COM or COG?

[Sathurn]

Several people have been using CoM (Center of Mass) on the forum, I have always heard it as CoG (Center of Gravity).

Is one used more with rockets than the other?

Or is a regional country thing?

[joppiesaus]

I think it is a regional country thinggy. I called it center of mass.

[Jm419]

We've always referred to it as center of mass in classes, or the centroid.

[Guest]

Given a uniform gravitational field (g), then the values are virtually interchangeable. When g is no longer uniform, then I favour centre of mass over centre of gravity.

This article does a good job of differentiating the two: https://en.wikipedia.org/wiki/Center_of_mass

Edited June 4, 2013 by Jack Wolfe

[Nikolai]

I don't think it matters too much. The point of language is to communicate, and both terms are unambiguous -- so even if they happen to refer to the same thing, that's okay. Mission accomplished.

Insisting too much on one or the other seems to me to be an example of what Fred called "the tyranny of the small". It's easy for people who feel passionately about a particular subject matter to insist on minutiae that pass completely beneath the radar of people on the outside. My favorite example comes from real-life rocketry. There are those who insist that the thing in the back of your rocket that makes it go ought to be called an "engine", and others insist that it ought to be called a "motor"... and there are those on both sides who are very, very passionate about their chosen side.

I love rocketry, but I couldn't care less. A solution I really like came from someone who pointed out that that thing has elements of both engines and motors and elements of neither, so maybe we should call it something new; he suggested "whoosh generator".

[JaySmoka]

I think it's not the same thing. CoM use the "body" as reference, the other CoG use a gravity system where the body is close as reference. May be I'm wrong... May be there's a Newton ghost here able to explain it better.

[Flixxbeatz]

I'm fine with both, but I'm already used to CoM.

[cesarcurado]

Center of Mass.

It feels correct and also that's how it is called in KSP.

[ZedNova]

Center of mass. I'm pretty sure that's the more accurate term.

[AmpsterMan]

I always assumed "Center of Mass" was a scalar property and "Center of Gravity" a vector. Ie CoG is where gravity is affecting you and it is always aimed downward. CoM is just where your mass is calculated to be (for game purposes)

I doubt it makes any real difference, or if there even is a difference, or whether my interpretation is correct, or whether I am actuall a duck, etc. etc.

Edited June 4, 2013 by AmpsterMan
for humor

[Eric S]

Definitely CoM when talking about KSP, since not everything with mass has gravity. Shifting towards using that term everywhere, since it's generally the mass that you're concerned with. Yeah, I know, real world, same thing, it still feels more correct on a gut level.

[Jokurr]

I suppose in the setting of KSP I suppose CoM is more accurate since your craft is not necessarily influenced by gravity. At work, I refer to it as a center of gravity since that is what everyone else calls it. And since the machines I deal with are always firmly attached to the ground, it is just as accurate.

[CalculusWarrior]

I suppose it's a little like the difference between a torque and the moment about a point. They both refer to the same thing, only they are used by different professions.

[theflyingfish]

I try to use CoM because, the way KSP shows it in the VAB and SPH, that's what it's displaying.

[dagothar]

CoM is not the same as the CoG if the gravity field is not uniform. I believe it's CoM you really want.

[SuperFastJellyfish]

I use both. Way back when I was in flight school we used CoG or CG exclusively. I find myself using CG for planes and CoM for rockets in KSP.

[Brotoro]

I've never dealt with a rocket in any situation where the center of gravity was not indistinguishable from the center of mass. So I can handle either.

[KevinW42]

They mean different things. CoM is the point about which an object rotates, and CoG is the theoretical point that gravity acts on an object. Under most normal circumstances, however, they happen to be in almost exactly the same place, and I believe they're always the same in KSP.

[OrbitusII]

I may be American, but I prefer the S.I. unit system versus imperial and as a chemist I use mass more than weight. Mass is constant, weight is based on gravity, and constant measurements make more sense to my mathematical mind.

CoM

Edited June 4, 2013 by OrbitusII

[stupid_chris]

Well mass is universal to an object. Everything has a mass (well as far as we know) while gravity depends on where you are. They're basically the same but I think the convention is to say CoM since CoG assumes a gravitational force is applied to it.

[John FX]

Given a uniform gravitational field (g), then the values are virtually interchangeable. When g is no longer uniform, then I favour centre of mass over centre of gravity.

This article does a good job of differentiating the two: https://en.wikipedia.org/wiki/Center_of_mass

Same here. Gravity changes, mass tends not to. (I like your revenge quote, where is it from?)

[Sephta]

Center of mass is valid for one solid object (and for the reason it's solid, it's also the center of gravity . special case)

Center of gravity: normaly the (imaginary) point from where you calculate gravitational effects for a system with more than one object

Example for the latter: Earth + Moon they have one center of gravity (where the orbit around the sun is going through) but each object has it's own center of mass, which is not in the center of gravity for the system earth/moon.

As KSP is based on SoI, the center of mass is always the same as the center of gravity.

[Xaryn Mar]

Center of mass is valid for one solid object (and for the reason it's solid, it's also the center of gravity . special case)

Center of gravity: normaly the (imaginary) point from where you calculate gravitational effects for a system with more than one object

Example for the latter: Earth + Moon they have one center of gravity (where the orbit around the sun is going through) but each object has it's own center of mass, which is not in the center of gravity for the system earth/moon.

As KSP is based on SoI, the center of mass is always the same as the center of gravity.

The Earth and Moon also have a center of mass (located inside the Earth). The center of mass is just that: The center of the combined masses of the objects in the system. Whether or not they are one solid object or millions of particles (this is why we can talk about the center of mass of a gass cloud).

Most of the time though the CoM and CoG will be the same or very close together.

[TooManyErrors]

I always refer to it as the CoM because when you're in orbit, you don't feel the effects of gravity.

[Guest]

Same here. Gravity changes, mass tends not to. (I like your revenge quote, where is it from?)

It's something I saw years ago, and it drifted back into my head recently and found its way into my signature.