My dilemma

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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.
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And hence the pressure arises. How misterious!<img src=icon_smile_clown.gif border=0 align=middle>

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.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=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.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

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 <i>Pushing Gravity</i>. -|Tom|-

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.

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.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>[AB]: BTW, Tom, if I kick you (gently), you'll notice that force is more physical than acceleration.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

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. <img src=icon_smile.gif border=0 align=middle>

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=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.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

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|-