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Youjaes

USA
28 Posts

Posted - 19 Aug 2002 : 23:46:17

My experience in "The fLaw of Gravity" thread has been going around in my head and I've been getting stuck in a bit of a quandary, that goes as follows:

A uniform spherical shell of uniform mass is encompassed by a larger spherical shell of uniform mass with the centers of the two shells coinciding at the same point.

Now, for each and every unit of mass of the inner spherical shell, there is a net attractive force of zero exerted on it by the outer shell, yet for each and every unit of mass of the outer shell, there is a net attractive force inward that is non-zero. Therein lies my quandary.

We have a radially compressive force on the outer spherical shell towards the inner one, but not an opposite and equal decompressive force on the inner spherical shell towards the outer one. I'm stuck on reconciling this one at the moment....perhaps a night's sleep will clear the fog for me.

James

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AgoraBasta

Russia
234 Posts

Posted - 20 Aug 2002 : 07:33:34

quote:

We have a radially compressive force on the outer spherical shell towards the inner one, but not an opposite and equal decompressive force on the inner spherical shell towards the outer one.

And hence the pressure arises. How misterious!

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Jim

1607 Posts

Posted - 20 Aug 2002 : 11:28:57

The two mass centers can be at the same point and the two gravity centers not. The mass of a shell is spread out over a surface and so is the gravity force so the pressure and temperature will not be more at the center of mass than it is at the surface and will be greater at the midpoint between the two surfaces.

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tvanflandern

USA
2793 Posts

Posted - 20 Aug 2002 : 11:40:20

quote:
We have a radially compressive force on the outer spherical shell towards the inner one, but not an opposite and equal decompressive force on the inner spherical shell towards the outer one.

Newton's third law of motion is greatly over-rated because it doesn't really apply to gravity, even though we compel it to apply by creating unobservable fictions. In the above case, as AB says, the counter-force manifests itself as downward pressure.

But I prefer a different way of understanding all this. Suppose we have a series of masses all at the same distance from the Sun. Let's say these masses are: 1000-solar-mass black hole; Jupiter; comet; dust grain; and a single neutron. Ignoring the effect these masses have or fail to have on the Sun, the Sun causes all of them to accelerate toward itself at exactly the same rate. That is because, as Galileo's Tower of Pisa experiment showed, the rate at which bodies accelerate in a gravitational field is independent of their own mass. Heavy and light bodies fall at the same rate.

If Newton had formulated his law of gravitation in this way, we would have no confusion over this issue. But instead of describing the acceleration of bodies in a gravitational field, which we can observe and verify, Newton wrote his law as a force law, even though we cannot observe forces.

So if you wish to understand physics by explaining that the Sun's force on any of those different target bodies is proportional to the product of the masses of the Sun and target body, but the acceleration of each target body is the same because the "force of inertia" counters the "force of gravity", more power to you. But if you want to achieve a clearer understanding of nature and gravity, I recommend recognizing that the acceleration of gravity is independent of the mass of the target body, and that inertia is not operative at all in the case of gravity. So Newton's third law doesn't hold either.

You can read more about this way of thinking in Pushing Gravity. -|Tom|-

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AgoraBasta

Russia
234 Posts

Posted - 20 Aug 2002 : 11:50:05

Or one can consider the local gravity as being rigidly attached to the local aether of density proportional to the gravity, and get an equivalent of GR. Question then would be - how to account for a completely physical pressure?

BTW, Tom, if I kick you (gently), you'll notice that force is more physical than acceleration. Acceleration needs space and time to manifest itself, while force requires neither of those, thus being as basic a category as space/time themselves.

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Jim

1607 Posts

Posted - 20 Aug 2002 : 11:57:30

The 1000 solar mass will not move to the 1 solar mass the 1 solar mass will not move to the the .001 solar mass. The reason is what? If not inertia then what do you call the effect that makes the smaller mass move and not the larger mass.

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tvanflandern

USA
2793 Posts

Posted - 20 Aug 2002 : 12:22:19

quote:
[AB]: BTW, Tom, if I kick you (gently), you'll notice that force is more physical than acceleration.

That is true for electromagnetic and mechanical forces. My point is that gravity is different -- gravity produces inertia-free acceleration.

If I retaliate for the kick by sending my pet 1000-solar-mass "black hole" in your direction, you will never see or feel it coming. Your first clue that you are about to be sucked into oblivion will be a large acceleration relative to distant objects.

quote:
[Jim]: The 1000 solar mass will not move to the 1 solar mass the 1 solar mass will not move to the the .001 solar mass. The reason is what? If not inertia then what do you call the effect that makes the smaller mass move and not the larger mass.

Bodies of all masses fall toward the Sun at the same rate -- a rate that depends on the Sun's mass. If you wish to look at the reciprocal question of what happens to the Sun (which is irrelevant for purposes of my example), then we could replace the sun with a 1000-solar-mass black hole or a neutron, and the results are the same. Any of the above will accelerate toward the distant bodies at a rate that depends on each distant body's mass, but is independent of the Sun's mass (or the mass of whatever you put there in place of the Sun).

Gravitational acceleration depends on the mass of the gravitating body, but not on the mass of the body being moved. When two bodies are involved, the acceleration each produces in the other depends on its own mass, and not on the other's mass. -|Tom|-

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AgoraBasta

Russia
234 Posts

Posted - 20 Aug 2002 : 12:31:04

quote:

My point is that gravity is different -- gravity produces inertia-free acceleration.

That's a very big "if", had gravity proved "shieldable".

quote:

If I retaliate for the kick by sending my pet 1000-solar-mass "black hole" in your direction, you will never see or feel it coming. Your first clue that you are about to be sucked into oblivion will be a large acceleration relative to distant objects.

Hadn't those dogs been outlawed at this board?
Also, after the universe, not me, sinks into oblivion, I will get hit by whatsoever is there under horizon. Not fair!

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Jim

1607 Posts

Posted - 20 Aug 2002 : 12:35:14

Well for the third or fourth time I aggree with you about acceleration of gravity. The point is the small mass is moved and the large mass is not moved. This result is why the large mass has tides and the small mass has velocity. You can pick on the model here since both masses are effected the same in one sense and differently in another sense.

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tvanflandern

USA
2793 Posts

Posted - 20 Aug 2002 : 15:56:47

quote:
[Jim]: The point is the small mass is moved and the large mass is not moved.

That statement is in error. The small mass makes the large body move too, in proportion to its own small mass. For example, both Earth and Moon raise tides on one another. Earth makes the Moon move in a large orbit, and the Moon makes the Earth move in a smaller orbit (81 times smaller, because the Moon has 81 times less mass).

So the Earth-Moon barycenter (the point that orbits the Sun in an elliptical orbit) is located about 1000 miles inside the Earth, about 1/4 of the way to the center. And the center of the Earth revolves around the barycenter once each lunar month. -|Tom|-

AgoraBasta

Russia
234 Posts

Posted - 20 Aug 2002 : 16:16:17

Tom,

Is there any consistent data on the Sun's motion wrt the Solar system barycenter? Also, is there any difference in motion of the Sun's geometric centre and its magnetic field centre?

Hardly this question belongs in this thread, but I just have no idea where to stick it.

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tvanflandern

USA
2793 Posts

Posted - 20 Aug 2002 : 16:29:37

quote:
[AN]:Is there any consistent data on the Sun's motion wrt the Solar system barycenter? Also, is there any difference in motion of the Sun's geometric centre and its magnetic field centre?

The Sun's motion wrt the solar system barycenter is calculated because the barycenter cannot be observed. In fact, its location is a matter of arbitrary definition, and not of physics. For example, if we include Alpha Centauri along with the Sun and its planets in our dynamical system, the barycenter moves half-way to Alpha Centauri without any observable consequences.

I am unaware of any observable distinction between the Sun's geometric center and its magnetic field center. Such a thing might exist, as it does for planets; but I've not heard of it for the Sun. -|Tom|-

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Jim

1607 Posts

Posted - 20 Aug 2002 : 20:42:24

The moon then by this logic orbits 81 times faster than the Earth? So the GM=RV2 rule must be false since it indicates the moon orbits 9 times faster. And is the moon tide very much bigger than the tide on Earth?

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Larry Burford

USA
1355 Posts

Posted - 22 Aug 2002 : 07:41:24

[ab]
Is there any consistent data on the Sun's motion wrt the Solar system
barycenter? Also, is there any difference in motion of the Sun's geometric centre and its magnetic field centre?

[tvf]
The Sun's motion wrt the solar system barycenter is calculated because the barycenter cannot be observed. In fact, its location is a matter of arbitrary definition, and not of physics. For example, if we include Alpha Centauri along with the Sun and its planets in our dynamical system, the barycenter moves half-way to Alpha Centauri without any observable consequences.

======

This is rather unexpected. I'm intrigued.

Assuming that you are not pulling our leg, WHY would we want to allow for the possiblilty of including Alpha Centauri when determining the barycenter of "the Solar system"? Doesn't your example describe the approximate location of the barycenter of the Solar/Alpha Centauri "system"?

I've never seen a formal definition of barycenter - from context I had always assumed it was the technical term for center of mass. My CRC doesn't mention it and the Internet says "...center of mass...".

The location of a center of mass obviously depends on which masses you are talking about. Specifying "the Solar system" should exclude things like the Centauri triplets or the Sombrero Galaxy.

What am I missing here?

Regards,
LB

Jim

1607 Posts

Posted - 22 Aug 2002 : 11:52:07

The barycenter is a model that balances the two mass centers as one would do on a balance scale. It is in common use today and no one thinks that is wrong because it has been in use for a few centuries. The error with this model is that in a dynamic system the static balance is not stable due to the fact the smaller mass is spinning way to fast to be offset by the slower and larger mass. The model is bogus.

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tvanflandern

USA
2793 Posts

Posted - 22 Aug 2002 : 23:26:56

quote:
[Jim]: The moon then by this logic orbits 81 times faster than the Earth? So the GM=RV^2 rule must be false since it indicates the moon orbits 9 times faster.

81 times faster is correct. The law you cite refers to the orbit of the Moon relative to the Earth, or to any real circular orbit and its center. It does not apply to motion around fictitious points such as the barycenter. No mass is located at the barycenter.

quote:
:And is the moon tide very much bigger than the tide on Earth?

The Moon tide is nearly 1000 times larger than Earth tides. That is partly because Earth's mass is so much larger than the Moon's, and partly because the Moon does not rotate, so the "tide" (or bulge, if you prefer) is permanent. -|Tom|-