Talk:Moment of inertia/GA2

GA Review

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Reviewer: RockMagnetist (talk · contribs) 00:53, 3 April 2013 (UTC)

GA review (see here for what the criteria are, and here for what they are not)

1. It is reasonably well written.

   a (prose): b (MoS for lead, layout, word choice, fiction, and lists):

   Most of the MOS guidelines are met, but the lead contains some significant topics (such as Newton's first law) that are not discussed in the body (the compound pendulum is discussed in a different context). Also, per WP:LAYIM, the text should not be squeezed between a pair of images, as occurs in a couple of places.

2. It is factually accurate and verifiable.

   a (reference section): b (citations to reliable sources): c (OR):

   Where there are references, they check out, although I had to correct some. There are large chunks of text with derivations that are not cited, and I did not check them all because of the extent of rewriting that I think is needed (see below).

3. It is broad in its coverage.

   a (major aspects): b (focused):

   The role of the moment of inertia in quantum mechanics and relativity is not discussed, nor effective moments of inertia for non-rigid bodies or fluids. Newton's first and second laws are both mentioned, but their relevance to the moment of inertia not really developed adequately; and the applications are mostly relegated to the images. There should be a section on applications of the moment of inertia in sports (ice skating, diving, tight-roping walking, ...); technology (flywheels, ...); and the planets (including the very interesting topic of inertial interchange true polar wander)

   I think the fundamental problem with this article is that it is not organized in a way that makes the material easy to understand or connects it clearly with the fundamentals or the applications. That results in discussions that are sometimes difficult to follow and duplication of material.

   A good introduction would begin with some simple observations like the effect of a skater pulling in her arms; then it would develop the concept of a rigid body, discuss Newton's laws, and use them to derive the moment of inertia for a simple pendulum. This could immediately be generalized to the expression for the moment of inertia of a collection of point masses about an axis. Instead, the overview has a brief mention of the second law followed by a discussion of a bicycle wheel that doesn't really aid the intuition. This is followed by several other elements that are presented before they can be explained properly (mechanics of machines) or separated from their applications, as Moment of inertia theorems is. The latter is particularly bad - it has a table of theorems with lists of definitions for the terms, followed by a set explanations for the theorems in a list-like form; the theorems are much better explained elsewhere in the article.

   Because of the poor organization, there are multiple discussions of concepts like the parallel axis theorem and unnecessary derivations. For example, a whole section (Moment of inertia#Moment of inertia around an arbitrary axis) is devoted to a derivation of an expression that reappears as a trivial result in the section on tensors.

4. It follows the neutral point of view policy.

   Fair representation without bias:

5. It is stable.

   No edit wars, etc.:

6. It is illustrated by images and other media, where possible and appropriate.

   a (images are tagged and non-free content have fair use rationales): b (appropriate use with suitable captions):

   There is a tag on File:Lever shear flywheel.jpg stating that the image might not be in the public domain outside the U.S. (I have added the attribution required by the licensing statement).

   The pendulum image is only in there because a pendulum is mentioned, but it in no way aids in understanding the text. There are multiple images illustrating real-life relevance of the moment of inertia, including three on flywheels and two on tight-rope walking, but most of the applications are not discussed in the text and the images seem to be arbitrarily located. The video of objects rolling down an inclined plane is an excellent illustration of moment of the effect of moment of inertia, but it is not discussed at all in the text.

7. Overall:

   Pass/Fail:

   This article has plenty of good material, but it needs substantial rewriting and reorganization to meet the GA criteria. I think this will take a lot of work; it would be easier for me to rewrite the article than adequately describe what changes are needed. However, I think the first steps would be to get rid of Moment of inertia#Moment of inertia around an arbitrary axis and Moment of inertia theorems; and combine discussions of the parallel axis theorem, perpendicular axis theorem and stretch rule with examples of how they are used to make calculations easier. I'll defer making a final decision until others have had time to respond.

Recent revisions

This is a summary of revisions provided in response the the GA review:

1. A new lead is presented that provides a simplified description of moment of inertia. The previous lead is now presented as the Overview.
2. The large sections of derivations show what cannot be described in words, which is how the movement of an extended body necessarily combines mass and geometry to form a complex parameter called moment of inertia.
3. The intended coverage is as follows: the meaning of moment of inertia, the definition of the scalar moment of inertia, its calculation and how this parameter appears in the planar movement of a body, and in planar rigid body dynamics, then the we shift to spatial movement where the scalar moment of inertia becomes the inertia matrix, the relationship between scalar moment of inertia and the inertia matrix is presented, the inertia tensor is described then the transformation of the inertia matrix to the body frame where the inertia ellipsoid is defined. This evolution from concept to spatial properties covers the use of moment of inertia in classical dynamics. Other applications can be considered but would probably be better placed in another article.
4. The original article involved the contribution by many authors, who had many different views of what was important. This article has streamlined this presentation. The fact that other contributors are not commenting on the many revisions should not be interpreted as agreement, but perhaps as tolerance.
5. This article was the focus of a lot of discussion many years ago. Over the past year most contributors have focussed on smoothing language and correcting notation.
6. Images of moment of inertia are difficult to find and often include definitions of notation that are not helpful. The inertia driven shear has a copyright warning, but it is such a dramatic demonstration of moment of inertia, that perhaps it can be given the benefit of the doubt.
7. The revisions to the article have been more aggressive than I would normally pursue, because I defer to the historical evolution of the article. However, I have never been able to justify the list of special theorems that claim to simplify the calculation of moment of inertia, and therefore I do not regret taking this opportunity to eliminate them. I have renamed sections to better describe the evolution of the article. The one section identified for removal was perhaps poorly presented. The result that was identified as a trivial demonstration of the same fact is not the same calculation, and actually depends upon the previous controversial calculation. Hopefully, this has been clarified. Overall the article now has a better structure, but it has been significantly edited and no doubt suffers from typographical and grammatical errors. It will take a little time to smooth this out.

Prof McCarthy (talk) 04:27, 5 April 2013 (UTC)

Prof McCarthy, sorry to take so long to reply. I have been thinking about this article a lot, but I haven't had the time to formulate my thoughts clearly. I'd just like to mention one point here: You seem to have misunderstood my critique of the lead. I was saying that it includes material that is not in the body, particularly Newton's first and second laws. The lead should adequately summarize the body of the article, so I don't think that shortening it and simplifying it was the solution. Moreover, the first line in the current version of the Overview, with all the synonyms for moment of inertia, really should be in the lead. What I'd like to see is a more explicit discussion of these laws in the body so there is something to summarize. RockMagnetist (talk) 22:07, 9 April 2013 (UTC)

Comment: I looked at this one with an eye to reviewing, and agree with the above. The article has a problem with (1) its organisation, which is confusing and (2) has large, unreferenced sections. Both are serious enough preclude promotion to GA status, but the real problem is that I cannot see either being quickly or easily corrected. Yet I felt that the article deserved better than a quick fail. Hawkeye7 (talk) 21:01, 9 April 2013 (UTC)

Comment: I too am reluctant to fail it without first providing a clearer idea of how I think it needs to be altered. Here are a few suggestions:

1. Convert the overview (which attempts to summarize the concepts, duplicating the lead) to an introduction (which discusses some well-known observations in a qualitative way).
2. Begin with the scalar treatment that is in Moment of inertia#Calculating the moment of inertia of a rigid body.
3. Convert Moment of inertia#Moment of inertia in planar movement of a rigid body to scalar form. That includes the discussions of angular momentum, rotational kinetic energy, center of mass and the various theorems. The nice thing about planar rotation is that it can be expressed in scalar form. Most of what can be said in one dimension still holds true in three dimensions.
4. The discussion of a compound pendulum is getting longer and longer, which doesn't help with the focus problem. It should be reduced considerably and moved either to a section on applications (see below) or combined with the section on measurement.
5. There are a lot of places where a reference vector is explicitly included in a long series of calculations (e.g., $\mathbf {r} _{i}-\mathbf {R}$). If the coordinates are chosen so the reference vector is zero, the calculations would be simplified considerably without sacrificing generality.
6. Add sections on applications to sports, technology and planetary and astronomical bodies.
7. Add section on the dynamic effects of having unequal principal moments (precession, nutation, unstable motion, ...)
8. Move the history section to the end (because it does not help introduce the subject).
9. Add more citations.

RockMagnetist (talk) 14:21, 10 April 2013 (UTC)

Revisions in response to the latest comments...

Here is a description of the latest revisions:

1. Convert the overview (which attempts to summarize the concepts, duplicating the lead) to an introduction (which discusses some well-known observations in a qualitative way).

• The overview is now an introduction that has been rewritten

2. Begin with the scalar treatment that is in Moment of inertia#Calculating the moment of inertia of a rigid body.

• Calculating moment of inertia depends on understanding the importance of mr^2. This is the purpose of the section on the simple and compound pendulums.

3. Convert Moment of inertia#Moment of inertia in planar movement of a rigid body to scalar form. That includes the discussions of angular momentum, rotational kinetic energy, center of mass and the various theorems. The nice thing about planar rotation is that it can be expressed in scalar form. Most of what can be said in one dimension still holds true in three dimensions.

• The scalar form of the moment of inertia have been collected under the section describing the simple and compound pendulums.

4. The discussion of a compound pendulum is getting longer and longer, which doesn't help with the focus problem. It should be reduced considerably and moved either to a section on applications (see below) or combined with the section on measurement.

• The section discussing the radius of gyration which is a commonly confused property that is used to define moment of inertia has been removed.

5. There are a lot of places where a reference vector is explicitly included in a long series of calculations (e.g., $\mathbf {r} _{i}-\mathbf {R}$). If the coordinates are chosen so the reference vector is zero, the calculations would be simplified considerably without sacrificing generality.

• A defining property of moment of inertia is the reference point used to compute it. This must remain a variable in the calculations so the importance of the choice of center of mass for this reference point is clearly demonstrated. Clearly showing the interaction of the shape of the body, with it associated distribution of mass, and the choice of the reference point should be viewed as one of the features of this presentation not a flaw.

6. Add sections on applications to sports, technology and planetary and astronomical bodies.

• Moment of inertia is such a fundamental property of the mechanics of bodies that showing how it appears in sports, technology, planetary and astronomical bodies is similar to showing how mass appears in each of these areas. It is not clear where to begin or end.

7. Add section on the dynamic effects of having unequal principal moments (precession, nutation, unstable motion, ...)

• The study of precession of a top is an important topic but it is better presented in a separate article. In fact several articles in Wikipedia attempt this presentation, such as Euler's equations (rigid body dynamics), Nutation, and Herpolhode. These are not particularly enlightening articles, which shows that this can quickly move away from the central topic of moment of inertia.

8. Move the history section to the end (because it does not help introduce the subject).

• The history section has been combined with the section on the simple and compound pendulums.

9. Add more citations.

• Every book on elementary dynamics, analytical dynamics and analytical mechanics includes a discussion of moment of inertia, just like every physics text describes mass. The references provided range from classical to modern and from elementary to advanced. It would help to know what statements are perceived as lacking references.
  • Misunderstanding here: what was meant was a shortage of inline citations. Normally, one expects to see at least one citation per paragraph. Both the book, and the page number. It's a start, but not quite good enough to reference a whole book; and there are still statements like This result was first shown by J. J. Sylvester (1852), and is a form of Sylvester's law of inertia for which no source is supplied. Hawkeye7 (talk) 09:40, 15 April 2013 (UTC)

Prof McCarthy (talk) 18:49, 11 April 2013 (UTC)

• Added references for Sylvester. However, the term inertia in Sylvester's theorem refers to the signs of the eigenvalues of a symmetric matrix. Prof McCarthy (talk) 03:31, 29 April 2013 (UTC)

Even more revisions...

The lead and the introduction have been revised to focus on the themes that run through the article, which is that moment of inertia is the combination of mass and geometry measured relative to a specified axis. The section on the pendulum has been streamlined to emphasize a simple example of moment of inertia and how it is measured. The section on calculating moment of inertia has been revised to focus on a rod combined with a disc. The example calculation for the ball can be removed if it is a distraction. The next sections on planar movement and spatial movement have been streamlined to simplify the notation and provide the basics of moment of inertia as it appears in elementary through advanced dynamics. The last couple sections provide useful specialized information, but the main point of the article is achieved with the definition of the inertia matrix. Prof McCarthy (talk) 05:51, 15 April 2013 (UTC)

How's this review coming? No progress in a month it seems.. Wizardman 04:17, 15 June 2013 (UTC)

With regard to "progress," I have tried to address all of the issues of concern that I am aware of, but I may not have been successful. Prof McCarthy (talk) 19:20, 25 June 2013 (UTC)