# The Naked Scientists Forum

### Author Topic: Gravity... are we making a grave mistake?  (Read 16460 times)

#### DoctorBeaver

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##### Gravity... are we making a grave mistake?
« on: 19/12/2005 11:31:32 »
If you have a ball bearing, it will have a certain amount of gravitational force associated with it that acts as if concentrated at its centre of mass. Now, if you take a load of ball bearings & glue them together, the total gravity of your construction would act from the centre of mass of the whole.
As the gravitational attraction of this construction would be uniform in all directions, that implies that a ball bearing on the far side is contributing as much gravitational attraction as 1 on the near side. But, as gravity follows an inverse square law, the individual gravity of the ball bearing on the far side should be proportionately less than that of the ball bearing on the near side. This, in its turn, implies that whatever gravitational attraction each ball bearing has, is, as it were, sucked into the middle of the whole mass & then re-emitted. This would certainly appear to be the case were gravity a particle-based phenomenon.
Furthermore, the strength of the gravitational field of an object does not alter with time. The logical conclusion from this is either that no particles are being emitted or that for every particle that is emitted, another is absorbed. Any particle that is absorbed would have to have been emitted by another object. However, were our construction zapping out particles all over the place in the hope of finding another object to exchange with, that, surely, would gradually reduce its overall energy until, eventually, there were no more particles to emit. This is clearly not the case.
That means that somehow our construction "knows" when another object is close enough to exchange with. If that is so, it means that the force of gravity comprises 2 parts; the 1st, that which detects the other object and 2nd, the exchange of gravity particles. This would imply that gravity is not, in itself, a fundamental force and that something even more fundamental is at work. Hence, any theory that unites gravity, as no more than an attractive force between 2 objects, with the other forces must still be incomplete.
Another option is that there is no exchange of particles at all. Relativity says that mass distorts spacetime such that any object entering the gravitational field of another object is, in effect, going downhill (I know that's not a good analogy but it's the best I can manage). This means there is no actual attractive force between the objects; there is no direct interaction between them, no exchange of particle - just this "dip" in spacetime. Does this imply that gravity is the force that distorts spacetime? If so, that still means that gravity is not a force of attraction.
Right... that's the easy stuff out of the way. Now let's get to the dodgy bit! I'll use the rubber sheet analogy as I think everyone is familiar with that.
I've come across a few theories that propose that gravity leaks into the 4D universe with which we are familiar, from an extra dimension (I've seen recent reports of a forthcoming experiment to detect gravitons escaping back to their home dimension). However, to produce the effects of gravity as we see them, more must leak in where there is more mass.
So, what if all mass acts as a "gateway" into that extra dimension. Think of the rubber sheet - an extremely thin  1. Mass resting on that sheet causes it to "tear". Obviously, the greater the mass, the larger the tear. Now, gravity leaks in through that tear, so the bigger it is, the more can get through. This is where the sheet analogy fails. That tear is a function of the interaction between the mass and the whatever-it-is that separates us from the gravitic dimension. Therefore as the mass moves, the tear moves with it and so does the leakage of gravity.
But even this means that there is some force apart from gravity at work - the force that causes the tear. In conclusion, it seems to me that whichever you look at it, uniting gravity with the other forces will not produce a GUT.
I'm sure that more-able minds than mine have pondered this, but I can't find any mention of it anywhere.

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##### Re: Gravity... are we making a grave mistake?
« Reply #1 on: 19/12/2005 12:57:35 »
Am I alone in thinking that all gravity comes from the past, that being the singularity at the begining of our universe.
That while Time flows in one dirrection the attractive force of gravity acts as if it is trying to take us back in time.

Tomorrow lies outside our universe without it there would be no tomorrow.

#### DoctorBeaver

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##### Re: Gravity... are we making a grave mistake?
« Reply #2 on: 19/12/2005 13:04:51 »
Yes, you probably are
« Last Edit: 19/12/2005 14:13:37 by DoctorBeaver »

#### another_someone

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##### Re: Gravity... are we making a grave mistake?
« Reply #3 on: 19/12/2005 20:53:01 »
As far as I can tell, the difference is that if you have two ball bearing, a distance 2r separating them, and a distance d away from you, then the gravitational force should be: 2g/(r^2 + d^2).

If d is significantly larger that r, then this would approximate to 2g/d^2, which is what you would expect.

Clearly, if d is comparable in magnitude to r, then it would be a different matter, but then it is clear that in the extreme, that d is smaller than r (i.e. one ball bearing is one side of you, and the other is the other side of you) then it is logical that the approximation of assuming all the gravity emanating from the centre of gravity of the two ball bearings would obviously break down.

In other words, the assumption that the gravitational pull is from a point between the two ball bearings is only relevant at great distance.

#### DoctorBeaver

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##### Re: Gravity... are we making a grave mistake?
« Reply #4 on: 19/12/2005 22:05:15 »
another_someone - I appreciate what you're saying, but I was thinking of a construction of many ball bearings pressed together. You wouldn't then be able to get between them. The gravitational attraction is measured as if it all emanated from a single point at the centre of mass.
However, in your example of being between 2 balls, your mass would have to be added to the system. If the balls were of equal weight and equidistant from you, then the centre of mass of the system would be somewhere inside you.

#### another_someone

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##### Re: Gravity... are we making a grave mistake?
« Reply #5 on: 19/12/2005 22:31:31 »
quote:
Originally posted by DoctorBeaver

another_someone - I appreciate what you're saying, but I was thinking of a construction of many ball bearings pressed together. You wouldn't then be able to get between them. The gravitational attraction is measured as if it all emanated from a single point at the centre of mass.
However, in your example of being between 2 balls, your mass would have to be added to the system. If the balls were of equal weight and equidistant from you, then the centre of mass of the system would be somewhere inside you.

But the point still stands, that I would believe that the inverse square law (from a nominal centre of gravity) only applies when the distance between you and the object is large in comparison to the size of the object.

The inference is that the inverse square law would not neatly apply to object on the surface of the Earth.  In fact, since gravimeters are used to measure variations of the Earth's gravity at its surface.  Clearly, these variations could not exist if the gravity of the Earth was as if it originated from a point source.  Thus the assumption that gravity works as if it was from a point source does not work when close up.

This is true, even if you are not actually getting below the surface of the Earth, or between the ball bearings of your example; but merely close enough that the size of the object is of the same order of magnitude as the distance between you and the object itself.

#### DoctorBeaver

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##### Re: Gravity... are we making a grave mistake?
« Reply #6 on: 19/12/2005 22:40:44 »
But it has been shown that the gravitational pull of the earth is fractionally less at the top of a mountain than it is at ground level. That shows that gravity cannot be coming from the surface.

#### another_someone

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##### Re: Gravity... are we making a grave mistake?
« Reply #7 on: 19/12/2005 23:32:17 »
quote:
Originally posted by DoctorBeaver

But it has been shown that the gravitational pull of the earth is fractionally less at the top of a mountain than it is at ground level. That shows that gravity cannot be coming from the surface.

What it shows, not surprisingly, is that much of the Earth's gravity does not emanate  from the surface – because most of the Earth is below its surface.  It does not show that the Earth behaves, at close quarters, like a point source of gravity.

If you were to say that gravity, as you ascend a mountain, were to accurately follow the inverse square law as if all the gravity emanated from the centre of the Earth, then you would have a point; but merely saying that it is less, without describing the rate at which it declines as you ascend, does not prove your point.

Even if all the gravity did emanate solely from the surface, you would still expect a reduction of gravity as you climbed a mountain in Scotland (even if only a very slight decline in gravity) as you move away from the gravitational pull of Australasia.   But, in fact, you would anyway still expect a lot of gravitational pull from the solid mass of Earth all the way between Scotland and Australasia.
« Last Edit: 19/12/2005 23:38:54 by another_someone »

#### DoctorBeaver

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##### Re: Gravity... are we making a grave mistake?
« Reply #8 on: 20/12/2005 00:24:06 »

Your question is more complicated than it might appear. Gravity, as
best is known, is always attractive. Every particle in the Universe
attracting every other particle in the Universe. Newton's law:
F = G * M * m / R^2 where 'F' is the attractive force, 'M' and 'm' are the masses of two objects and 'R' is the distance separating the two objects.
The constant of proportionality 'G' = 6.67259x10^24 N*m^2/Kg^2. The AVERAGE radius of the earth is ~ 6.38 x 10^6 meters and its AVERAGE mass is such that: F = g * m where 'g' is the collection of AVERAGE constants G*Mearth/(Rearth)^2. has the familiar value 9.8 m/s^2. Among many web sites you can find the math worked out at http://hypertextbook.com/facts/2002/SamanthaDong2.shtml
Now this is the value used in introductory physics to work problems. In actual fact the force due to gravity varies with separation and the
density of the matter between the two bodies. Geologists use sensitive instruments that measure the change in the force of attraction at various places on the earth. For the purpose of your calculation, you only have to consider the 1/R^2 dependence to calculate the change in weight between a valley and a mountain, but "the real" calculation would have to take into account the fact that as you climb the mountain you are putting more "mass" between you and the center of the earth, so things are more complicated.

Vince Calder

Doesn't explain it in full, but does show that it's not at all straightforward. However, it says at the end "...between you and the centre of the Earth". Is that, then, not to be taken literally and is it, in fact, no more than a way of simplifying the maths?
« Last Edit: 20/12/2005 00:32:59 by DoctorBeaver »

#### another_someone

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##### Re: Gravity... are we making a grave mistake?
« Reply #9 on: 20/12/2005 00:43:55 »
http://en.wikipedia.org/wiki/Gravity
quote:

Thus gravity is proportional to the mass of each object, but has an inverse square relationship with the distance between the centres of each mass.
Strictly speaking, this law applies only to point-like objects. If the objects have spatial extent, the force has to be calculated by integrating the force (in vector form, see below) over the extents of the two bodies. It can be shown that for an object with a spherically-symmetric distribution of mass, the integral gives the same gravitational attraction on masses outside it as if the object were a point mass.

#### DoctorBeaver

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##### Re: Gravity... are we making a grave mistake?
« Reply #10 on: 20/12/2005 00:50:15 »
OK. Thanks for the clarification.

However, it doesn't negate my original argument, it just alters it slightly & shows that I phrased it a bit wrong.

#### Soul Surfer

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##### Re: Gravity... are we making a grave mistake?
« Reply #11 on: 20/12/2005 08:22:21 »
As "another someone" has pointed out the description of gravity acting from the centre of mass of an object is a convenient simplification of what is really a very complex situation.  Many of the familiar discriptions we use of physical phenomena are also similar approximations and only applicable within limited regions.  Many of the ideas and discussions hre come as a result of extending simplifications like this beyond their region of applicability.

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#### Soul Surfer

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##### Re: Gravity... are we making a grave mistake?
« Reply #12 on: 20/12/2005 08:28:38 »
One other very interesting simplification that is not always understood or used is the fact that if you have a hollow spherical shell of uniform density the gravitational field inside the shell is Zero!  This means that the as you go down towards the centre of the rearth the acceleration due to gravity drops linearly to zero at the centre of the earth.

This fact also means that if you have a very large uniform body of indefinite extent (for example a simple universe!) the net gravitational field inside the body is zero.  You would only get gravity if you could detect that you were nearer one edge than the other and if the universe had started expanding very rapidly (as ours appeares to do) this information may not have reached us yet and may never reach us.  This is what is meant by having a flat universe

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« Last Edit: 20/12/2005 08:43:35 by Soul Surfer »

#### DoctorBeaver

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##### Re: Gravity... are we making a grave mistake?
« Reply #13 on: 20/12/2005 11:53:31 »
Ian - thanks for that. So I can blame those buggers who over-simplified it in the 1st place! grrrrrrrr

quote:
This is what is meant by having a flat universe

I thought that meant the geometry of the universe was basically Euclidean rather than Riemannian.

#### another_someone

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##### Re: Gravity... are we making a grave mistake?
« Reply #14 on: 20/12/2005 12:18:37 »
quote:
Originally posted by DoctorBeaver

Ian - thanks for that. So I can blame those buggers who over-simplified it in the 1st place! grrrrrrrr

quote:
This is what is meant by having a flat universe

I thought that meant the geometry of the universe was basically Euclidean rather than Riemannian.

But if there is no detectable gravity, then would we not in fact have a  Euclidean geometry, since there would not be any gravity to curve space?

#### DoctorBeaver

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##### Re: Gravity... are we making a grave mistake?
« Reply #15 on: 20/12/2005 13:22:57 »
quote:
But if there is no detectable gravity, then would we not in fact have a Euclidean geometry, since there would not be any gravity to curve space?

Good point [V]

#### Soul Surfer

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##### Re: Gravity... are we making a grave mistake?
« Reply #16 on: 20/12/2005 16:37:30 »
Beaver, Rules of thumb and approximations are very valuable when it comes to understanding something and doing a bit of original thinking its just that you've always got to remember the conditions in which they apply and not stray too far out of the box, or, if you do want to push the limits look for a new model or approximation.  I have experienced more problems with mathematical pedants bashing their head against a brick wall of their own construction than I have with original thinkers getting an approximation wrong, finding out, and getting round the problem to create something worthwhile.

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#### Solvay_1927

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##### Re: Gravity... are we making a grave mistake?
« Reply #17 on: 23/12/2005 22:52:05 »
Eth,
going back to your original posting above, one thing occurs to me. (I'm not fully sure if this is one of the things you were getting at or not - the wording in your original posting was a little ... er ... confusing in places.)

If you stick a million identical ball bearings together in a spherical shape, their combined gravitational attraction will be 1 million times that of a single ball bearing.

The ball bearings at the surface of the sphere will obviously be able to interact gravitationally with external bodies (i.e. other masses outside the sphere).  But those trapped well inside the sphere will just spend their time interacting with others inside the sphere (especially if, as you say, the exchange of gravitational force might be mediated by particles - "gravitons" or whatever).

So the question is, how can the total gravitational attraction of the sphere be proportional to the total number of ball bearings in it (1 million)? How can the ball bearings on the inside contribute to the total attraction, given that the inner ones can’t “see out” to interact with the external world?  Why isn’t the total gravitational attraction of the sphere proportional to the number of ball bearings (c.50,000) on the surface instead?

Or is this just a stupid question that betrays my complete ignorance of physics?[:I]

Can anyone help me to understand this?

Ta.
Paul.

#### another_someone

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##### Re: Gravity... are we making a grave mistake?
« Reply #18 on: 23/12/2005 23:30:25 »
quote:
Originally posted by Solvay_1927

Eth,

So the question is, how can the total gravitational attraction of the sphere be proportional to the total number of ball bearings in it (1 million)? How can the ball bearings on the inside contribute to the total attraction, given that the inner ones can’t “see out” to interact with the external world?  Why isn’t the total gravitational attraction of the sphere proportional to the number of ball bearings (c.50,000) on the surface instead?

Simple answer – do you know any way of blocking out gravity – if you do, I'd patent it real quick, you've just made a fortune inventing an anti-gravity machine

My impression of DoctorBeaver's thoughts on gravitons was that the balls on the outside would channel the gravitons in the same way that iron channels a magnetic field, but not that it would block the gravitons (maybe absorb and re-emit – he will have to explain that – but not just swallow up and not disgorge again).

quote:

Or is this just a stupid question that betrays my complete ignorance of physics?[:I]

Probably – but we'll allow you special dispensation for that (as if I don't ask enough stupid questions myself [:I]).
« Last Edit: 23/12/2005 23:39:33 by another_someone »

#### Andrew K Fletcher

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##### Re: Gravity... are we making a grave mistake?
« Reply #19 on: 24/12/2005 09:18:54 »
Why does a compass point towards a mountain when close to it if the force at the top of a mountain is presumed to be less than at ground level? You may find that the actual gravitational force at the top of the mountain is greater, but counterbalanced by the centrifugal force of the planets rotation perhaps?

Eth, when people with multiple sclerosis experience high altitude standing on top of a mountain, their condition improves greatly. I found two cases in a magazine sharing their experiences, finding that they both could walk normally at the top of the mountain, leaving behind their sticks for support, yet when they got back to sea level they could not walk unaided. Fascinating when we were able to get the same results by tilting a bed to make use of gravity on the nervous system.

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#### Soul Surfer

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##### Re: Gravity... are we making a grave mistake?
« Reply #20 on: 24/12/2005 09:47:21 »
Andrew what you are saying is not true and I doubt that any effects on people with MS have anything to do with gravity.

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#### Solvay_1927

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##### Re: Gravity... are we making a grave mistake?
« Reply #21 on: 24/12/2005 17:46:30 »
quote:
--------------------------------------------------------------------------------

Or is this just a stupid question that betrays my complete ignorance of physics?

--------------------------------------------------------------------------------

Probably ...

I could go off you, another someone! [:p][:p][:p]

OK, so I know it was a really stupid question.  But I suppose what I'm getting at is why can't you block gravity?  How can the gravitational influence (on me, say) of each ball bearing not be affected in any way by the presence of other ball bearings which are in the way?

None of the other forces work like that, do they?  (Magnetism/electromagnetism - or even the weak & strong forces?)  And no particles can bypass all matter? (OK, neutrinos come close in that most of them can pass through the entire earth without intereacting with anything - but they don't all get through.)

Don't worry, I'm not expecting anyone to have an answer.  I'm just thinking aloud.

#### another_someone

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##### Re: Gravity... are we making a grave mistake?
« Reply #22 on: 24/12/2005 19:54:02 »
quote:
Originally posted by Solvay_1927

OK, so I know it was a really stupid question.  But I suppose what I'm getting at is why can't you block gravity?  How can the gravitational influence (on me, say) of each ball bearing not be affected in any way by the presence of other ball bearings which are in the way?

None of the other forces work like that, do they?  (Magnetism/electromagnetism - or even the weak & strong forces?)  And no particles can bypass all matter? (OK, neutrinos come close in that most of them can pass through the entire earth without intereacting with anything - but they don't all get through.)

Don't worry, I'm not expecting anyone to have an answer.  I'm just thinking aloud.

Gravity is a kind of peculiar force.  Firstly, the quantum physicists have not really come to grips with it, so it clearly doesn't behave like anything else, but conversely, the relativity guys are really only able to deal with gravity.

Secondly, I don't know about how (or even if) you can block strong or weak forces (they're such short range that I can't imagine there's much practical need for it).

The electrical force can only be blocked by an opposite charge (i.e. a positive charge will attract a negative charge to cancel it out), but simply putting a piece of non-conducting material in the way of an electric field will not block an electric field.  Gravity is totally different because it does not have an opposite (there is no positive and negative polarity of gravity – if there were, things could get very interesting).

Electromagnetism is something else, because it is an oscillating field, and it induces an opposing oscillating field (in the appropriate materials) which reflects the incoming field.  What happens with gravity waves, assuming we ever get to see them, is another matter (maybe someone else knows the answer to that, but I don't).

One interesting question, since gravity is associated with mass, which is also associated with inertia; if you could somehow block out gravity, what would you expect to happen to inertia?

Another interesting question is what would the gravitation field of a tachyon look like?  Ofcourse, how you might pin down a tachyon is another matter.

#### Andrew K Fletcher

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##### Re: Gravity... are we making a grave mistake?
« Reply #23 on: 26/12/2005 19:00:33 »
quote:
Originally posted by Soul Surfer

Andrew what you are saying is not true and I doubt that any effects on people with MS have anything to do with gravity.

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Local magnetic anomalies
(0-90 degrees; 3-4 degrees frequently)
Predictive geomagnetic models such as the World Magnetic Model (WMM) and the International Geomagnetic Reference Field (IGRF) only predict the values of that portion of the field originating in the deep outer core. In this respect, they are accurate to within one degree for five years into the future, after which they need to be updated. The Definitive Geomagnetic Reference Field (DGRF) model describes how the field actually behaved.

Local anomalies originating in the upper mantle, crust, or surface, distort the WMM or IGRF predictions. Ferromagnetic ore deposits; geological features, particularly of volcanic origin, such as faults and lava beds; topographical features such as ridges, trenches, seamounts, and mountains; ground that has been hit by lightning and possibly harboring fulgurites; cultural features such as power lines, pipes, rails and buildings; personal items such as crampons, ice axe, stove, steel watch, hematite ring or even your belt buckle, frequently induce an error of three to four degrees.

Anomalous declination is the difference between the declination caused by the Earth's outer core and the declination at the surface. It is illustrated on 1:126,720 scale Canadian topographic maps published in the 1950's, which included a small inset isogonic map. On this series, it is common to observe a four-degree declination change over 10 kilometers (6 miles), clearly showing local anomalies. There exist places on Earth, where the field is completely vertical; where a compass attempts to point straight up or down. This is the case, by definition, at the magnetic dip poles, but there are other locations where extreme anomalies create the same effect. Around such a place, the needle on a standard compass will drag so badly on the top or the bottom of the capsule, that it can never be steadied; it will drift slowly and stop on inconsistent bearings. While traveling though a severely anomalous region, the needle will swing to various directions.

A few areas with magnetic anomalies (there are thousands more):

-North of Kingston, Ontario; 90° of anomalous declination.

-Kingston Harbor, Ontario; 16.3° W to 15.5° E of anomalous declination over two kilometers (1.2 miles); magnetite and ilmenite deposits.

-Near Timmins, Ontario, W of Porcupine.

-Savoff, Ontario (50.0 N, 85.0 W). Over 60° of anomalous declination.

-Michipicoten Island in Lake Superior (47.7 N, 85.8 W); iron deposits.

-Near the summit of Mt. Hale, New Hampshire (one of the 4000-footers, near the Zealand Falls hut on the Appalachian Trail) ; old AMC Guides to the White Mountains used to warn against it.

-Around Georgian Bay of Lake Huron.

-Ramapo Mountains, northeastern New Jersey; iron ore; compass rendered useless in some areas.

-Near Grants, New Mexico north of the Gila Wilderness area; Malpais lava flows; compass rendered useless
http://www.geocities.com/magnetic_declination/

And gravity does have an effect on multiple sclerosis!

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#### DoctorBeaver

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##### Re: Gravity... are we making a grave mistake?
« Reply #24 on: 28/12/2005 13:38:41 »
quote:
So the question is, how can the total gravitational attraction of the sphere be proportional to the total number of ball bearings in it (1 million)? How can the ball bearings on the inside contribute to the total attraction, given that the inner ones can’t “see out” to interact with the external world? Why isn’t the total gravitational attraction of the sphere proportional to the number of ball bearings (c.50,000) on the surface instead?

That's sort-of what I was getting at.
Let's see if I can explain things better.
Instead of a sphere of ball bearings, just take 3 joined in a line across in front of you. Assume you are 4r from the centre of the middle bearing, therefore 4r from the centre of mass of the 3-bearing structure. The gravitational attraction would therefore be 3m/4r^2.
Now, turn the structure so that it's lengthwise away from you but still at 4r to the centre of the middle bearing, hence still 4r from the centre of mass. The calculation, therefore, is still 3m/4r^2, so the gravitational attraction should still be the same. But do the calculations individually for each bearing and it doesn't add up (g=m/2r^2+m/4r^2+m/6r^2). If you assume m=1 and r=1, you get a value for g of 0.1875 for the 1st calculation and 0.34 when you sum the individual values - almost twice as much!. (at least, they are the results from the spreadsheet I did)
Now, if you take a bar that is the same length and mass as the 3 bearings, what would be the result?
« Last Edit: 28/12/2005 13:45:41 by DoctorBeaver »

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##### Re: Gravity... are we making a grave mistake?
« Reply #24 on: 28/12/2005 13:38:41 »