CG/CM?

[Sampa]

Ok, I am trying to explain the difference between Center of Gravity and Center of Mass and how they work with an airplane or rocket to my friend in the AMT school.  Unfortunately, I am struggling to find adequate ways to explain it acurately.  Can anyone help me out?

[Tex_NL]

You are most likely struggling to find a difference between them because their basically is none.

Centre of Mass is the average of location of mass within a body. Centre of Gravity is the same but multiplied by gravity. However if  gravity is not constant over the entire body (e.g. body is extremely tall) CoM and CoG can differ.

[Sampa]

2 minutes ago, Tex_NL said:

You are most likely struggling to find a difference between them because their basically is none.

Centre of Mass is the average of location of mass within a body. Centre of Gravity is the same but multiplied by gravity. However if  gravity is not constant over the entire body (e.g. body is extremely tall) CoM and CoG can differ.

oh, ok.  I think part of the problems could be coming from the definitions I am using (from an aviation dictionary, btw)

Center of Gravity: Concentration of weight

Center of Mass: area of the plane where the mass is in equilibrium

[ZooNamedGames]

Yeah, the only time you see this defer is with something like a space elevator which as you should already know is huge. The CoM would be much higher than the CoG.

Now on a normal person, this is so minute, they're basically identical... as well as what @Tex_NL said. I think they summed it up a little too perfectly  left no one any other room to chime in.  Ah well, it means they did a great job! Can't complain about that can we?

[mikegarrison]

11 minutes ago, ZooNamedGames said:

Yeah, the only time you see this defer is with something like a space elevator which as you should already know is huge. The CoM would be much higher than the CoG.

Now on a normal person, this is so minute, they're basically identical... as well as what @Tex_NL said. I think they summed it up a little too perfectly  left no one any other room to chime in.  Ah well, it means they did a great job! Can't complain about that can we?

Gravity doesn't change all that much over the length of a space elevator. The apparent gravity (net of actual gravity and centrifugal effects) does.

But generally speaking, people use CG and CoM as synonyms. The traditional term in aeronautics is CG. This is because aeronautics is mainly concerned about it as a force. The location of center of gravity versus the center of lift establishes a pitching moment that needs to be controlled.

Edited September 26, 2016 by mikegarrison

[Ten Key]

It has been a long time since high school physics, but, can't you use offset centers of gravity and centers of mass for passive attitude control? 

https://en.wikipedia.org/wiki/Gravity-gradient_stabilization

[magnemoe]

35 minutes ago, Ten Key said:

It has been a long time since high school physics, but, can't you use offset centers of gravity and centers of mass for passive attitude control? 

https://en.wikipedia.org/wiki/Gravity-gradient_stabilization

Not passive, you could use it to keep one side towards earth. 
problem is that it make orbital positioning far harder. 

[Sampa]

Yeah, the definitions I posted were from a Jeppessen Aviation Dictionary, so, I suppose so.  but it is just a pain in the neck to explain to a friend about the difference and find proper examples.  So, thanks guys for chiming in!

[mikegarrison]

9 hours ago, Ten Key said:

It has been a long time since high school physics, but, can't you use offset centers of gravity and centers of mass for passive attitude control? 

https://en.wikipedia.org/wiki/Gravity-gradient_stabilization

Despite what the always authoritative wikipedia has to say, I think that is basically a tidal phenomenon, having to do more with the difference of "desired" orbital velocities at different altitudes than really having to do with gravity itself falling off at 1/R. It's like tidally locking a moon, except for something as small as a spacecraft it doesn't take a billion years.

[Nibb31]

Dragon uses a sled with a moving ballast to modify the CoM of the capsule on reentry.

[p1t1o]

2 hours ago, mikegarrison said:

Despite what the always authoritative wikipedia has to say, I think that is basically a tidal phenomenon, having to do more with the difference of "desired" orbital velocities at different altitudes than really having to do with gravity itself falling off at 1/R. It's like tidally locking a moon, except for something as small as a spacecraft it doesn't take a billion years.

Im pretty sure that gravity falling off proportional to 1/R is exactly why you have different orbital velocities and the resulting tidal effects.

[mikegarrison]

2 hours ago, p1t1o said:

Im pretty sure that gravity falling off proportional to 1/R is exactly why you have different orbital velocities and the resulting tidal effects.

Hmmm. Yes. That's true.

OK, that was a silly thing for me to have said.

Edited September 27, 2016 by mikegarrison

[Green Baron]

On 26.9.2016 at 9:34 PM, Sampa said:

Yeah, the definitions I posted were from a Jeppessen Aviation Dictionary, so, I suppose so.  but it is just a pain in the neck to explain to a friend about the difference and find proper examples.  So, thanks guys for chiming in!

Do i get this right ?

CoM is an attribute of the body. If you hang it at the CoM without any forces applied it should be in equilibrium.

Now if you apply forces to the body the CoG comes into play. If the resultant of the force goes through the CoM the body will move in a straight line, away from the force. If not, the body will start to turn and move, the CoG will shift with the resultant of the applied forces and the body, with it's CoM, will follow that movement. CoM is usually fixed, in case of a rocket it moves with the loss of mass.

Without forces CoG and CoM are identical.

With our rockets we applying forces in a directed manner using thrust/gimbal/SAS/RCS to steer the CoG around the CoM to set it on the planned course.

Edit: I got that wrong. That happens when one expects things to be more complicated than they actually are :-)

 

Edited September 28, 2016 by Green Baron

[wumpus]

Note that in the case of Philae (project that bounced on a comet) and Osiris-rex (mission that ends with "landing"/grabbing a sample from an asteroid) are dealing with gravity from masses small enough to have CoG and CoM differ enough to influence maneuvering.  It takes some pretty extreme sizes for this to be true, and typically only possible where the object producing the gravity in question is *tiny*.

[kerbiloid]

Put a barbell along your orbital radius.
Its CoM is always right in the middle. It's CoG is near the middle, but closer to the lower end, because GM/(r-x)² < GM/(r+x)² 

Edited September 28, 2016 by kerbiloid

[RCgothic]

Centre of mass is the average position of all the masses in a body. It is always fixed unless you add/remove mass or allow those masses to move around with respect to one another. If the resultant force of all applied forces acts through the centre of mass the body will translate without rotation. Any resultant moment will cause the body to rotate about its CoM without translating unless there is also resultant force.

Centre of Gravity is the average position of every mass weighted by the gravity field at the position of that mass. It is a concept that makes visualising resultant forces more easy by allowing a single bulk force to be applied at the CoG but it can be misleading. For instance, considering Gravity as applied to the CoG of an airliner will help you find the pitching moment, but it won't remind you that the weight of the wings still need to be supported back to the fuselage.

In a uniform gravity field, CoM and CoG occupy identical positions.

For shallow gravity fields (I.e. Earth's), the positional difference is usually negligible except for ridiculously precision applications or ridiculously large objects.

Edited September 28, 2016 by RCgothic

[Green Baron]

Hi,

so, only forces from an outside gravitational field influence the CoG. My assumption that forces from the body itself like thrust, moving internal masses influence the CoG is wrong, correct ?

I should know that by now, but i'm not ashamed to ask :-)

 

[Tex_NL]

What @RCgothic is saying is correct. And pretty much what I tried to condense into two sentences.

I appreciate all the rep/likes I received for my answer but I must admit I simply googled the answer. And you could have done the same. Google isn't scary, Google doesn't bite. Just literally type your question and click.

LMGTFY (Let Me Google That For You) difference between centre of mass and centre of gravity: http://lmgtfy.com/?q=difference+between+centre+of+mass+and+centre+of+gravity

Edited September 28, 2016 by Tex_NL

[monstah]

On 26/09/2016 at 3:45 PM, ZooNamedGames said:

Yeah, the only time you see this defer is with something like a space elevator which as you should already know is huge.

Wait, we're forgetting near black holes!

[Green Baron]

I see. Only the (parallel) forces of the gravitytional field.

How boring ... :-)