View Full Version : How can the centre of gravity of an object exist outside the object?


Aaaaaaaaaaaaaaaargh!
11th Jun 2008, 22:13
If you think about it, it's a little strange.

I wonder how different the unoiverse would be if there had been a physical law that said that such a situation couldn't exist.



chksix
11th Jun 2008, 22:22
In space there are often system cg's that lie somewhere in the middle between planets or stars that rotate as pairs.

I can't think of an example on earth though.

ORAC
11th Jun 2008, 22:22
Depends what your definition of an object is; which depends on many factors.

Beyond a certain range the solar system can be considered an onbject, as the internal forces become insignificant. Similarly beyond certain ranges the same can be said of the Mars/Phobos/Deimos or the Earth/Moon etc.

Of course, inside these ranges they come into play, which is why the moons of planets can be snatched away and the axial tilt and orbits of their closed systems can be changed.

Tell you what, give me a generic solution to the three body problem and I'll give you a fiver.... ;)

tony draper
11th Jun 2008, 22:29
The common center around which twin Planetary systems such as the Earth Moon rotate is called the Barrie (sp?)Center,I believe in the case of the Earth and Moon it is a just few kilometers below our surface.
:)
Not many people know that.

Loose rivets
11th Jun 2008, 22:34
tides are caused by gravitational sheer, not gravity per se, non?

there's alink, me thinks.

BlueWolf
11th Jun 2008, 23:02
In the depths of my memory is a tale about an inquisitive French chap (at least I think he was French) who dug a couple of wells some distance apart and dangled weights down them. Measuring the distance between the tops of the holes and the distance between the weights at the bottom (how, I have no idea), he was surprised to find the bottom distance to be the greater; from this, and the corresponding angle, he deduced the earth's centre of gravity to be some 4,000 miles out in space.

Weird, huh.

MadsDad
11th Jun 2008, 23:18
Then you get LaGrange points,here the gravitational pulls are equal between two object (so if you put something there it would stay there, with rqual gravitational forces acting on it).

For instance I would have thought there would be a LaGrange point between the Earth and the Moon, about 6/7th of the way towards the Moon in a direct line (in terms of mass Moon = 1, Earth = 6 so the balance would be there). It appears there are 5 LaGrange points. One of which is on the opposite side of the Earth to the moon.

I think I'll just have a lie-down to think about this.

G-CPTN
11th Jun 2008, 23:37
I can't think of an example on earth though.Fie, Sir - imagine a shape like a crescent moon - the CofG lies outside the body of the shape (though admittedly within the envelope).
It isn't beyond imagination to weight the limbs of a random skeletal structure such that the CofG falls outside the envelope of the shape . . .

Close:- http://www.youtube.com/watch?v=F0Mg0n-4UQg

tony draper
11th Jun 2008, 23:38
Once watched a rather silly Brit Sci Fi movie about a twin earth on the exact opposite side of the sun to the Earth ergo folks on Earth had no idea twere there because the Sun always sat in front of it,impossible of course, such a point would be unstable as it is not one of the La Grange points.
Sadly one's noggin no longer works like it should due to decades of overuse otherwise one could have told you the location of all the LaGranian points int Earths orbit,one has to use google like the rest of you now.
:(

Thirty06
11th Jun 2008, 23:41
Think doughnut.

Okay then, half a doughnut.

If your proposed law of physics existed, then you'd only be able to get the jam ones.

Davaar
11th Jun 2008, 23:53
With all these Australians around, how can the boomerang be overlooked? Or any hollow ball.

Dan D'air
12th Jun 2008, 00:05
Bugger. I did an OU degree in Physics and Astronomy when I was In the RN and this thread is now doing me in. I still believe that if you accelerate enough, then you can get past light speed. Oops.

Bullethead
12th Jun 2008, 00:10
Quote,
I can't think of an example on earth though.

How about on a horse then? :}

tony draper
12th Jun 2008, 00:11
Nope that would not work Mr D'air, twould be easier to stand still and get the rest of the Universe to accelerate past you at the speed of light.
:rolleyes:

Whirlygig
12th Jun 2008, 00:12
Yeah Dan, but it won't do much for your figure; might take years off you though!!

Cheers

Whirls

Dan D'air
12th Jun 2008, 00:28
Oh I wish Whirls, luckily i'm not elongated yet, am still (luckily) 5'10 and 190lbs so hope that all will be well.................

James 1077
12th Jun 2008, 01:28
Isn't it due to Man's general incapability of thinking on anything other than human sized scales?

As an atom is predominantly nothing it is perfectly possible for the centre of gravity of a bunch of atoms to be somewhere in the middle of nothing? Likewise with a solar system or a galaxy - its centre of gravity is also likely to be somewhere in the middle of nothing it is just difficult to imagine such a concept with our piddly brains that only really comprehend the idea of 4 dimensions (and even then can't really do time very well).

Also there is a concept of "matter"; just because two objects appear to us to be seperate, and therefore making us question how their centre of gravity exists outside of them, it could be perfectly possible for them to be simply one object when viewed with different eyes that see things that we do not.

Or somethin' like that. I always hated physix and philosophy.

Spodman
12th Jun 2008, 01:52
Bedevire: "And that, my liege, is how we know the Earth to be banana-shaped." Proving that the center of gravity of the Earth is outside the Earth.

On the other hand, the center of gravity of the Earth and the Moon is inside the Earth.

My Brain Hurts:ugh:

Aaaaaaaaaaaaaaaargh!
12th Jun 2008, 02:54
Once watched a rather silly Brit Sci Fi movie about a twin earth on the exact opposite side of the sun to the Earth ergo folks on Earth had no idea twere there because the Sun always sat in front of iThat was Doppleganger and I think it was an episode of Space 1999.

The guy got lost and ended up on the other world, and realised his predicament when he was in the bathroom and relaised he could read the label on the after shave in the mirror ... brecause everything was backwards.

It was pretty shite though.

BlueWolf
12th Jun 2008, 03:50
...and then this one time when I was stoned, I had this way cool experience, it was like the time I was communicating with this tree, and the tree took me down through its roots into the underworld, and there was as much tree underground as above it, and like this whole other world; like that, only cooler.

What happened was that I was communicating with this piece of greenstone, and then the greenstone was a river of light going deep into the ground, and I went with the river, and the centre of the earth was another earth, and the outside was the inside, and the inside was the outside, and the macrocosm and the microcosm were one and the same, and the earth was the mother but also the womb, and we are the child, and the earth is our mother, but we must be as her parent, and it was way cool.

Don't remember what gravity was doing, I don't think I was experiencing it at the time.

Sorry, what was the question again?

BlueWolf
12th Jun 2008, 03:52
Also, I think someone else may be using my login. Or at least I hope it's someone else :uhoh::ooh:

Bushfiva
12th Jun 2008, 03:54
The Fosbury Flop lets the body pass over the high jump bar while the cg passes under it. A good flopper can get the cg some 15-25 cm outside the body.

Beatriz Fontana
12th Jun 2008, 06:41
The trees know everything, Blue Wolf...

You know why we can't get this? Because we're like fish trying to understand water - too deeply immersed in it...

Now my head hurts.

Blacksheep
12th Jun 2008, 07:37
It isn't beyond imagination to weight the limbs of a random skeletal structure such that the CofG falls outside the envelope of the shape . . .
A simple exeriment can prove this. Drink twelve large glasses of Duvel (or the equivalent for your own locality) while seated. Then stand up, at which point you will find that your centre of gravity has moved outside the envelope of your random skeletal structure.

I now need independent researchers to confirm my findings, (but you have to provide your own Duvel.)

Farmer 1
12th Jun 2008, 07:54
Where would the c. of g. of a hydrogen-filled balloon be?

BombayDuck
12th Jun 2008, 08:05
What do you mean by "outside"?

If you look at a simple ring, the CoG will be it's geometric centre. But that is not *on* the ring, so it is outside the physical structure. Not hard to imagine now.

But the CoG will never be outside the boundaries set by joining all the extremities of the object to each other.

There is this awesome experiment in which one sticks two forks' pointy ends and a matchstick into each other. Then balance the entire thing via somewhere along the matchstick on the rim of a glass. Then burn the rest of the match away so that the system stabilizes on the tip of a matchstick. So it is that a single matchstick point holds up the weight of two forks on the rim of a glass.

Ah... found a link (http://www.wikihow.com/Make-a-Fork-and-Spoon-Appear-to-Defy-Gravity) :)

BlueWolf
12th Jun 2008, 09:12
I've tried that Bombay, and the forks fell down. :sad:

Also, a wide mouth with a tapered bottom well, I've known a few women like that, and none of them helped me defy gravity :sad::sad:

Also Blacksheep, I have done this, with Guinness, whilst young and silly in the UK, and what you say is true; now, however, that my physical boundaries have expanded, if I try it again, will my CoG remain within them?

We should be told.

Also also, the person who was using my login before :cool:, has now remembered that the river of greenstone/light which went into the ground, was also an umbilical cord. I think that that's way way cool.

Hokulea
12th Jun 2008, 10:29
A simple exeriment can prove this. Drink twelve large glasses of Duvel (or the equivalent for your own locality) while seated. Then stand up, at which point you will find that your centre of gravity has moved outside the envelope of your random skeletal structure.

I now need independent researchers to confirm my findings, (but you have to provide your own Duvel.)My experiments suggest that the centre of gravity can no longer be attributed to Newtonian physics and that it's likely more due to quantum effects; specifically the random shifting of the centre of gravity whenever one has had a drink or two. It's been explained by Heisenberg so I'm on solid ground although I think he forgot to mention the moving floor phenomenon in his papers.

Fitter2
12th Jun 2008, 10:50
The CofG of a helium ballon is approximately in the middle of the balloon. (Just because its mass is less than an equivalent volume of 20% O2, 80% N2 does not affect this).

I convinced myself of this while drinking from my coffee mug. Having emptied the mug, I then considered where its CofG was located....................

tony draper
12th Jun 2008, 10:54
The simplest way to picture gravity is the mass distortioning space model, rather then the action at a distance one, it fits common sense to picture a heavy lead ball sitting in the dint it's own weight has caused in the middle of a stretched rubber sheet,of course shifting this mental picture from the two dimensional to the three or four is the buggah.
:E

Blacksheep
12th Jun 2008, 11:16
shifting this mental picture from the two dimensional to the three or four is the buggah.Fitting it into just the four isn't too much of a problem, but once you go beyond the four that we can detect with our limited senses, it does get a trifle complicated.

With everything in the universe being in motion with respect to everything else (in many cases at high sub-light velocities) there cannot be a universal time constant. With every celestial object existing on a different time base to every other, there must therefore be an infinite number of possible dimensions. What we observe right now is merely the four dimensional frame within which everything we observe happens (or happened) to be in relation to everything else at a specific interval of our own time.

Celestial objects may almost certainly be appearing from or vanishing to other dimensions beyond our detected frame of existence (the space-time continuum as some, but not I, would have it) all the time and Mankind has simply not existed for long enough to observe the changes. What part gravity's virtual rubber sheet may play in this jolly jape of the gods is certainly hard to untangle. Perhaps that's why the gods gave us Single Malt Highland Whisky?

Dushan
12th Jun 2008, 18:13
I know I shouldn't link this to aviation, being JB and all, but imagine an aircraft on a giant conveyor belt.......


(coat, hat, asbestos suit...)

jayemm
12th Jun 2008, 18:24
I knew a lady at Uni whose C of G was most definitely outside of her body.:ok:

Aaaaaaaaaaaaaaaargh!
12th Jun 2008, 21:41
If you look at a simple ring, the CoG will be it's geometric centre....well, assuming the ring had uniform properties, such as thickness and density :8

tony draper
12th Jun 2008, 21:54
If you want a real puzzle ponder upon the C of G of rapidly rotating bodies such as yer Gyroscope,they seem to defy gravity.
:rolleyes:

John Marsh
12th Jun 2008, 23:50
That reminds me of an odd anecdote.

A man removed a device containing gyroscopes from the cockpit of an aircraft and walked backwards with it down the steps. As he stepped onto terra firma, he felt a sharp blow to his back. No one had hit him.:ooh:

Blacksheep
13th Jun 2008, 07:38
As an RAF Instrument Technician I worked with gyroscopes in the Calibration Lab. Fascinating things they are, even to those who understand the theory. One became quite adept at toppling them just the right amount when performing erection rate tests (now-now! No remarks please; :suspect:) I've wondered what effect gyroscopic forces from the rapidly rotating and massive Upkeep weapon had upon the controls of the 617 Sqn Lancasters during Operation Chastise. The cross coupling between roll and yaw must have been quite a problem. Helicopter pilots do it all the time of course, but these guys were novices in that department.

P.S. Apologies for raising aeronautical matters in JB and especially in this thread, but gyroscopes are so queer they don't really belong anywhere.

ShyTorque
13th Jun 2008, 15:05
A flat sheet (for example, made of metal) has its C of G in the middle of its mass.

The same sheet, bent into a curve, has its C of G outside of its mass.

Surely, that's fairly straightforward? Or bent forward.... :confused:

ft
13th Jun 2008, 21:41
Take one airplane, 30 feet long.

Let the reference datum be at the nose of the aircraft.

Remove 300 kgs of weight at a position 15 feet from the nose of the aircraft.

Moment change -300*15 = -4500 kg ft.

Add 310 kgs of weight at a position 20 feet from the nose of the aircraft.

Moment change 310 * 20 = 6200 kg ft.

Net mass change 310 - 300 kg = 10 kg, so you have added ten kg.

Total moment change = -4500 + 6200 kg m = 1700 kg ft.

The center of gravity of the added ten kgs must thus be at the position

1700/10 ft = 170 feet from the nose of the aircraft, or 140 feet behind the tail of the aircraft.

Now, try to explain that to project management and you will see why being in charge of W&B is no fun. :}

gizmocat
13th Jun 2008, 22:52
String theory anyone ?