Talk:Center of mass/Archive 1

old comments
"The centre of mass of an object is the point through which any plane divides the mass of the object in half."

Are you sure this is true? If the center of mass is a weighted sum of all the points in an object, and the distance is part of the weighting, then the plane would not divide the mass in half, but the mass times distance. Later on in the page, the Jupiter-sun system is mentioned. This can be viewed as one object without loss of generality (think of the matter connecting them approaching zero mass in the limit The center of mass is outside of the sun, so a plane perpendicular to the line between the objects would certainly not divide the mass of the object in half!  I think I will wait for some comment and then remove the sentence from the article.

Other than that, it looks like a good article. It has a lot of good examples.

Cos111 04:38 24 Jul 2003 (UTC)


 * I agree, I changed it. - Patrick 08:54 24 Jul 2003 (UTC)


 * For mass that is distributed according to a continuous, nonnegative density $$\rho(\mathbf x)\ge0$$...

We are not likely to encounter substances with negative density, but if we did, these integrals could still be evaluated and the result would be physically correct.


 * It seems to be a reminder that density is not negative, and also a clarification that we can integrate over the whole space, not just the masses, since we allow density to be zero. --Patrick 01:00, 23 Feb 2004 (UTC)

Also, &rho; doesn't have to be continuous to be integrable. In fact, being composed of point masses (quarks and electrons) matter is never distributed continuously. More to the point, &rho; is often discontinuous at the interface between two materials. A better formulation might be:


 * ''For a physical body with mass distribution $$\rho(\mathbf x)$$...


 * However, if you integrate over point masses, you will exclude them from your integral since you they are multiplied by a measure of 0. So, not continuous, but piecewise continuous.


 * The Lebesgue measure allows point masses and continous masses in one formula.--Patrick 13:38, 4 November 2005 (UTC)

If the Earth-Moon distance is rounded to one significant digit (400000 km), it's silly to come up with 4 significant digits in the answer (4877 km). I call this 'calculator blindness'. I'm not fixing it because the Earth/Moon example is duplicated in the existing article on barycenter. Since that term seems to be in use primarily in celestial mechanics, wouldn't it be more logical to move all the astronomy stuff to the barycenter page? Herbee 19:18, 2004 Feb 22 (UTC)

Example removed from page
I removed the following example from the page: The first sentence of this example is incredibly vague, and doesn't apparently have to do specifically with center of mass. Because of this vagueness, the second sentence is inherently confusing: the influence on center of mass is proportional to distance, while the gravitational influence is inversely proportional. The same confusion holds in the third sentence with "smaller" versus "larger". Dbenbenn 10:21, 2 Jan 2005 (UTC)
 * To calculate the actual motion of the Sun, you would need to sum all the influences from all the planets, comets, asteroids, etc. of the solar system. The influence of each is approximately proportional to the product of mass and distance. That of Jupiter is largest, its large mass more than compensates its smaller distance to the Sun than several other planets. If all the planets would align on the same side of the Sun, the combined center of mass would lie about 500,000 km outside the Sun surface.


 * I have clarified it, but to say things accurately makes the sentences somewhat complicated. Since you seem to understand the matter, you could have improved the formulation instead of just deleting everything. You are welcome to further improve the formulation.--Patrick 00:00, Jan 3, 2005 (UTC)

Mixing up centers of mass and gravity?
The paragraph on Archimedes' discovery and the section on aeronautical significance both use the term "center of gravity", which is actually a subtly different thing from "center of mass". To my inexpert eye it looks like just replacing all the instances of "gravity" with "mass" would correct this, but I'm not certain enough at this time to just go ahead and do it. So I've tagged the problem with an HTML comment and am appealing to anyone with this article on their watchlists to give me a yea or nay before I tinker. Bryan 23:55, 18 October 2005 (UTC)


 * There appears to be a difference. The center of gravity weighs the mass density by the gravitational field somehow, while the center of mass just assumes a homogeneous field. For non-gravitational forces the center of gravity should thus not be used. However this is all based on disputed information. --MarSch 16:16, 4 November 2005 (UTC)


 * Yeah, that disputed information was quite incorrect, and the difference between "center of mass" and "center of gravity" has been greatly exaggerated. As I understand it, "center of gravity" seems to be the historically preferred term, so it is appropriate in a historical context. I'll restore the paragraph and work on explaining the difference. Melchoir 01:12, 20 April 2006 (UTC)

Mixes up center of gravity with geometric center
The center of mass of a celestial object only tends to be at the geometric center because the mass is pretty evenly distributed around the center. This configuration requires the least energy. If you consider a two-body system in which one body is much larger than the other (e.g. the Earth and the Moon), the center of mass is nowhere near the geometric center of the system. In fact, because the Earth is much more massive than the Moon, the center of gravity remains within the Earth at all times. —The preceding unsigned comment was added by 131.136.202.27 (talk • contribs) 22:48, 4 November 2005.


 * It depends on what you mean by geometric center of the system: the centroid, or halfway between the Earth and the Moon. The centroid is near the center of mass (not the same point because the densities differ).--Patrick 01:00, 5 November 2005 (UTC)

Distinguishes between center of mass and center of inertia
Inertia is the tendency for an object to resist an acceleration. It is proportional to the object's mass.

Gravitation is an (apparent) attraction between objects. It is proportional to the product of the masses of the objects in question and inversely proportional to the distance between them.

There are therefore two ways to determine an object's mass. The inertial mass may be determined by applying a force to the object and measuring the resulting acceleration. The gravitational mass may be determined by measuring the gravitational attraction between the object and another object. We usually measure the attraction between an object and the Earth, and call this value its weight.

In all cases, inertial and gravitational masses are identical. Much of Einstein's theory of general relativity is based on the idea that the acceleration produced by gravity is identical to that produced by application of a force. One (experimentally confirmed) consequence is that a gravitational field causes the paths of photons to be deflected, although they have no mass.

In short, the center of mass of an object is also its center of inertia, and the article should not distinguish between the two. —The preceding unsigned comment was added by 131.136.202.27 (talk • contribs) 22:48, 4 November 2005.


 * I am not aware of a reliable source that even defines such a thing as a "center of inertia". Melchoir 01:06, 18 April 2006 (UTC)

Definition of center of gravity not rigorous
The article states that "The center of gravity of an object is the average location of its weight." This is misleading. It is a weighted average (no pun intended), with the contribution of each part of the object being proportional to its mass. If an object's mass is not distributed uniformly, its center of gravity tends to be closer to its denser portions. For example, if a metal bar were composed of two cubes, one of them made of aluminum and one made of lead, the center of gravity would be within the lead cube, since lead is denser than aluminum. You could safely place all of the aluminum end over the edge of a desk, but not the lead end.

I would like the article to include a mathematical formula that expresses this idea. It's sure to be an integral. —The preceding unsigned comment was added by 131.136.202.27 (talk • contribs) 22:48, 4 November 2005.


 * "The average location of its weight" expresses that. An integral for the centroid is given, expressing this idea.--Patrick 01:07, 5 November 2005 (UTC)

I do not not like/understand the integral definition as given (common in physics ?). The result of an integration (as usually defined) is a scalar not a vector. In other words you do the integration on every coordinate and that should be indicated (and not left to context knowledge of the reader).

See http://mathworld.wolfram.com/GeometricCentroid.htm for a more clear use of the integral notation. If the integral in the article is simple a use of standard notation of an expanded integral concept, i think it should be noted/commented or otherwise replaced by a mathworld style notation. --84.132.233.173 12:30, 28 January 2006 (UTC)


 * That link is broken... Anyway, integration of vectors is extremely common in physics and is quite well-defined. Melchoir 01:02, 18 April 2006 (UTC)
 * Oh, I see: the link went here. Melchoir 01:09, 20 April 2006 (UTC)

Center of Mass vs. Center of gravity
"The path of an object in orbit depends only on its center of gravity."

As illustrated so beautifully later in the article, this is absolutely not the case. Only in an (imaginary) isolated system consisting of a single rigid body in a uniform gravitational field could this be true. The path of the moon relative to the earth alone depends only on the moon's center of gravity; however, its "absolute" path (since the earth is orbiting the sun) is a squiggly line about the barycenter of the Earth-Moon system; this barycenter depends on the masses of both the Earth and the Moon. The Earth-Moon system does not have a center of gravity, since it is comprised of multiple discrete bodies in a non-uniform gravitational field (they attract each other and are also attracted by the sun, so the gravitational field is constantly changing). Likewise, the sun is orbiting the center of the galaxy, and the path traveled depends very little on the center of gravity of the moon.

Thus, it may be more accurate to say that the path of an object in orbit depends on its center of mass, as well as on the barycenters of the system(s) in which it orbits. It would be most accurate to say that the path of an object is affected by every other mass in the universe, but most significantly by the masses of bodies with which it participates in orbital motion.

Because of this confusion, I don't think it would be a good idea to merge this article with Center of Gravity.

Pcress 07:16, 26 December 2005 (UTC)


 * I agree. Do not merge. --Rebroad 12:39, 22 January 2006 (UTC)


 * I also agree. Do not merge. The two quantities are distinct.Outofmine 15:04, 26 January 2006 (UTC)


 * Do not merge. Totally different concepts. Steve Max 21:33, 12 April 2006 (UTC)


 * I have removed the statement from both articles, as it accomplished the rare feat of being meaningless and yet still wrong. As for merging, it has become clear to me that there is nothing worth keeping in the Center of gravity article, so to "merge" it here would really be more like a redirect. I'll work on improving this article first to fix the various confusions. Melchoir 00:45, 18 April 2006 (UTC)

Disputed tag
There is a disputed tag on this article, but there seems not to be any point of dispute raised on the talk page. Should the tag be removed? Outofmine 15:07, 26 January 2006 (UTC)
 * I've relocated the tag to the most problematic section. Melchoir 00:58, 18 April 2006 (UTC)

Center of Mass vs. Center of Gravity
Well what I think is that the definition of center of mass/gravity in a way it is done: In physics, the center of gravity (CG) of an object is a point at which the object's mass can be assumed, for many purposes, to be concentrated. is a little vague.

Let's talk about center of mass (CM). For two points it is quite simple $$\mathbf{r^*} = \frac{m_{1}\mathbf{r}_1 + m_{2}\mathbf{r}_2}{m_{1} + m_{2}}$$. Generalizing for N points isn't far more complicated either:

$$\mathbf{r^*} = \frac{\sum_{i=1}^{N}m_{i}\mathbf{r}_{i}}{\sum_{i=1}^{N}m_{i}}$$    (*).

Here the $$\mathbf{r^*}$$ means the position vector of the CM and $$m_{i}$$ resp. $$\mathbf{r}_{i}$$ is the weight resp. position of i-th point.

Now try to imagine that on each point in our N-points system acts an external force $$\mathbf{f}_{i}$$. According to Newton's third law hte i-th point will start to move with an acceleration $$\mathbf{a}_{i}$$ depending on the given force and mass: $$\mathbf{f}_{i} = m_{i}\mathbf{a}_{i}$$. Summing this up for N particles gives: $$\mathbf{F} = \sum{\mathbf{f}_{i}} = \sum{m_{i}\mathbf{a}_{i}}$$    (**).

After differentiating (two times) the eqution (*) we get:

$$M\mathbf{a^*} = \sum{m_{i}\mathbf{a}_{i}}$$    (***),

where $$M = \sum{m_{i}}$$ is the total weight of the system and $$\mathbf{a^*}$$ is the acceleration of its CM.

Comparing equations (**) and (***) leads to final result:

$$\mathbf{F} = M\mathbf{a^*}$$.

OK, what is it good for? The last equation shows the physical meaning of center of mass. Simply said: Consider an N-point system under some external forces. The movement of this system (given by the set of N equations) can be simplyfied by assuming the concept of center of mass in this way: take the total weight of the system, put it into the center of mass, sum all external forces. The CM will then move like a single point (with mass M) under single force (F). So we have reduced the N-point system to only one point.

Note 1: For solid bodies everything is the same but you have to use integration instead of summation.

Note 2: The center of gravity is the point with this behavior: when we support the body in CG it will not move (better said it will be well balanced). What does it mean? Take the N-point system again. We are looking for some point where when we act with some force F (supporting force) the system will not move. The system wants to move due to gravitational forces acting on each point (for non-uniform gr. field they aren't the same size nor direction but it doesn't matter). The movement of this system can be solved easily using the CM. Put total weight into CM and act with the sum of all gravitational forces. Now it should be clear that the balancing of this system will be achieved by supporting the system in its CM. Result: Center of Mass IS THE SAME THING as Canter of Gravity. --147.175.20.101 13:25, 6 February 2006 (UTC) umer


 * You're right that the vagueness needs to be purged from the article, But the handwaving argument in part 2 fails to account for torques, which are what centers of gravity are all about; see Talk:Center of gravity. I intend to edit this article to make the point clear. In full generality, an object's center of mass does not need to be a center of gravity. Melchoir 00:52, 18 April 2006 (UTC)

Without reading everything, it seems to me that large gravitational field gradients are so far from common experience that "center of mass" and center of gravity should be directed to the same page. Which title the page should have depends on a choice between common usage and technical jargon. David R. Ingham 06:05, 22 March 2006 (UTC)


 * I'm pretty sure "center of mass" is better. It's the correct jargon, and it does pretty well on Google. Most important is to avoid confusion: the center of mass is unambiguous, and of those sources that make a distinction with a center of gravity, it's the "center of mass" concept that agrees with the content of this article and that dominates the discussion. Melchoir 00:57, 18 April 2006 (UTC)

Note: Merging the concepts of "center of mass" and "center of gravity" may be done. Keep in mind that the concept of "center of mass" pertains to mass, which is common to all physical objects. Also the concept of "center of gravity" pertains to gravity, which is specific to space near a very massive object, earth, sun, etc. Hence, the "center of mass" concept is more general where the "center of gravity" concept is a sub-concept concerning mass under the influence of gravity. J.A.T. 12:46am, 13 April 2006

Center of gravity now incorporated
I've edited the article to explain that "center of gravity" is a limited synonym for "center of mass". I've also removed the merge tag and the disputed tag, and I'll make Center of gravity a redirect to here after I sort out the interlangs. This doesn't mean that discussion about the two concepts is over; everyone, feel free to yell at me if you disagree, but please read the new article first! Melchoir 07:52, 23 April 2006 (UTC)

Oh, and Talk:Center of gravity has some material that we might want to merge into this article. Melchoir 08:16, 23 April 2006 (UTC)

Another way of looking at it
Departing from the acceptance that Newton's Laws are valid we can examine a stationary raisin loaf. For the loaf to remain stationary we deduct that the sum of all forces and moments impacting on the loaf are equal to zero, otherwise the loaf would rotate or move in some direction. For purposes of analysis any fixed reference point Pt is selected and all the forces and moments about that point can be identified and equated to zero, assuming clockwise as positive. The sum of moments of each raisin and piece of dough about the X, Y & Z axis provide three equations which each equate to zero. The sum of the forces in the direction of each of the X, Y & Z also provide three equations that equate to zero. The total mass of the loaf M exercises a moment about point Pt and per definition it is Rx, Ry, & Rz from point Pt. Analysis of the equations yield results for Rx, Ry, & Rz that are the point we are trying to locate, which in the case of a doughnut is somewhere in space.. Gregorydavid 06:52, 16 August 2006 (UTC)

Barycenter: Location within/without heavier body depends on more than just the mass ratio!
This --


 * In the case where one of the two objects is much larger and more massive than the other, the barycenter will be located within the larger object. Rather than appearing to orbit it will simply be seen to "wobble" slightly. This is the case for the Moon and Earth, where the barycenter is located on average 4,671 km from Earth's center, well within the planet's radius of 6,378 km. When the two bodies are of similar masses (or at least the mass ratio is less extreme), however, the barycenter will be located outside of either of them and both bodies will follow an orbit around it. This is the case for Pluto and Charon, Jupiter and the Sun, and many binary asteroids and binary stars.

-- is a nonsense!

mMoon:mEarth = 0.0123, mJupiter:mSun = 318/333,000 ~ 0.001 - how on Earth (!) is this mass ratio "less extreme"? The Sun-Jupiter barycenter is outside the Sun only because the distance between them >> than the radius of the Sun! If Jupiter were in Mercury's orbit, the barycenter would be close to the Sun's center: r1 = 5,500 km -- much less than the Sun's radius.

--Ant 09:16, 17 August 2006 (UTC)

In the end, I was moved to completely rewrite the section. Thanks to Frankie1969 and Srleffler for catching the typos in the table! What's an order of magnitude between friends? --Ant 22:53, 18 August 2006 (UTC)

Barycenter...moves at different points in the orbit?
I don't understand this assertion:
 * Astronomers expect to find hundreds of Pluto-sized objects in the outer solar system. If one has a satellite that is round, and which has a certain eccentric orbit -- meaning the two objects come very close together at one point and then diverge greatly -- then the barycenter could dip inside the larger object during part of the orbit, Laughlin explained.
 * In such a case, the smaller object would be defined as a moon part of the time and a planet the rest.

How does this make sense? Wouldn't it just end up looking like the last animation? Or is there another animation which could demonstrate this situation? —pfahlstrom 21:42, 19 August 2006 (UTC)


 * Try imagining each ball in that animation to be three times its current size. Sometimes it'll contain the CM, and sometimes it won't. Melchoir 22:14, 19 August 2006 (UTC)
 * Ah! Thanks. That does help. —pfahlstrom 21:37, 20 August 2006 (UTC)

Barycenter Independant
Was "Barycenter" origionally it's own article? I think it's a related topic, but should be it's own article. Waarmstr 00:45, 1 September 2006 (UTC)
 * The problem is that we don't have a reliable source that even distinguishes between the phrases "barycenter" and "center of mass". For all we know, they mean exactly the same thing and are interchangeable. Given that, it's hard to justify having separate articles. What I would support is an article on barycentric motion, which would explicitly apply the CM/barycenter concept to celestial mechanics. Melchoir 04:37, 1 September 2006 (UTC)

Many-body problem?
The many-body problem seems to refer to quantum mechanical situations. I presume that orbital physics, centre of mass, etc, etc, is about classical mechanics, and so I've changed the link in Barycenter section to the n-body problem, which is classical. Tez 18:37, 10 May 2007 (UTC)

Symbol?
Is the symbol that looks like the BMW logo a center-of-mass symbol? (Or perhaps a center of effort symbol? I've seen it drawn on sails in books on sailboat physics.) It's common on diagrams and as markers in crash tests. I can't find the Unicode symbol for it or any information about it; the closest symbol I could find is the circled plus, ⊕. The BMW page says the logo is a roundel, but that's a broad class of symbols. —Ben FrantzDale 15:46, 25 June 2007 (UTC)
 * SolidWorks appears to call it the "dowel pin symbol". 155.212.242.34 17:46, 9 November 2007 (UTC)

Animations
The animations help understand the concept as it applies in astronomy. Please do not remove them. --Shkedi 02:00, 16 November 2007 (UTC)

The animations are useful, but at the scale shown, they do not relate what is really happening as the dominate planet becomes larger, and the subordinate planet becomes comparatively smaller.

Several changes occur: The dominate planet becomes larger, its orbital velocity decreases, and it will spiral inward toward the common barycenter. The subordinate planet in the animation becomes smaller, it's velocity increases, and it will spiral away from the common barycenter. The absolute value of the distance between the two planets will increase beyond the limits possible in such a small animation, and the two planets will orbit one another far more slowly with increasing distance between the two.

This can be demonstrated by thinking about the real three pairs of celestial objects. Pluto and Charon co-orbit their barycenter in hours, The Moon and Earth co-orbit their barycenter in days, and the Sun and Jupiter co-orbit their barycenter in years. Their distances increase by 19,700 km (P-C) to 384,401 km (E-M) and 5.2 Au (S-J)respectively.

The animation with the two elipses is valid when a third party like a star forces the two planets into elipses. Without the third party, the orbits will become circular.

MWC - Golden Colorado —Preceding unsigned comment added by 75.71.153.113 (talk) 04:38, 16 October 2008 (UTC)

Torque about the Center of Mass Induced by a Uniform Gravitational Field.
The gravitational torque acting on a system was cited as being Mg x R = Σmig x ri. But, isn't this the negation of torque? The convention for positive gravitational torque should satisfy "the right-hand rule". Also, the linear combination of torque for a system of particles shouldn't switch the place of r and F because the derivative in respect to time of the angular momentum is r x F (since v x mv = 0) thus the total torque for every particle in the system would be Σri x Fi. I think the switched orientation of r and F is a minor error since it is still perfectly possible to show that the center of mass is the center of gravity in a uniform gravitational field since the net gravitational torque about the center of mass is zero and the negation of zero is obviously zero, so in this case the negative of the gravitational torque about the center of mass equals the gravitational torque about the center of mass. However, for the sake of convention, the orientation should be switched so everyone agrees on sign in our "right-handed" universe. Russmoore86 (talk) 07:23, 30 January 2008 (UTC)


 * Fixed. Melchoir (talk) 07:34, 30 January 2008 (UTC)

Locating the centre of gravity
I loved the examples of how to determine the centre of gravity of various objects. What would be most interesting is the same treatment for a) a portion of a bisected circle (ring) and b) a portion of a bisected hollow sphere (shell). :-) 89.243.77.119 (talk) 06:31, 14 July 2008 (UTC)

Center of gravity - no useful general definition
Regarding "center of mass" versus "center of gravity": they are equivalent when the gravitational field is uniform (as is the case near the surface of the Earth), but not equivalent in non-uniform fields. The best discussion I have seen so far is by Symon in his textbook "Mechanics." There he shows that an extended object feels a net gravitational force due to a point mass (external to the object) that can be considered to be acting at a specific point, which is the center of gravity. However, the location of that point depends on the location of the external point mass. Therefore, he says, "The center of gravity of the body relative to the point P is not, in general, at the center of mass..." Also, "For two extended bodies, no unique centers of gravity can in general be defined, even relative to each other, except in special cases, as when the bodies are far apart, or when one of them is a sphere....The general problem of determining the gravitational forces between bodies is usually best treated by means of the concepts of the field theory of gravitation..." These quotes appear on page 260 of the 3rd edition.

Given all this, it has become accepted practice, in physics and engineering, that "center of mass" and "center of gravity" are synonyms, since most important applications occur when the gravitational field is sufficiently uniform. MarkReynolds667 (talk) 18:04, 24 July 2008 (UTC)

Ship-related engineering
In ships, the center of mass must be below a certain point or the ship will capsize. Similarly, in a spinning hard-boiled egg the center of mass is below a certain point or the egg up-ends. These points would be encyclopedically interesting here. Simesa (talk) 01:46, 25 August 2008 (UTC)

Centre of Mass vs Centre of Gravity
I agree with the points stated in the second section on this page and would like to add that 'centre of gravity' should absolutely NOT redirect here. They are separate concepts and I came to Wikipedia to distinguish between the two, not to be directed to the other page! I would write a 'centre of gravity' page if I had the knowledge; is there anyone willing to do this? For now though I would immediately remove the redirection as it will cause confusion. - Nessa Ancalimë ♥ (talk) 19:19, 20 September 2008 (UTC)
 * It probably suffices to target the redirect to an appropriate section of the article, rather than to (the top of) the article, if the Rdr'n is problematic. If the current section boundaries won't accommodate that, the sections should be restructured to do so.  But IMO, another article would reduce understanding, since many readers will go to Center of Gravity (the better known title), and have to be told follow a Lk to where they won't believe they should go, and perhaps won't do. Hmm, unless we make Center of gravity a Dab of some sort that i haven't yet envisioned.... Nah, naming that violates common sense is a bad idea, even when common sense is mistaken.  What about following a single sent lead 'graph with a second 'graph, still in the lead sec'n, that says roughly "except for purposes seldom considered even by engineers and most professional physicists, CoG may be adequately approximated by CoM", and lk'g to a section abt non-uniform G-fields?  One last thot, tho i don't think i like it: make both titles Rdrs to Center of mass and center of gravity. --Jerzy•t 06:21, 1 October 2008 (UTC)

CopyVios?
My first impulse was to remove both the Feynman and Symon quotes as Copyvios, without discussion. They certainly cannot stay here without discussion of why. First of all, if your impulse is along the lines of citing educational use, (a) go read WP:Copyvio thoroughly, and (b) your response will be moved to another section if you comment without getting it. Except for educational use, which it is WP's purpose not to be restricted to, quotation, in order to be fair use, (outside of certain inapplicable cases) requires either (a) there being no other way to express the idea, or (b) quotation for the purpose of discussing the content of the quoted material. On the plus side, i'll say that other than perhaps discussing poetry, i can't recall a WP case as close to being a clear basis for a quotation of this size. On the minus side, (1) the copyright statute says there's no formula to make you sure your case for fair use is sound, so (1a) our for-profit users may be deterred from using our content if their impression is we are overconfident, and (1b) every time we use a quote where paraphrasing is feasible, we provide ammunition to lawyers trying to show that the broad pattern of our quoting practice is one that is casual in invoking fair use in gray areas, and to lobbyists and legislators who would like to see the fair use criteria tightened because we do free what commercial publishers and professional writers do, thereby damaging life, liberty, and the pursuit of profit, and in fact the whole American Lay of Wife. And (2) quotes are bad for the content, at least bcz they force us to adjust what leads up to and follows the quote, to make those words that are beyond our control fit into our article. (In fact, there's an excellent chance that our many eyes, informed by these two pros, can give more time and thot than either the presumably brilliant but busy Feynman or Syman did, to the problem of wording this. Using a quote deprives us of the chance to find out whether we can do better -- if i propose alternate wording here on the talk page, very few editors will make efforts to make it better and better, bringing it from shit to Shinola, but each editor who reads what we replace the quote with will be tempted to tinker.) Also, you may well share my view that (whatever is legally the case) we are violating the spirit of fair use if we use the quotes bcz we can't express ourselves adequately without quoting a genius. Wait, did i say "spirit"? Yeah, that's simply what i think. Fair use covers what copyright law requires. And i'd like to hear why anyone thinks it doesn't require paraphrasing, to escape Feynman's & Syman's wordings of these public domain ideas. --Jerzy•t 06:21 & 06:25, 1 October 2008 (UTC)

Improving on Syman
In the spirit of my call, above, for paraphrasing the quotes here's two points toward eventual improvement on Syman, if i'm not mistaken: These are afterthoughts, but they certainly flesh out my abstract thot in the previous section that a quote creates a no-go zone for the tuning of the article's text. --Jerzy•t 07:32, 1 October 2008 (UTC)
 * 1) "Far apart" is not a "special case", but an approximation; what they're getting at is "decreasingly significant as separation increases." My first thot for a suitable truly special case would be a specially shaped massive object, which would have to be concave, i think, on the "working" side, that actually created a perfectly uniform G-field inside a finite volume big enuf to contain the (universe's only) other object. But on second thot -- i hope i'm not superstitiously confusing this with the infinite energies involved IIRC in any filter with sharp corners on its attenuation-vs.-frequency curve -- is my gut right to be worried about discontinuous derivatives at the edge of the "finite volume"? If so, i think you need a slab with uniform thickness and density profile, and, uh, infinite extent in two dimensions. But what about a uniformly accelerated frame of reference, can that buy you something in this problem?
 * 2) I can't imagine "sphere" could be enough, though IIRC "rigid sphere with radially symmetric density" is equivalent to "uniform sphere".

Astronomical Barycenter
The barycenter concept in astronomy goes beyond the two-body problem. Stars in globular clusters orbiting the center of a galaxy are themselves orbiting a common barycenter. Otherwise, the cluster would collapse in on itself. Astronomers are looking for black holes at the centers of galaxies but, even there, a concentrated mass wouldn't be needed to form a galaxy into the spiral shapes typically seen. Recommend that the discussion of astronomical barycenter be expanded beyond the localized description now there. Virgil H. Soule (talk) 16:56, 6 October 2008 (UTC)

"not the center" of mass
Under the Definition section is: 'If an object has uniform density then its "not the center" of mass is the same as the centroid of its shape.' I have no idea what the "not the center" of mass is. I'll probably simply remove the quotes and "not the" if no-one has reasons against. Or, feel free to do it yourself if you agree. Or, put an explanation so that people know what we mean by "not the center." Kineticscientist (talk) 19:45, 26 November 2008 (UTC)


 * It's vandalism by an anonymous IP.- (User) Wolfkeeper (Talk) 21:21, 26 November 2008 (UTC)

Article Overload
This article tries to be too many things to too many people. It has sections on mathematics (Definition, Derivation of center of mass, etc.), physics (Rotation and centers of mass, CM frame), engineering (Engineering, the navbox on Automotive handling related articles) and astrophysics/astronomy (Barycenter in astrophysics and astronomy), as well as whole sub-sections about "Locating the center of mass" at the end of the article that could be considered how-to manuals and removed.

In my opinion, this article should either (a) be better organized (if possible) or (b) split into multiple disambiguation articles, like Center of mass (mathemetics), Center of mass (astronomy), etc. For example, if we're going to try re-organizing the article then the Definition section should have explanations from engineering, astronomy, etc., as well as mathematics.

Does anyone agree with me? I can handle the astronomy perspective but the math and engineering are beyond me. My guess is that this revamping will be a fairly long job, say two weeks to a month or even more. We should probably create a separate subpage (or subpages) where we can work on the new article(s) until they're ready to roll-out. --RoyGoldsmith (talk) 14:36, 28 September 2009 (UTC)


 * I whole-heartedly agree. I was frustrated by this article but not motivated to fix it.  I'll provide feedback but I don't really have the time to work on this article.  Good luck. MarcusMaximus (talk) 06:55, 1 October 2009 (UTC)


 * I can't do it alone. At a minimum, someone who knows math and engineering and I should get together and decide whether the single article is fixable or if we should begin cut-and-pasting separate disambiguation articles. Either way, it seems to me that some mention of mathematics will wind up introducing sections under astronomy, engineering, physics, etc. So we have to have that talk first. Otherwise our revision(s) will be worse than the current article. --RoyGoldsmith (talk) 15:56, 4 October 2009 (UTC)

Solar System Barycenter diagram
While reviewing this article, I noticed that the diagram "Motion of Barycenter of solar system relative to the Sun" was a relatively poor quality gif file, the source URL was to a PhotoBucket account that is now disabled, and it was for the years 1945-1996. Therefore I created two new diagrams and uploaded them to Wikimedia for you: 1) File:Solar System Barycenter 1944-1997.png is a complete recreation of the diagram in the article with the same years and path to verify validity of my algorithms, and 2) File:Solar System Barycenter 2000-2050.png which is a more "current" diagram for the years 2000-2050. Feel free to use one or both of these public domain png files if you want to replace the older diagram. Larry McNish, Calgary Centre of the Royal Astronomical Society of Canada. Larry McNish (talk) 12:45, 8 January 2010 (UTC)

Somewhat Dysfunctional Article
As an armchair philosopher with, among other sins, formal training in physics, and having immediate access to ancient texts of scientific wisdom in my library which I have consulted,  I am inclined to mildly suggest and regard that: center of mass, and center of gravity are DIFFERENT concepts relating to the discipline of PHYSICS; that centroid is a analogous but distinctly different term most familiarly used in MATHEMATICS/GEOMETRY; and that barycenter (and its British first cousin barycentre) are more ancient and ambiguously used but probably should take the modern definition related to solar system dynamics accepted and used by by NASA/JPL and IAU. While these are all weighted averages of analogous kinds, they are still DIFFERENT kinds. In an encyclopedia the distinctions should be made clear. Alas, it is not so here and now. I therefore suggest that those engaged in the editing of this article attempt to reestablish the page for center of gravity (physics). The redirection of center of gravity has made that edit non-trivial. The appended discussion concerning centroid  should be simply mentioned as a RELATED concept and redirected to the centroid pages. The solar system barycenter discussion may also deserve a tweek in light of these comments.

As a point of scholarly clarification, the center of gravity is defined IN PHYSICS (sub/mechanics), as that point CG within a distributed mass body for which a test mass m at point P would experience a resultant (ie net) 'gravitational force' as though the entire mass of body O was concentrated at point CG. (See the referenced texts in Mechanics). Math-wise (let ri, CM, CG be position vectors, M is total mass)

CM = Sum [ mi x ri ]/M (CM = center of mass)

CG / CG^3 = Sum [ mi x ri / ri^3 ]/M (CG = center of gravity);

F(P) = mGM x CG/CG^3 (gravitational force on mass m at P due to distributed mass M)

Further commentary: The center of gravity is not, in general, fixed but depends on the location P as well as the distribution of mass. In contrast, the center of mass CM of a body is characteristic of and determined entirely by the distribution of mass in the body, independent of location P. Spherically symmetric distributions, for example, have CM = CG, proof of which is left as a student problem (and a distinction Issac Newton originally sweated over before finally publishing his theory of gravitation ), while an oblate spheroid, such as the Earth is just ever so slightly off, explaining the comment in the article concerning Earth satellites. For distant objects (dimensions of body O insignificant) CG~CM. I hope these points clarify matters and will be of assistance in the improvement of this article. --LeChiff200 (talk) 14:25, 3 October 2010 (UTC)

The second hit on the center of gravity is NASA which says: "The center of gravity is a geometric property of any object. The center of gravity is the average location of the weight of an object." This article make the concept unnecessarily complex. I am not planning to use my wheelbarrow anywhere except on the surface of the earth CaptCarlsen (talk) 13:14, 20 April 2011 (UTC).


 * All men are born ignorant. By the grace of God's some lack wit and must remain ignorant, while others deliberately choose the safety of darkness. Pity neither, for it is the fool who seeks the light who ends up burned by it. Icarus. Concerning wheelbarrows: (1) Center of gravity (geometry) and center of gravity (physics) are different concepts. It is error to confuse these ideas. (2) Center of gravity (physics) and center of mass (physics) are also different concepts, and it is also also error to confuse these. (3) Properly loading a wheelbarrow in a flat gravitational field (the earth's surface) is obviously a matter of physics, not geometry. In the case of wheelbarrows, boats, and airplane loading one is concerned about balancing torques, that is, balancing the forces acting through lever arms, ie F x r in two dimensions.  For landscape architects loading wheelbarrows on the Earth's surface... CM and CG are conveniently in the same place anyway, so either will help with balancing the torques, but remember that torques are what is being balanced, not mass. For the rocketeer loading his spacecraft, best practice is to understand what you are doing and get it right. Just a general safety tip for rocketeers, for what its worth. GenKnowitall (talk) 22:44, 7 May 2011 (UTC)

Barycenter inside Sun
Why does this page say that "it is possible in some systems for the barycenter to be sometimes inside and sometimes outside the more massive body ... Note that the Sun-Jupiter system ... just fails to qualify" yet in the page for the 22nd Century, it claims "March 10, 2130: At 07:32 UTC, Sun passes through solar-system barycenter " (and links to this article)? —The preceding unsigned comment was added by 194.221.133.226 (talk • contribs) 20:04, 10 November 2006 (UTC)


 * It does sound like an inconsistency. Is there a source for either statement? Melchoir 20:14, 10 November 2006 UTC


 * Excuse if I misunderstand, but it seems to me a correct statement to say that 'at the present time' the Jupiter-Sun barycenter lies inside/outside the sun's photosphere (visible surface) but it is a separate and different thing to say where the solar system barycenter is or will be in 2130. Of the planets, Jupiter mass/position has the most significant effect on the SS barycenter, but does not decide it. --LeChiff200 (talk) 14:25, 3 October 2010 (UTC)

Misleading introduction
The first sentence reads, "The center of mass of a system of particles is the point at which the system's whole mass can be considered to be concentrated for the purpose of calculations." This is misleading - it depends on what calculations you are doing. For example, a spinning wheel has non-zero kinetic energy, even though its centre of mass is stationary. The sentence holds if the motion is linear (ie not rotating). Simplifix (talk) 21:59, 25 April 2010 (UTC)
 * Be bold. —Ben FrantzDale (talk) 12:21, 26 April 2010 (UTC)


 * Simplifix's point is a very good one. Center of mass is a very useful concept in statics and linear motion, but it is irrelevant in angular motion for the reason Simplifix has explained.  Dolphin  ( t ) 23:24, 26 April 2010 (UTC)


 * Well... the moment of inertia of system is at a minimum about the center of mass, and you can use the moment of inertia there to calculate the moment of inertia at any point from that. So CofM is really important in rotational analysis as well.- Wolfkeeper  00:21, 27 April 2010 (UTC)


 * The article presently says CofM can be used for certain calculations. Surely it is possible for us to be more specific than at present?  Dolphin  ( t ) 00:28, 27 April 2010 (UTC)


 * Just because "a spinning wheel has non-zero kinetic energy, even though its centre of mass is stationary" does not mean that the center of mass is not applicable to the calculations. The only way, short of integrating each mass element, that the total kinetic energy of the wheel can be correctly calculated is it to know the location of the center of mass. If it happens to be stationary, then it makes no contribution, but that cannot be known without knowing its location. That some centers of mass are easier to locate than others or are temporarily stationary does not diminish their importance.


 * To attempt to limit the utility of center of mass to statics and linear motion seems like an overreaction, especially given the utility of center of mass to such non-stationary and non-linear-motion fields as orbital mechanics. -AndrewDressel (talk) 01:42, 27 April 2010 (UTC)


 * When I listed statics and linear motion in the article I was not attempting to limit the utility of center of mass. (I will assume we all agree that center of mass is indeed relevant to statics and linear motion.)  It seems there is agreement that center of mass is also relevant to some aspect of orbital mechanics, so let's add that to the list.  A list of relevant fields is a lot more encyclopedic than the present text which says only that CofM can be used in certain calculations.  Dolphin  ( t ) 03:39, 27 April 2010 (UTC)


 * The center of mass can be used in almost any calculation, which is also true for any other point, so long as you define your equations using the correct kinematical relationships. In a number of cases the mass center results in simplified equations, but not always.  MarcusMaximus (talk) 07:00, 27 April 2010 (UTC)


 * Agreed. It might be a simpler task to enumerate the fields in which center of mass is not relevant. As I believe I've shown above, it is indeed necessary for calculating kinetic energy, even of spinning wheels. The task would be similar to listing the fields in which potential energy is relevant because otherwise a reader might mistakenly try to use potential energy to calculate the kinetic energy of a spinning wheel. In some situations it would work, and in others it would not be helpful. -AndrewDressel (talk) 12:11, 27 April 2010 (UTC)
 * For example, here are the dynamics equations provided in the back of my undergrad "Introduction to Statics and Dynamics" textbook:
 * $$\mathbf{P}= M\mathbf{v}_\text{cm}\,\!$$
 * $$\dot \mathbf{P} = \ M\mathbf{a}_\text{cm}\,\!$$
 * $$\mathbf{L}=\mathbf{r}_\text{cm}\times\mathbf{P}_\text{cm} + \mathbf{I}_\text{cm} \boldsymbol{\omega}$$
 * $$\dot \mathbf{L} = \mathbf{r}_\text{cm}\times\dot \mathbf{P}_\text{cm} + \mathbf{I}_\text{cm} \dot \boldsymbol{\omega} + \boldsymbol{\omega} \times \mathbf{I}_\text{cm} \boldsymbol{\omega}$$
 * $$ E_k = \tfrac{1}{2} mv_\text{cm}^2 + \tfrac{1}{2} I_\text{cm}\omega^2 \, $$
 * Notice how each one references the center of mass. -AndrewDressel (talk) 18:14, 27 April 2010 (UTC)


 * How about we dispose of the first sentence entirely and use something truthful, like "The mass center is the mean location of all the mass in a system", and then if we want to talk about how its use often simplifies equations, just say "Its use often simplifies equations". MarcusMaximus (talk) 19:51, 27 April 2010 (UTC)
 * Good solution. -AndrewDressel (talk) 21:30, 27 April 2010 (UTC)

MM's edit is a good one. The Introduction is much improved. Dolphin ( t ) 23:11, 27 April 2010 (UTC)

Sun-Jupiter barycenter
How far above the surface of the sun is the Sun-Jupiter barycenter? 75.118.170.35 (talk) 17:24, 23 September 2010 (UTC)


 * This is not on topic on this article talk page. It might be a good question for the science reference desk. Good luck over there. DVdm (talk) 18:56, 23 September 2010 (UTC)


 * I agree with DVdm. Your question will quickly attract a number of responses at the Science Reference Desk.   Dolphin  ( t ) 02:25, 24 September 2010 (UTC)

Explanation
I removed this material:

"In the Middle Ages, theories on the center of mass were further developed by Abū Rayhān Bīrūnī, Zakaria Razi (Latinized as Rhazes), Omar Khayyám, and al-Khazini.[citation needed]"

The material had been added by an editor whose contributions are under examination for POV and exaggerated interpretations of referenced articles. The full story is in and

Please do not replace this material unless it is clearly justified by references to reliable sources. Macdonald-ross (talk) 09:59, 22 October 2010 (UTC)


 * Center of mass:
 * I have checked all contributions to this article, and the above "[1]" link is the only change made to this article by the problem editor (see overview). The reference in the removed material is not reliable. Johnuniq (talk) 00:29, 23 October 2010 (UTC)

Summary style
I'm going to try splitting some of the more detailed material in this article into other articles. For example, Barycenter was merged into Center of mass six years ago, and the material has grown since then, so it will help to move the details to Barycentric coordinates (astronomy). Melchoir (talk) 00:25, 17 October 2011 (UTC)

How about a somewhat different lead?
The topics of center of mass (CM) and center of gravity (CG) have a long and controversial history in this Wikipedia. They had been presented in one article, then separated, and now are merged again. A particularly lengthy discussion (sometimes heated, but finally inconclusive) was held on the CG topic before the merger. Some very good points were maid, and many of them are included into the present article.

Except that the major argument for the present merger is not fully reflected in the present article form, the way I see things. The argument, roughly, was that most people would be confused with separate CM and CG articles because in common usage the terms are used as synonyms; but nobody denied that physicists see them as different concepts.

I believe that the lead should in more detailed and ballanced way address these aspects, but without engaging in too technical details. On one hand, the lead should first give a simple introductory information on the topic to those people expecting the synonymous meanings; and this information should concentrate on the CM concept (any too technical details on CG specifics should be moved from the lead into later sections). On the other hand, a more comprehensive account on the conceptual and historical differences, as well as differences in usage, should be presented in the second part of the lead. Therefore, I propose the following lead (and the changes in the rest of the article can be discussed if we agree on this):

In various applications of physical concepts, the terms CM, barycenter and CG are most frequently used as synonyms, denoting a point that in many respects behaves as if the entire mass and weight of a body (or of a system of material objects) is concentrated there. In addition, any of these terms is sometimes used as a synonym for the centroid in geometry.

Mathematical definition given below shows how the CM position is calculated as a weighted arithmetic mean of distribution of masses in space. In the case of a rigid body, the center of mass is a fixed point in relation to the body, and it may or may not coincide with its geometric center. In the case of a loose distribution of masses, such as the planets of the Solar System, the center of mass may not correspond to the position of any individual mass.

The mass center often obeys simple equations of motion, and it is a convenient reference point for many other calculations in mechanics, such as angular momentum and moment of inertia. In many applications, such as orbital mechanics, objects can be replaced by point masses located at their mass centers for the purposes of analysis. The center of mass frame is an inertial frame in which the center of mass of a system is at rest at the origin of the coordinate system.

Despite the predominant usage, the terms CM, barycenter and CG are not fully synonymous. In particular, physicists recognize a conceptual difference between CM and CG. And in astronomy, the term baricenter is usually reserved for the CM of celestial bodies orbiting each other.

The conceptual difference between CM and CG is that the concept of CM is not related to the force of gravity, whereas the concept of CG is. This amounts to the following:
 * CM may loosely be described as the average location of the mass of the system. It is used to simplify description and analysis of the inertial properties of the system. Without the CM concept, Newton's laws of motion would have to be applied to each point of a body separately.
 * CG may loosely be described as the average point of application of the gravitational forces felt (or exerted) by the system in interaction with its surrounding. It may be used to simplify description and analysis of static equilibria in gravitational field. Without the CG concept, weight of a body would have to be calculated for each point separately.

When the concept of CG is defined in this way, as related to gravitational forces, it does not generally coincide with the CM, and in many circumstances it may not be possible to determine the position of CG uniquely (if at all). But in a uniform gravitational field, the positions of CM and CG of a body (or of a system of material objects) are exactly identical. For small bodies on the Earth, gravitational field of the Earth is very nearly uniform, so that CM and CG practically cannot be distinguished. According to a texbook example, Petronas Towers in Malaysia, 452 meters tall, have their CG only about 2 centimeters below CM.

Due to this almost perfect coincidence, there is no need to distinguish CG from CM in the Earth field in practical applications. Nor was any distinction possible in the early research, before gravitational and inertial properties of matter were fully known, as can be illustrated by a brief chronology of terms and their meaning:
 * "Center of weight" was the term used from the ancient Greece time until about Newton. In Greek it was "baricenter" (βαρύκεντρον) and in Latin "center of gravity", where "gravity" actually meant "weight". Archimedes introduced this concept analyzing static equilibrium of weights. His "center of weight" fully coincided with the present CM, as there was no conceptual understanding of the Earth's gravitational field nor could anybody observe its small non-uniformity. Then he extended the concept to geometrical figures, where it was subsequently used under the name "center of gravity" until the term "centroid" was introduced (first known usage in 1882, according to the Merriam-Webster dictionary).
 * "Center of gravity", as a term in English language, appeared in 1648 (Merriam-Webster).
 * "Center of mass", as a term in English language, appeared in 1862 (Merriam-Webster). About one century earlier, Newton himself still used the expression "quantity of matter" instead of mass. Though at his time "matter" finally acquired both its inertial and gravitational properties, the word "mass" did not yet become a physical term.

In summary, for about 2000 years only the physical concept of CG (or barycenter), understood as the "center of weight", existed. But due to the almost full uniformity of the Earth's gravitational field, CG was located exactly at the position of CM. Only in recent centuries, the newly introduced term CM (together with better understanding of gravitation) finally made it possible to conceptually distinguish the inertial properties of matter from its behavor with respect to gravitational forces. However, in common usage, the tradition of fully coinciding positions and synonymous meaning still prevails.

--Ilevanat (talk) 02:10, 7 March 2012 (UTC)


 * Well, that's quite a lot of detail! It would be very helpful to some of this material, but I worry that concentrating on the lead isn't the best way to approach editing the article. According to Manual of Style/Lead section, the lead should be a short summary of the body of the article. So, for example, I recommend that historical information be added to the "History" section before being summarized in the lead. Also, it's important to observe Verifiability and No original research. We should be careful to cite sources for historical facts, and we should also be careful not to synthesize those facts into a novel understanding of the history of the concept(s).
 * Generally, I think it's better to be conservative about what we write; it's better to avoid addressing an issue than to address an issue in an un-encyclopedic way. I think for those of us who have strong viewpoints on the topic, one of the best ways we can make ourselves useful is to remove unsupported and controversial claims, rather than always attempt to rebut them. Melchoir (talk) 01:37, 12 March 2012 (UTC)


 * ...For example, in, I change "the CG" to "a CG" and replace a weasel word "many". I really want to avoid directly comparing "the two points". Melchoir (talk) 01:47, 12 March 2012 (UTC)

You are right: the above material is too long for a lead. And now that CM and CG articles are split again (in some sense, at least), I am not sure where such information would fit anyway. But historical facts will remain facts: the ancient guys talked only about weights (and sometimes about "quantities" in about the same meaning), inertia was not known much before Galileo, and the term "mass" is only couple hundred years old.

Such facts should perhaps be more "cautiously" stated, if there is a danger if "synthesizing them into a novel understanding of the history of the concept(s)". But then to say that Archimedes had assumptions about gravity and to mention the concept of field in that context...

Anyway, let us get to the more serious issues with the present article. Several illustrations show where CG is: imagine you can switch of gravity, and they are useless - as is not the case with center of mass. (Not to mention that "two same points" is not exactly true, if applicable at all.)

Next: CM is defined as the average...etc. OK, that may be so. But then one does not offer link to "derivation" of CM (you do not derive definitions). And about derivation of its properties, neither attempt is fascinating. I would much prefer the one below (which I already mentioned in the CG article):

The Newton second law (as well as the first) originally refers to a point particle; say, for a particle 1 it can be written as:
 * $$\scriptstyle \vec {F}_{total-on-1} = m_1 \vec {a}_{1}$$

Consider a system of e.g. three particles. Then one adds three such equations to obtain (internal forces cancel, and the total force is the net external force):
 * $$\scriptstyle \vec {F}_{net-ext} = m_1 \vec {a}_{1} + m_2 \vec {a}_{2} + m_3 \vec {a}_{3}$$

It is then obvious how a fictitious point C (the center of mass) should be defined in order to obtain the right-hand side of the above equation after two consecutive time derivatives:
 * $$ \scriptstyle m\, \vec {r}_{c} = m_1 \vec {r}_{1} + m_2 \vec {r}_{2} + m_3 \vec {r}_{3}$$


 * $$ \scriptstyle m\, \vec {v}_{c} = m_1 \vec {v}_{1} + m_2 \vec {v}_{2} + m_3 \vec {v}_{3}$$


 * $$ \scriptstyle m\, \vec {a}_{c} = m_1 \vec {a}_{1} + m_2 \vec {a}_{2} + m_3 \vec {a}_{3}$$

And then we get
 * $$ \vec {F}_{net-ext} = m\, \vec {a}_{C} $$.

The result is that the Newton second law for a particle can be stated in the same form for a system of particles (e.g. for a body), using CM. In addition, the equation with velocities shows that total momentum is:
 * $$ \vec p = m\, \vec {v}_{C} $$.

And there is quite enough room for it in this article.--Ilevanat (talk) 00:27, 22 April 2012 (UTC)

Expected Value
Someone removed my addition of Expected value from the additional links as "not related". It is very closely related - the expected value of a random variable with probability density $$\rho$$ is the same as the center of mass of a physical object with mass density $$\rho$$. I'm putting the link back in. 70.113.68.242 (talk) 14:39, 5 May 2012 (UTC)
 * One might also note the connection between the physics and mathematics definitions for higher moments, such as the connection between the moment of inertia tensor of a physical object and the covariance matrix of a probability distribution (both second moments). 70.113.68.242 (talk) 14:54, 5 May 2012 (UTC)
 * This is user:Dger . If he does not understand what expected value means, it does not imply that the definition of the center of mass is not the case of that concept. Incnis Mrsi (talk) 16:38, 5 May 2012 (UTC)
 * Mathematically, these concepts are similar that does not make them the same. An expected value does not require mass in its computation. Obviously, centre of mass does. Expected value is another name for the arithmetic mean in statistics. I don't see the need in for this "connection". Dger (talk) 21:44, 5 May 2012 (UTC)
 * Well, mathematically they *are* the same - the concept of center of mass is just the application of the mathematical concept of first moments to physics. There's a reason why it's called the "first moment", the moment of inertia is called the "second moment", and so forth, and this is because of the connection to probability. This is connection is made in most probability courses to give intuition to moments, and we even have a short sentence about it on the expected value page.. I don't see why you are so opposed to including it here. Would you be opposed to including references to the mathematical derivative on pages about the physics concept velocity? 70.113.68.242 (talk) 23:45, 5 May 2012 (UTC)
 * Your concept of same is not the same as my concept of same. Indeed formulas for centre of mass and expected value are the same but that doesn't mean the concepts are the same. One is a purely mathematical object the other is a physical property. Mathematical concepts do not require any application to the real world to be valid. For that matter, centre of mass doesn't require mathematics for its determination. It is useful to make links between mathematics and other sciences but they are not necessary for our understanding of the science or mathematics. Dger (talk) 16:58, 9 May 2012 (UTC)


 * If expected value is another name for the arithmetic mean in statistics, then measure is another name for the weighted sum in the measure theory. BTW read some books on the latter, you probably will realize that the "center of …" points and quantities cannot be defined correctly with your three arithmetical operations for continuous bodies. And don't try to convince yourself that real-world bodies are not continuous because consist of atoms – atoms itself are not material points. Incnis Mrsi (talk) 19:39, 7 May 2012 (UTC)
 * Expected value is NOT another name for arithmetic mean. Arithmetic mean is an unweighted average whereas expected value is a weighted average. If you are dealing only with numbers there is no difference, but if you are dealing with real objects each object may have a different weighting factor and therefore contributes differently to the mean. Dger (talk) 16:58, 9 May 2012 (UTC)
 * An arithmetic mean is the sum of a group of numbers divided by the number of numbers. That is NOT the definition of an expected value nor the centre of mass. Using arithmetic mean in the definition of centre of mass is quite misleading. A better definition, used later in the article, is a weighted average; even though weight implies a gravitational force. Weight is used in a different sense here. Dger (talk) 21:48, 9 May 2012 (UTC)


 * If you go to the article on expected value you need to scroll down very far to find its relationship to centre of mass, where you find one brief paragraph. Seems like a red herring to the average reader. That's why I took out the link. In fact the description there is really about a centre of gravity. Centre of gravity is the point where the moments of force on one side of the rod balances the other. If there is no gravity, from for example the Earth, there would be no "balancing" of the rod. There would, of course, still be a centre of mass. Dger (talk) 16:58, 9 May 2012 (UTC)


 * Well this discussion got heated rather quickly and that was not the original intent. The tone gives the impression of ganging up on Dger so I apologize for that. I still think we should include the connection to expected value.


 * As an aside, you might find it interesting to know that for random variables in infinite dimensional spaces, the usual concept of expected value breaks down because there is no lebesgue measure on infinite dimensional space. The usual generalization actually defines the expected value as the point which is a "balance point" in every direction. Ie: if under every linear functional, the pushforward of a point is the expected value of the pushforward measure, then that point is defined to be the expected value)! 70.113.68.242 (talk) 22:44, 9 May 2012 (UTC)

Definition of center of mass
I would like to add the following equivalent formulation to the definition of the center of mass. In particular, for a continuous body the coordinate vector R that identifies the center of mass has the property that the integral of density weighted relative position vector r-R is zero. This is written as
 * $$\int_V \rho(\mathbf{r})(\mathbf{r}-\mathbf{R})dV = 0.$$

This equation can be solved for the coordinate vector R to yield the formula,
 * $$ \mathbf{R} = \frac{1}{M}\int_V \rho(\mathbf{r})\mathbf{r}dV,$$

so the formulas are equivalent. However, I would propose that the addition of the first equation supports the insight that this point is the center of the mass distribution. Prof McCarthy (talk) 22:11, 4 October 2012 (UTC)

Center of gravity
This article is titled the center of mass but its first section discusses the center of gravity. The center of mass can be determined for any mass distribution and does not depend on gravity. This section starts wrong with the statement "center of mass is often used interchangeably with the term center of gravity". The problem is that neither center of mass or center of gravity has been defined and clarifying the differences not blurring their distinction should be the focus of this section. The definition of center of gravity as an average weight distribution is simply wrong, it is the point in the body where there is no resultant gravity torque. In a parallel gravity field, it happens that this corresponds with the center of mass. However, it is not true for satellites in earth orbit. I recommend that this entire section be deleted. Prof McCarthy (talk) 15:22, 5 October 2012 (UTC)


 * According to "Stability and Trim for the ship's Officer" CoG is "that point at which all the vertically down-ward forces of weight are considered to be concentrated"


 * According to a NASA web site:The center of gravity is a geometric property of any object. The center of gravity is the average location of the weight of an object.


 * Britannica: n physics, imaginary point in a body of matter where, for convenience in certain calculations, the total weight of the body may be thought to be concentrated. The concept is sometimes useful in designing static structures (e.g., buildings and bridges) or in predicting the behavior of a moving body when it is acted on by gravity.


 * Frankly I can't make head nor tails of the definition here, is there any good reason to take what seems to be a relativity simple concept and make it so complex? I would say that if the definition is both complex and incorrect it should be deleted.CaptCarlsen (talk) 02:38, 6 October 2012 (UTC)

Please make any changes you feel are appropriate. The concept of center of gravity is not central to this article. All calculations associated with center of gravity arise from the summation of forces of gravity in a near earth gravity field, and define the point where the resultant torque is zero. However, please feel free to write this in the way that you feel is best. Prof McCarthy (talk) 05:45, 6 October 2012 (UTC)


 * Thank you for your comments. The torque = 0 is simple enough. The problem with deleting the entire section is that "Center of Gravity" redirects here, which is how I arrived. The first and third definitions each contain a phrase ("are considered" and "for convenience")  that apparently are intended as a sort of disclaimer to avoid the problem caused by the differences between CoG and CoM. CaptCarlsen (talk) 11:48, 6 October 2012 (UTC)

Position (vector)
[]

There is nothing wrong in this change of wording, but the link position (vector), as well as a link on the word "weighted", were lost. It is not an improvement. Incnis Mrsi (talk) 12:33, 20 January 2013 (UTC)
 * [] thanks. Incnis Mrsi (talk) 08:22, 21 January 2013 (UTC)

Earth-Moon barycenter
In the section "Astronomy" is a wrong statement, that the barycenter is approximately 1,710 km (1062 miles) below the surface of the Earth. This is correct for a point mass body (calculated from a mass ratio only). But if this center is inside one body and if we are taking into account the density differences inside the Earth (input data are here ), so we will obtain about 1,400 km. 195.113.87.138 (talk) 11:06, 8 February 2013 (UTC)
 * It is silly. The density/depth dependence does not influence the CoM location at the (geometrical) centre of Earth (if shape of the planet is centrally symmetric, which is true with the precision of several tens of metres). The CoM location of a composite body depends only on masses and CoM locations of constituents – learn mathematics better. Incnis Mrsi (talk) 13:34, 8 February 2013 (UTC)

Linear/angular
The variable V has been left undefined in the Linear and Angular Momentum section. If it's the velocity of R, then "The total linear and angular momentum vectors relative to the reference point R are" isn't precise language. And I'm not sure what's going on with the 2nd term in the angular momentum equation.

173.25.54.191 (talk) 18:02, 1 November 2013 (UTC)

Lede needs improvement
The very first sentence of the lede is wrong or at least inadequate:"In physics, the center of mass of a distribution of mass in space is the unique point where the weighted relative position of the distributed mass sums to zero." It is rubbish. The phrase "weighted relative position" is completely meaningless since no weighing function is specified. I could weigh it based on its distance (which is the 'correct' function), √(distance), or distance², log(distance), -- even sin(distance) or some other nonsense. Weighing of a set involves a function f(x) on the elements x of that set and so the weighing is x*f(x). Additionally, "relative position" is meaningless, since it too is not defined. This isn't just an objection because the precise technical definitions are lacking, I object on the grounds that even for the general audience this sentence is effectively meaningless. (Or should I say "ineffectively"?) Why not say that "the unique point which is the average position of the mass of a distribution of mass is the center of mass"? Perhaps even add that in a uniform gravitational field, the center of mass is the point at which the mass can be balanced on a fulcrum. (I understand that 'uniform gravitational field' is problematic.)Abitslow (talk) 18:26, 4 November 2013 (UTC)

Two very important theoretical applications missing
The "Applications" section contains some useful real-world applications, but the entire article is devoid of the center of mass's two most important theoretical applications (in my opinion). The first is its use for calculating the moment of inertia via the parallel axis theorem. The second is the use of the center of mass frame as a way of doing ideal elastic collision problems much more efficiently. I think both of these applications deserve great attention in the article, perhaps above the existing "Applications" section.72.177.229.234 (talk) 09:24, 3 February 2014 (UTC)

Ships
This aticle needs a subsection on ships, as it has with planes. Ships being in two fluids, air & water, have more complicated motion. Lentower (talk) 01:02, 12 June 2014 (UTC)


 * Try these articles instead: Metacentric height and Buoyancy. Dger (talk) 02:24, 12 June 2014 (UTC)