# Talk:Hooke's law

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## Comment on hashing does not belong here

I think the comment about hashing would be better placed on the anagram page.
Charles Matthews 12:13, 20 Mar 2004 (UTC)

Hooke's use of this anagram can be thought of as an early and primitive example of a Cryptographic hash function. A modern example is the case of an industrial research organization that may later need to prove, for patent purposes, that they made a particular discovery on a particular date; since magnetic media can be altered so easily, this may be a nontrivial issue. One possible solution is for a researcher to compute and record in a hardcopy laboratory notebook a cryptographic hash of the relevant data file. In the future, should there be a need to prove the version of this file retrieved from a backup tape has not been altered, the hash function could be recomputed and compared with the hash value recorded in that paper notebook.
was misplaced, reverting a few hours later that article contrib by an IP.
--Jerzyt 06:36, 11 March 2010 (UTC)

## Excessive historical detail

The core of the accompanying article Hooke's law is the wide proportionality of stress and strain, and its limits and implications. The time of its discovery, and the discoverer (since it bears his name) should also be included; likewise the lag between discovery and publication, since it may be crucial to an occasional user. These are things that are not as reasonable to cover elsewhere.
But the minutiae of the means of establishment of the priority misplaced in the article: they are much more relevant to Hooke's bio (they say far more about his practice of science than about the core topic), and they can be linked to Robert Hooke without cluttering the article as the current language does.
--Jerzyt 06:36, 11 March 2010 (UTC)

On the other hand, now the historic material is insufficient. This classic topic deserves "History" section, not limited to Hooke. For example, who extended the law beyond simple springs and axial stresses? --Jorge Stolfi (talk) 15:32, 25 January 2013 (UTC)

## Anagram or analogy?

Maybe I'm being dumb here, but I thought an anagram was a linguistic term for a word or phrase that forms another word or phrase when the letters are rearranged. Perhaps analogy is meant here? Visiting both pages only confirms my suspicion, but its source, citation 3 seems to refer to it as an anagram too. Who's wrong? I don't have the balls to correct something that contradicts a source and both previous posts. -Keith (Hypergeek14)Talk 19:24, 4 May 2011 (UTC)

I get it now. It actually was an anagram for the latin word for 'law' -Keith (Hypergeek14)Talk 00:01, 14 May 2012 (UTC)

That's it: ceiiinossssttuu as an anagram of Ut tensio, sic vis; the u and the v were interchangeable. Perhaps Hooke was trying to establish "first use" without actually giving anything away to others. Why didn't you change it back again? A rv follows.
--Old Moonraker (talk) 07:10, 14 May 2012 (UTC)

## Only an approximation

Would it be appropriate to mention here that Hooke's Law is only a linear approximation (albeit one that is very good up to the elastic limit)?
CyborgTosser 01:04, 23 Aug 2004 (UTC)

I agree, plus I think the use of stress/strain should be toned down. It's not the main idea behind Hooke's Law.
--Dan 15:02, 3 August 2006 (UTC)
Done. --Jorge Stolfi (talk) 01:14, 24 January 2013 (UTC)

## Tensor notation

Hi,
Does anyone know whether in the generalized form, some indices covariant and some other are contravariant? So should we write something like

$\sigma_{ij} = C^{ij}_{kl} \cdot \varepsilon^{kl}$

(random proposition)?
I never saw something like this in my books, nobody cares in continuum mechanics, but if we want to be strict and stick to the math conventions...
--Cdang 15:34, 26 Nov 2004 (UTC)

• Probably more like
$\sigma_{ij} = C^{kl}_{ij} \cdot \varepsilon_{kl}$
actually.
(PS There is something suspicious about the strain tensor, though, as I read in another encyclopedia.)
Charles Matthews 16:37, 26 Nov 2004 (UTC)
• I got another answer from fr:, someone proposes σji because it should be covariant in j (number of the face) and contravariant in i (axes of the force). Any other opinion ?
Concerning your suspicion about the strain tensor, pleasewrite them down at the talk page.
--Cdang 13:44 & 14:09, 6 Dec 2004 (UTC)
The following contrib was inserted inside the preceding signed contrib but has been moved here.
other opinion: the force is of same type than the conjugate momentum and thus of opposite type than the coordinate; the faces are also "dual" to the coordinates (via the completely antisymmetric tensor of rank 3), thus both indices should be of the same type. (But maybe I made an error in my reasoning...). But without considering opinions, one could just check its transformation properties under rotations...MFH: Talk 18:12, 20 Jun 2005 (UTC)
The following contrib was positioned at the left margin. Perhaps a colleague will offer an opinion of what earlier contrib it should be construed as commenting on.

As it is written currently on strain tensor, the latter should have one upper and one lower index (but maybe rather xj is meant to be there instead of xj, and idem for u).
Next, upper and lower indices are different only if the metric is different from δij, else this tensor and its inverse allow to raise and lower indices. However, in 3 dimensions people usually consider xj and xj the same. Without fixing a convention about the index position of coordinated and derivatives, it is quite useless to ask this question for more complicated derived tensors (imho).
MFH: Talk 17:38, 20 Jun 2005 (UTC)

The following contrib was positioned at the left margin. Perhaps a colleague will offer an opinion of what earlier contrib it should be construed as commenting on.

I also noticed that in the article tensor indices are messed up. IMO, in such a compressed article sticking to a particular convention is not at all important (since we can always raise/lower indices whenever we want to). What is important is that in any tensor product you must always pair a covariant index with a contravariant one, otherwise an operation doesn't make sense. I suggest that you either rearrange indices in a plausible fashion, or refrain from using Einstein notation in this article altogether. 212.75.204.52 (talk) 13:05, 19 June 2009 (UTC)

## Microscopic Interpretation of Hooke's Law

I am missing any reference to the microscopic interpretation of Hooke's law in the article, and in particular to the fact that it starts to fail already at about 1% of the value that should be expected if molecular forces are responsible. I have discussed this issue on my page http://www.physicsmyths.org.uk/hooke.htm and suggested there that in fact plasma polarization fields due to free electrons in the material might actually be responsible for the linear stress/strain curve in the Hooke-region.
Thomas
—Preceding unsigned comment added by 81.102.225.127 (talk) 15:14, 20 September 2005

Hooke's law is an observation expressed in simplistic linear math. You are trying to derive it from atomic-level analysis and have a hypothesis. Good luck, that is research.Cuddlyable3 19:16, 12 February 2007 (UTC)
• "Good luck, that is research." is ambiguous to me, in both its phrases. Specifically,
"Good luck" could carry ironic content, suggesting that it is far-fetched that the hypothesis will be fruitful, and
the significance of "that is research" seems vague.
Cuddlyable3 may have intended something equivalent top the following, which is appropriate for placement on this page (tho neither sarcasm nor denigration of interest in the hypothesis would be):
You imply that that hypothesis is original research, which is not acceptable as article content. Nevertheless, pursuit of such hypotheses outside WP is consistent with WP's policies about itself, and the vast majority of WP editors seem to value and admire the pursuit, outside WP, of such hypotheses.
I presume no offense was intended in the ambiguous answer, and i am presumptuous enuf to apologize, for myself and on behalf of my colleagues, for it having taken this long to elicit comment.
--Jerzyt 19:48 & 21:10, 10 March 2010 (UTC)
Independently of these quibbles, the article should say a bit more about the physical origins of the linear stress-strain response. --Jorge Stolfi (talk) 15:32, 25 January 2013 (UTC)

## Citation

I've added the box, inside the contrib, enclosing the quoted material without changing any markup within it.Jerzyt 21:10, 10 March 2010 (UTC)

i don't know why this was added to the end of the article:

Bibliography citation (MLA style): Various Authors. “Hooke’s Law.” Wikipedia. 18/08/2005. Wikipedia, 19/08/2005 <http://en.wikipedia.org/wiki/Hooke's_law>

. — Omegatron 13:35, 22 September 2005 (UTC)

## (Caption)

The picture shows a compression spring so the text under the picture should surely say Hooke's law accurately models the physical properties of common mechanical springs at small extensions or compressions. As a newbie I don't know how to edit the text myself.
(cuddlyable3)
84.210.139.189 11:07, 12 December 2006 (UTC)

I've changed it to Hooke's law accurately models the physical properties of common mechanical springs for small changes in length.
- EndingPop 13:01, 12 December 2006 (UTC)

## Why talk about a prismatic rod?

"For many applications, a prismatic rod, with length L and cross sectional area A, can be treated as a linear spring."
Under the heading "Details" the above sentence with its reference to an unfamiliar "prismatic rod" confused me. The term "prismatic rod" just links to "bar" which has a lot of meanings. Apart from that obscure word "prismatic" I think the unclarity is because the writer has a diagram in his head that we can't see. That diagram would show that the discussion has changed from a coiled spring like in the picture to an elemental block of material, and that this is being extended (or compressed) in the direction of its length L, which is not obvious.
(cuddlyable3)
84.210.139.189 15:00, 11 December 2006 (UTC)

In structural analysis a prismatic rod usually refers to a rectangular prism. Regardless, this statement about the linearity of the stress-strain curve is true for any bar of constant cross-section. I've made this change in the article.
-EndingPop 18:07, 11 December 2006 (UTC)
As you see my objection is to using the word "prismatic" which can send an ordinary reader (equipped with an ordinary dictionary and not privy to the jargon of structural analysis) into a spin. In the real world the statement is NOT "true for any bar of constant cross-section". Uniform cross-section is not even a requirement for Hooke's law, only for the elemental analysis to follow. I also dislike the introduction words "For many applications..." which smack of pedantic vagueness.
As a friendlier alternative I suggest:
We may view a small rod of any elastic material as a linear spring. The rod has length L and cross-sectional area A.
(cuddlyable3)
84.210.139.189 10:57, 12 December 2006 (UTC)

## Treatment of zero-length springs

The discussion of zero-length springs seems confused, and badly obscures the simple definition of a zero-length spring, namely a spring for which L0 is zero in the expression force = (spring constant) * ( length - L0 ). The article also misleadingly asserts that zero-length springs do not obey Hooke's law: in a typical zero-length spring's operating range, the force is indeed a linear function of the length, and that is Hooke's Law. One might wonder whether Hooke's Law requires that the linear relationship extend all the way to zero force, but even if one asserts that there is such a requirement (which is not stated in the current article, by the way), it does not rule out Hooke's-Law behavior of zero-length springs, since a zero-length spring can be constructed to encompass the zero-force region within its linear regime. Thus, non-Hooke's-Law behavior is not an essential attribute of zero-length springs.
I would propose that the discussion of zero-length springs adds nothing to the discussion of Hooke's Law, and should appear as a standalone Wikipedia entry. Since the only discussion of zero-length springs that I've ever seen relates to gravimetry, the web page at http://jclahr.com/science/psn/zero/winding/gravity_sensor.html would make an excellent starting point.
Peter 22:06, 11 January 2007 (UTC)

• There was an article on zero-length springs, but as it was a stub and said nothing that was not already stated in this article, I deleted it and replaced it with a redirect here. Heliomance 12:53, 12 January 2007 (UTC)
• The discussion on zero-length springs is not confused, it is incorrect. As can be found from the references, the zls is designed to be linear, but have an (apparent) length of zero. The constant-force element comes from the way it is used in the LaCoste suspension, which is a particular geometry.
Pedant543 06:41, 5 November 2007 (UTC)

## Relationship to Harmonic oscillator

This page should link to Harmonic oscillator at some point and discuss the relationship. —The preceding unsigned comment was added by 132.206.14.212 (talk) 20:51, 25 January 2007 (UTC).

As long as the oscillation stays within the range where the spring obeys Hooke's law, the oscillation is so-called Simple Harmonic Motion. That means it follows a sinusoidal function of time in the ideal case (always modified in practice by friction and/or transient disturbance, i.e. we can't make it a perpetual motion device). Be wary of HagermanBot's proposal because of the following nemenclature tangle: "harmonics" in music and electronics are overtones (multiples) of the fundamental frequency. From that viewpoint, the Simple harmonic motion of the ideal Hookean spring oscillator means Without harmonics. I hope there is a less confusing way to explain the above.Cuddlyable3 16:56, 8 February 2007 (UTC)

## IP edit, can somebody verify

Referring to this recent edit, can this (no edit summary) be verified or should it be reverted because no verification was given? --Berland 11:10, 18 May 2007 (UTC)

Ok, I had a look at it myself, and the change made the formula consistent with the preceding, so the edit is hereby verified. --Berland 11:14, 18 May 2007 (UTC)

## (Thank you)

Thank you for this Article you guys really saved my ass. —Preceding unsigned comment added by 74.232.151.240 (talk) 04:13, 10 September 2007 (UTC)

## Tensor notation part perhaps a bit unprecise?

Am I the only one who thinks, that the last part of the article is, from a physical point of wiev, a bit wierd? The point of writing up Hookes Law in tensor notation is not just so the eggheads can use their fancy math, but it is needed to make a precise description of the elastic properties in 3D (what the derivation shows). Since the page already use expressions from Landau & Lifshitz, why not go overboard and use a thermodynamic derivation like theirs? That would 1. Show the importance of the 3D expression 2. Give some useful connections to thermodynamics via. the identity:
$\left(\frac{dU}{d\varepsilon_{ij}}\right)_S=\sigma_{ij}$
I would gladly write it, if someone else would correct my wiki-errors. I suspect I'd make quite a few...
Bierlich (talk) 19:37, 21 December 2008 (UTC)

## C = Stiffness tensor

As [K] is the stiffness matrix it would be less confusing to call [C] the elasticity tensor or material matrix (tensor) instead of stiffness tensor. —Preceding unsigned comment added by 129.187.218.187 (talk) 11:03, 9 March 2009 (UTC)

Unfortunately, the stiffness matrix and compliance matrix have, by a quirk of history, been written as [C] and [S] by the mechanics community (which is the set of people most likely to come by looking for these terms on Wikipedia). The term elasticity tensor does not distinguish between a stiffness tensor and its inverse (both are elasticity tensors) and is not in regular use by the community. Also, there are several elasticity tensors and a separate page for those would be ideal. Bbanerje (talk) 21:50, 10 March 2009 (UTC)

## Content to be moved

The section on Multiple springs should be moved to another article that deals with the specific topic of springs, e.g. Linear spring (which currently redirects to this article, or Spring (device). The purpose of this article should be to explain the concepts behind Hooke's law and to show the mathematical formulation used in Linear elasticity. But it should not include particular applications, such as multiple springs. Any comments on where to moved this content? sanpaz (talk) 23:26, 21 May 2009 (UTC)

I moved the content of zero spring. This is a very particular section about a "type" of spring. It has nothing to do with the explanation of Hooke's law. sanpaz (talk) 23:35, 21 May 2009 (UTC)
Done. the section was moved to a new article, series and parallel springs. Some related comments were moved too. See its talk page.

## SI units

The SI unit of length is "metre" not "meter" and changes to "meter" need to be reverted. — Preceding unsigned comment added by 82.153.241.42 (talk) 16:32, 20 June 2011 (UTC)

The SI unit of length is "metre" in British spelling, but "meter" in U.S. spelling. The original spelling in this article was U.S. English, which is why the U.S. spelling is used in it. —Stephen (talk) 00:49, 21 June 2011 (UTC)

SI units are international, not British or American and spelling is not optional, see SI base unit — Preceding unsigned comment added by 82.153.139.222 (talk) 09:48, 23 June 2011 (UTC)

The convention used on Wikipedia in terms of spelling (American vs British) is to be consistent with the original article. Having said that, this has turned into an edit war. I suggest going through arbitration rather than just continually reverting each other. If not, you both risk disciplinary action by an admin. PhySusie (talk) 19:23, 23 June 2011 (UTC)

I am the only one who risks disciplinary action, since the anon's IP address changes after each edit. I have no preference in regard to spelling, but I think it is unethical for someone to change the correct spelling of the original author to that preferred in his own country. I think arbitration is an excellent idea. —Stephen (talk) 01:31, 24 June 2011 (UTC)
• One UK/USA spelling difference (meters/metres) is a very small teacup to have a storm this size in. This page has a long history before that of IPA edits followed by reversions. Semiprotect to keep IPA users out? Anthony Appleyard (talk) 05:20, 12 July 2011 (UTC)

Hi folks, I've agreed to facilitate the discussion here on behalf of the Mediation Committee. It has been observed that one of you has a dynamic anonymous IP address. That makes it more difficult to mediate. Would that individual be willing to open an account? There are specific advantages to having an account. Whatever the decision on that, here are some specific questions regarding the dispute:

1. How would you summarize what the Manual of Style says about units of measurement applicable to this case? (Ref: WP:UNIT).
2. What specific examples in the article are at issue?
3. How could the dispute be resolved?

Would you be willing to address these questions for starters? Sunray (talk) 17:10, 12 July 2011 (UTC)

1. American spellings of unit names (e.g. meter or liter) should be used on pages written in American English.
2. The earliest example of the metric unit that I was able to find in the article was spelled "meter".
3. Decide which national variety of English was used first, or used for most of the article.
I don’t have a preference for one national spelling over another, but I would not change the variety that an original author wrote in. —Stephen (talk) 00:02, 13 July 2011 (UTC)

This article spells "behavior" and "idealize." It thus uses AE, in which the unit is spelled "meter". Choyoołʼįįhí:Seb az86556 > haneʼ 01:45, 13 July 2011 (UTC)

I agree with Stephen's summary of the guideline. Seb has described the use of American English in the article. That seems fairly conclusive to me. Sunray (talk) 06:20, 13 July 2011 (UTC)

## Semi-Protection

I concur with the analysis this article has a history of some anon disturbances. While I'm not sure if it's straight up vandalism, edit warring or someone trying to sock to get around a ban it seems little storms keep erupting. I have semi-protected for 30 days, protection can then be lifted and if it starts up again it should be extended. --WGFinley (talk) 13:42, 13 July 2011 (UTC)

## Add Medical mathematics section to Hooke

I am researching a medical section to this article that describes application of Hooke's law to the contraction and relaxation of the heart guided by immersed boundary method mathematics. Seen as a spring weighted on one axis but not the other, the plurality of coiled springs within the myocardium is astounding. Dr. Francisco Torrent-Guasp and Dr. Carolyn Thomas revealed much of the structural foundations describing illustration and physics of the mammalian myocardium. Work done by this set of unique muscular fibers is readily applied to physics such as string theory and angular velocity. Editing in adding a medical wringing of this article is welcome.

## Too technical

I strongly agree with the above tag. Please don't forget that Wikipedia is meant to be an encyclopedia, not a collection of textbooks and course notes. Hooke's law is hugely relevant to all sorts of readers, but this article seems to be written for the benefit of graduate students in physics and mechanics. The article should be made interesting to people who have only a high school math education. It is OK to give the main formulas of continuum elasticity in in tensor form, but most of the details should be pushed to separate articles, such as stress tensor, strain tensor Young modulus, etc.. Such splitting would also benefit the specialist readers who seek the advanced material, as they will not have to scroll through pages of irrelevant stuff. (Note that those readers will hardly ask for "Hooke's law" if what they need the stress tensor in spherical coordinates.) --Jorge Stolfi (talk) 01:14, 24 January 2013 (UTC)

Specifically, I propose to move most of the continuum mechanics material (sections "Tensor expression" and "Thermodynamic basis") to linear elasticity and other specialized articles, leaving here only a short section that presents the general idea as being the ultimate generalization of Hooke's law. --Jorge Stolfi (talk) 15:35, 25 January 2013 (UTC)
I am not proposing to throw away anything, just to reorganize what is there (and add more, as I have been doing these few days). No, I do not use linear elasticity every day, but I had to learn it for a couple of projects in computer graphics and image processing. But that is precisely the point. Wikipedia should not be written *only* or even *primarily* for the specialists, but rather for people like me, who are the vast majority of its readers. The so-called "encyclopedias of XXX" that you mention are abusing the term, and are definitely not a model that Wikipedia should follow (even though their contents can and will be here somewhere). All the best, --Jorge Stolfi (talk) 18:09, 27 January 2013 (UTC)
I agree with Bbanerje - You need to stop thinking of those who understand more than you as "specialists", and that you represent the majority of readers. The page should be designed for almost everyone. Someone who understands little of the subject should get an idea of things from the introduction. Those with a better background can go further, and so on, and so on. Writing down a 3 dimensional second rank tensor as an explicit 3x3 matrix is helpful in the beginning, but this should not be a page that teaches tensor algebra, and to go further, the reader needs to learn tensor algebra, on another page. This page should not be a reflection of your present understanding of the subject, it should offer some understanding to anyone, with links on how to get up to speed to go further at the point where they get stuck. If you get stuck at a certain point, resist the urge to collect everything you don't understand and put it in a page for "specialists". PAR (talk) 07:09, 6 February 2013 (UTC)

## Formulas using 6×6 matrices

In section "Isotropic materials" there are several formulas that use 6×6 matrices to represent the two main tensors of an isotropic medium. Is this a good idea? It may perhaps save a few keystrokes in matlab (one equation instead of two), but it wastes lots of space in the article, hides the underlying physics and math, and must terribly inefficient in any implementtation -- even in matlab. Note that since those 6×6 matrices are two independent 3×3 matrices in block arrangement, and one of them is actually a scalar. So, merely splitting each equation in two and eliminating the duplicates gets us downto 10 matrix elements instead of 36. As for computation cost, an n×n matrix product requires n^3 operations, so the savings would be huge. --Jorge Stolfi (talk) 22:30, 25 January 2013 (UTC)

I suggest that you get a minimal understanding of the practical application of elasticity before making significant changes to the article. There is something to be said for convention - it makes reading easier. Bbanerje (talk) 04:49, 27 January 2013 (UTC)
Well, please educate me. I have some experience in using the theory for physically sound animation of elastic objects. Using three 6x6 matrices instead of two 3x3 tensors and two scalars would probably have made that part of computation 4 to 8 times more expensive. Even for non-isotropic media the penalty may be 2x to 4x. So when is the 6x6 approach effective? --Jorge Stolfi (talk) 18:21, 27 January 2013 (UTC)
OK, for 21 coeffs out of 36 the extra computational cost of the 6x6 matrix approach is not THAT large, perhaps about 2x. Still, by hiding the decomposition of the tensors it obscures the physics... --Jorge Stolfi (talk) 18:41, 27 January 2013 (UTC)
I agree - the article is about the physics of Hooke's law, and should use notation that facilitate that understanding. A separate section on efficient computation might be included, but the physical theory itself should not be presented in terms of the most efficient computer algorithm. PAR (talk) 04:20, 28 January 2013 (UTC)
@Jorge: A good starting for anisotropic elasticity is Lekhnitskii's "Theory of anisotropy of an anisotropic elastic body". The treatment is straightforward but not very physically intuitive. To get some physical intuition into the reasons for the use of a 6x6 matrix and the physical meaning of each of the constants in that matrix see any modern book on composites engineering. Bbanerje (talk) 20:29, 28 January 2013 (UTC)
• Thanks, sorry for my ignorance. I take back my objections, except that the article(s) should better explain the point of the 6x6 notation, and/or that contents should be moved to the more specialized articles.
The same constants appear in the two 3x3 matrices, so this is not the point. The 3x3 matrices represent tensors. Tensors are "physical" - vectors (rank 1 tensors) are "physical", in the sense that the relations and operations on them are independent of your choice of coordinate system. The whole point is not the meaning of the individual entries, it is the transformation properties under changes of coordinates. Its the independence of the relationships under translations, rotations, time shifts, which yield the conservation laws of momentum, angular momentum, and energy. This is the "physics" stuff, and trying to deal with it in terms of 6x6 matrices is an immense and useless complication. Its like the difference between $\sqrt{2}$ and $\sum_{n=0}^\infty 1/2^{n+1}$. For theoretical purposes, use the uncomplicated $\sqrt{2}$ along with knowledge of its properties, for computation, use the much more complicated series expression. PAR (talk) 22:04, 28 January 2013 (UTC)
The current version of the article seems to be rehashing arguments about force vs. stress that were resolved in favor of the stress concept a couple of centuries ago. Good references are Truesdell's "Essays in the History of Mechanics, Springer-Verlag, 1968." and "The rational mechanics of flexible or elastic bodies, 1638-1788. Leonhardi Euleri Opera Omnia, Series secunda (Opera mechanica et astronoca) (1960)". Bbanerje (talk)

## "Spring physics" redirect

"Spring physics" currently redirects to the "The spring equation" section of this article, which doesn't exist anymore. How should it be fixed? Is it fine for me to simply remove the anchor from the redirect, since Hooke's law is basically "the spring equation"? --SoledadKabocha (talk) 22:46, 8 February 2014 (UTC)

Yes, that would be good. I edited the redirect to drop "#The spring equation". Glrx (talk) 00:31, 9 February 2014 (UTC)

## Strange

I learned that $\sigma$=E * $\epsilon$, and that $\sigma$=F/A (force in Newton divided by the area of the cross section of the specimen in mm2). And that $\epsilon$= $\delta$L / L (the relative extension, without unit). E (Young'sw module) is depending of the material. In all F/A = E * $\delta$L / L. Unit for e is also N/mm2 (or N/m2 or Pascal). But the article suggests that F = E * $\delta$L / L , if I understand it correctly.

Mechanical tensity as function of relative stretching , during a common tensile test. Hookes Law applies only to the linear part of the graph.

Boeing720 (talk) 23:19, 2 March 2014 (UTC)