Nobog - I have to disagree with you. The 'center of gravity' for that motorcycle, or car, or anything, is a point where the gravitational forces of the motorcycle even out - it's a point of balance. Center of gravity is, basically, the center of gravitational pull for that particular object. A motorcycle, or car, or whatever, with a low center of gravity is more stable than one with a higher center of gravity -- a motorcycle with a low center of gravity will be less likely to tip over than one with a higher center of gravity.
If you climb on a ladder, the higher you climb, the higher your center of gravity becomes - and the less stable (and more likely the ladder, with you on it, is likely to fall) you become. (Your center of mass is also higher, because most of the mass is coming up the ladder - but center of mass and center of gravitational forces aren't the same thing).
The gravitational center of the earth is an entirely different thing - and, to me, can't be confused with a thing's center of gravity.
I'm not entirely sure what center of mass means -- it sounds to me as if this is something WITHIN the item, rather than a force that occurs outside of the item. Take that same motorcycle, I would guess that, because there's probably more metal in the engine than in the frame, the center of mass for that motorcycle would be somewhere around, or inside of, the engine, while the center of gravity would be, one would hope, a few feet UNDER the bike.
I look forward to your posts more than many others because I usually learn something from them, but I don't agree that the center of gravity is the gravitational center of the earth (it applies to any item that you're evaluating - not the gravitational center of the earth), or that center of mass relates to a spot that is probably below the motorcycle that you mention.
And - one more thing - I suspect that the center of mass for this planet is probably at the same spot as the center of gravity - but the further out from this center you go, the more variance will exist between center of mass and center of gravity.
Hey, administrator - this isn't politics; it's off the original subject of this thread; and it's been friendly all the way through.
Perhaps Nobog and I will discuss this further, before you close this thread.