I just got around to scanning my April AMA magazine and found some very curious statements in the "Safety Comes First" article.
The author starts the article talking about a rogue group of modelers who are building and flying aircraft that have no CG at all, and that everyone must be sure to install a CG in their aircraft.
Just how do you "install" a CG? Unless he's talking about something other than "Center of Gravity", I don't see how an aircraft can NOT have a CG, nor how you could install one if it doesn't.
Or should I just assume this is an April Fool's joke?
It is after all April... but it immediately got me thinking about controlling a model with a variable CG. A weight shift control would work for a model that was essentially a FF design. Response might be kinda slow in some attitudes but it could still be fun to play with the idea. Bob
On Wed, 7 Apr 2010 08:51:08 -0500, "MJKolodziej" wrote in :
Hmm.
It operates within earth's gravitational field (you have to go along way toward the moon to reach the point where the earth's gravity is precisely counterbalanced by the moon's).
The shuttle does not become massless when it possesses a velocity and a trajectory sufficient to offset the pull of gravity.
The fellows who designed the thrusters certainly had to think about the center of mass of the shuttle.
There is no center of lift in the vacuum, of course, so our ordinary anxieties about the relationship between center of lift and center of gravity (or center of mass) is not an issue.
Bottom line for me: a mass in motion within a gravitational field will have a calculable center of mass and center of gravity even if its motion is such that it is not accelerating toward the barycenter.
Yikes! Fr. Marty must be studying to be an IT tech. (Some of those ppl seem to enjoy confusing mere C.S. mortals).
These are honest questions:
What is the difference between center of mass and center of gravity? (I think I know, but I want to see what Fr. Marty is emphasizing).
This is the first time that I have seen THREE terms for a single(?) concept - what on earth, or in the universe is "barycenter" and how does it differ from center of mass or center of gravity?
"...will have a calculable center of mass and center of gravity..." Calculable? Maybe just say it will HAVE a center of mass and center of gravity.
"...even if its motion is such that it is not accelerating toward the barycenter." Stationary objects (on earth, wrt the earth) have a center of mass and center of gravity. I therefore believe what you said about "not accelerating", etc., but what is your point?
Are you a physics teacher? I have been a mathematics teacher, so I love lucid explanations. Thanks for any clarification.
On Thu, 08 Apr 2010 00:07:49 -0700, snipped-for-privacy@given.now (Joe) wrote in :
I'm root admin on a couple of websites and webmaster on several others. I'm co-chair of the Big-8 Management board. So, yes, there is a bit of IT in me. ;o)
The center of mass is a point located in 3-dimensional space. The body in question is balanced along all 3 axes around that point.
The center of gravity as we use it with flying machines is concerned only with the fore-and-aft axis of an aircraft. The location of the CG with respect to the center of lift will affect how stable the aircraft is in pitch. The other two axes (roll and yaw) are neglected for the purposes of that calculation.
The barycenter is the point around which two bodies rotate (e.g., the earth and the space shuttle or the earth and the moon). The SYSTEM's barycenter is not at the center of mass of either body.
OK.
The original question was whether the space shuttle has a CG. I say it does have such a point even though there may not be much need to worry about it when the shuttle's trajectory and momentum hold it in equilibrium against the tug of gravity so that it is in a stable orbit. We speak about it as "weightlessness" because of the balance of forces but it is not "masslessness" nor is it "gravitylessness."
I am a systematic theologian by trade. I am interested in theological epistemology, which, in turn, gets me involved with philosophy and especially the philosophy of science. I co-authored the first biography of Michael Polanyi, who was a medical doctor, physical chemist, economist, social analyst, philosopher and theologian (of sorts).
Nothing. Center of gravity is simply an imprecise name for center of mass.
Obviously not raised Catholic or Protestant...;-)
The barycenter (literally, center of heaviness) of a single rigid object is simply its center of mass. The barycenter of a system of two or more objects is the center of mass of that system, in their mutual orientation at the instant it is evaluated.
The barycenter of the Earth is at the center of the Earth. The barycenter of the Moon is at the center of the Moon. The barycenter of the Earth-Moon system is on the line that joins their centers of mass, about 4670 km from that of the Earth (which means it's well inside the Earth).Actually, "barycenter" is hardly ever used in connection with a single object, because "center of mass" is perfectly adequate. The "orbit of the Earth" about the sun is actually the orbit of the barycenter of the Earth-moon system.
Pellets coming out of a shotgun have a barycenter, too. In the absence of gravity and aerodynamic forces, their barycenter would fly straight along the extended centerline of the bore.
I once read that there is a theoretical difference:
Imagine a very, very tall object standing on the earth. The earth's gravity will be weaker at the top of the object than at the bottom. Therefore, the center of gravity will be closer to the earth than the center of mass.
In other words, the center of gravity will move depending on the object's location in the gravity field(s), while the center of mass will remain fixed relative to the object.
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