of velocity. For that reason, it is very useful to decompose any force into two components. One component is perpendicular to the velocity ($$\scriptstyle \vec F_n $$, called normal, radial or centripetal) and changes its direction. The other component lies along the line defined by the velocity (the line of motion or tangent line); it is called the tangential component $$\scriptstyle \vec F_t $$ . This component changes the speed, and only this component does work."

Most of these arguments actually belong to an article on force. But if all other Wikipedians prefer to keep these things the way they are (and if the people from the WikiProject Physics have no time for the most elementary basic concepts, which are the worst part of Wikipedia), I give up.--Ilevanat (talk) 01:49, 17 December 2011 (UTC)