Talk:Moment of inertia

message: It seems that on February 6, 2013 User:Coretheapple recommended deletion of the Wikipedia article NACA Technical Note No. 1629, and nine days later it is gone.  This article refers to perhaps one of the best presentations of an experimental method to determine physical properties that are critical to the dynamic analysis of mechanical systems.  I have not found another reference that provides as detailed and practical a method for determining moment of inertia that is as complete and useful as this reference.  One reviewer's comment caught my eye:  "Delete. WP:not for source texts. As to transwiki, is Wikisource really interested in reproducing a random piece of technobabble from the 1940s, of no apparent importance of any kind? If you think so, go ahead and do it, but don't expect others to do the work."  I have worked for a couple years now to provide clear and correct presentations to articles on machines, rigid body dynamics and even moment of inertia.  However, I am beginning to realize that while Wikipedia has a lot of these articles, they may actually be out of place.  A request to have the article moment of inertia evaluated as a good article yielded the response that Wikipedia is not a textbook.  Perhaps it is correct that Wikipedia should be reserved for every detail of a sports franchise or video game series, and not waste space on the basic principles of science and mathematics that are important enough to appear somewhere else. Prof McCarthy (talk) 07:07, 5 March 2013 (UTC)
thread_title:  NACA Technical Note No. 1629. 

message: Hi Professor McCarthy, I am quite confused about the following issues in the paragraph:
#If angular velocity ω is a scalar, then why can it be used in a cross product.  If it is a vector, why isn't it in bold font?  What is the font style used for specifying a scalar or a vector?
#What does the x inside the equation  stand for?  It's not defined in the paragraph.
Thanks for clearing up these issues.--LaoChen (talk)06:48, 6 April 2013 (UTC)
replies: text: An equivalent to this question has been asked previously, so I have added a note in the text.  In elementary presentations of planar movement, the cross product is not used.  However, the vector formulation of angular velocity and angular acceleration, even for planar movement, allows the use of the geometric properties of the cross product to present the interaction of mass and geometry that defines moment of inertia.  I hope this is helpful. Prof McCarthy (talk) 19:11, 6 April 2013 (UTC)
replies: text: This is a question about the use of fonts?  I have made ω and α boldfaced. Prof McCarthy (talk) 13:01, 7 April 2013 (UTC)

text: One minor point here. I see three exact copies of the statement "The moment of inertia I_C about an axis perpendicular to the movement of the rigid system and through the center of mass is known as the polar moment of inertia", yet the sources don't seem to support this definition. The more common definition seems to be equivalent to Second moment of area. RockMagnetist (talk) 22:10, 9 April 2013 (UTC)
replies: text: This is often a source of confusion, so it seemed worthwhile to address it directly in the article. Prof McCarthy (talk) 07:53, 10 April 2013 (UTC)
thread_title: Moment of inertia in planar movement of a rigid body

message: Looking at the dyadic construction of the inertia tensor,
it seems to me it is type (2-0), qualified as a bivector generally at 'Tensor' article.
Here is written 'binor' - I didn't find other reference in wikipedia for this term.
As for the bivectors matrix transformation it follows : I'=T*I*TT ,
same is here, but this article 
http://www.kwon3d.com/theory/moi/triten.html
has derivation where I can't understand
how we jump from inverse to transpose for last term.
I could accept it only for ONB transformation.
Otherwise it confuses me more, as I see I'=T*I*T-1 for liner transformation,
or tensor of type (1-1). Would appreciate someone to help me about this catch.
Twowheelsbg (talk) 09:31, 3 June 2013 (UTC)
replies: text: The transformation T is a rotation matrix so its inverse is its transpose T-1=TT.  I hope this helps. Prof McCarthy (talk) 14:24, 3 June 2013 (UTC)
thread_title:  The Inertia Tensor 

message: Prof McCarthy:
My math and science is probably just barely good enough to help with a review on this article. But having looked at the /GA1 and /GA2 review pages a bit, I thought I might make the following comments. Perhaps it might give you some ideas for rebooting this a bit, even before I would consider diving into the math.
#Simplify the sections you can simplify. In particular, I'm thinking of the article lead, the introduction, and the first paragraphs on the simple pendulum. There is language that just doesn't have to be so complicated. If you want anyone but physicists (so to speak) to be willing to read even that far, these would help. For example:
#*In the translation of the sentence of Principia, use remain instead of persevere. You are not bound by requirements of sourcing and verifiability to use the exact wording of someone else's translation from the Latin. If anything, I would argue that the most common sense of persevere today has an overtone of meaning wherein a person patiently remains in place. I do not think people use persevere much for inanimate bodies these days.
#*Next paragraph: You don't need extended body; use body. I would change mass is constrained to rotate to mass rotates.
#*In a couple of places: You talk about the mass and shape of a body "combining", resulting in the MoI. That's true, but only in the sense of integrating the point-s over the whole body. To a lay person, that "combination" isn't obvious. I might say something more like "The MoI of a body is a property that depends on the mass and shape of the body."
#*Similarly: "The MoI of a rotating flywheel is used in a machine to resist variations in applied torque ..." The MoI can't be used for that; it's a mathematical proportionality scalar (or vector or tensor). The fact that a flywheel has rotational inertia is what is used for that purpose. To me, it is confusing to say that a mathematical constant does these things.
#:I hope you get the general idea about this. If I were a formal reviewer, I would want these things changed before passing the article on prose.
#I very much agree with . There is just too much here. Focus on the core topic, and let side topics go.  More to the point, though ...
#Consider the critiques of WP:NOTTEXTBOOK seriously. 
#*If this is a general encyclopedia, I think an article like this deserves, at minimum, a reasonable treatment of practical examples of the subject. You've provided a number of them: the wire walker, the skater, and so forth. I'd put them up front, so that the general reader can get to them promptly and see why the subject is important, and not just so much academic windage.
#*I'm not sure, per WP:NOTTEXTBOOK, whether this is the place for a heavy mathematical treatment at all. But other articles have such, and who am I to argue that? But what I would do is put them in a second-level header entitled something like "Mathematical treatment". Then anyone who wants that can just look at it, and anyone else knows that the general descriptive matter is finished.
I hope you find these suggestions helpful. StevenJ81 (talk) 17:45, 31 July 2013 (UTC)
thread_title:  Comment on GA nomination 

message: The definition has: "... angular acceleration along a principal axis of the object ..." Shouldn't that be "around a principle axis" or "about a principle axis" rather than "along"?  — Preceding unsigned comment added by 139.68.134.1 (talk) 14:04, 12 March 2014 (UTC)
thread_title:  Definition 

message: This article illustrates perfectly the problem with technical articles on Wikipedia.
Virtually every elementary textbook introduces moment of inertia with conservation of momentum through a simple example such as an ice skater pulling in her arms to spin faster   1,     2,   3,    4.   A simple scalar equation I\omega = L\, which a general reader can understand.   But why explain things simply when you can explain them in a complicated way?  This article introduces moment of inertia through - wait for it - the angular momentum of a pendulum??? Using vector cross products???  Hope the introductory reader is familiar with the BAC-CAB rule for vector triple products.  I'm sure he will be. 
The advanced sections of this article offer plenty of opportunity for the editor to show off his vector and tensor skills.  But Wikipedia is a general-reference encyclopedia, and the nontechnical readers, who are going to be the majority of readers coming to this article, deserved an introductory section in plain English, with a simple example such as the ice skater, or a diver curling into a ball, that could give them an idea of what moment of inertia means.   After all, the introduction is the only part of this article they are going to be able to understand.  But no.  It seems ego triumphed over the duty to write a comprehensible article. --ChetvornoTALK 01:18, 30 September 2014 (UTC)
replies: text: Thanks, Professor.  Looks good to me.  Sorry I was sarcastic.  I've been sensitive about this issue ever since a reader called an article I edited "...one of those Wikipedia articles only the person who wrote it can understand." --ChetvornoTALK 19:06, 30 September 2014 (UTC)
replies: text: Your recommendations were very good.  Thank you. Prof McCarthy (talk) 06:32, 1 October 2014 (UTC)
thread_title:  Article is not comprehensible to nontechnical readers 

message: In the section named "Calculating moment of inertia about an axis", shouldn't the integral and text say
:  I_P = \int_V \rho(\mathbf{r})\,\mathbf{R}^2 \, dV.
R is the distance to a point r in the body from the specified axis (nearest point of the axis), and (r) is the mass density at each point r
:instead of what it says? There it seems as if the r in  \rho(\mathbf{r}) and the r in  \mathbf{r}^2.--84.125.11.33 (talk) 12:53, 28 December 2014 (UTC)
thread_title:  Integral variables 

message: What is mean by centre of mass? Muruveena (talk) 17:34, 15 July 2017 (UTC)
replies: text: @Muruveena: I've linked the first mention of the term to the article center of mass to explain it. —C.Fred (talk) 17:38, 15 July 2017 (UTC)
thread_title:  Physics 

message: The introduction isn't quite right: it is the moment of inertia tensor that is a tensor, while the moment of inertia itself is a scalar defined with respect to a specific axis of rotation.  This is explained at http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html.  Can someone fix this? --Brian Josephson (talk) 10:50, 18 August 2018 (UTC)
thread_title:  Tensor or scalar? 

message: Editor is attempting to remove "perpendicular" from the long-standing definition in the lead:  "for a point mass the moment of inertia is just the mass times the square of the perpendicular distance to the rotation axis". Editor claims it is redundant. 
"Perpendicular" is only redundant if one assumes that distance must be the shortest possible (i.e. perpendicular) distance. That may not be clear to general readers, and I think we should stick with the long-standing lead wording. Note that we also use "perpendicular" in the Definition section of the article. Meters (talk) 05:35, 19 February 2019 (UTC)
thread_title:  distance vs perpendicular distance in definition 

message: This article seems to me to be a really good one for Wikipedia, that is, currently comparable to a second draft of an undergraduate textbook chapter, with the great virtue of being written to be accessible to someone studying at that level. 
I only want to point out that the distinction between 2-dimensional and 3-dimensional calculations is not always well maintained. Case in point: leading up to the section labeled "Definition," the text is developing a 3-dimensional framework involving matrices. But the Definition section immediately drops back to 2 dimensions without announcing that change. 
Later in the article there is again considerable attention given to 3 dimensions.
I suggest that "Definition" should begin by clarifying that in that section the aim is to develop 2-dimensional equations and descriptions, and that thereafter the distinction between 2-d and 3-d analyses be maintained in the remainder of the article. 
I refrain from making the change that I am suggesting, because lately my changes to technical articles are always reverted by some editor, perhaps (or perhaps not) in every case for good reason, but in any case wasting both my time and that of the editor. So where (even slightly-substantive) technical changes are concerned, I currently confine myself to suggestions rather than edits.
Dratman (talk) 16:50, 10 January 2022 (UTC)
thread_title:  Distinction between calculations in 2 vs 3 dimensions 

message: For scalars Ir >= Ic making the minus sign strange. It seems as if the text uses some vector formulation which may be very elegant and useful and the minus sign may be correct in this formulation. However the vector formulation may be unnecessary and impractical for many wiki readers so the scalar formulation ought to be given too and the existing note on the minus sign extended to comment on the diefference between the scalar and vector formulations.150.227.15.253 (talk) 14:34, 15 June 2022 (UTC)
thread_title:  Parallell axis theorem 

message: Compare the moment of inertia of a various giant wheel types 182.73.209.66 (talk) 11:46, 27 January 2023 (UTC)
replies: text: Why would we analyze include that specific case? Meters (talk) 12:01, 27 January 2023 (UTC)
thread_title:  Compare the Moment of inertia of inertia of a various giant wheel types