Where is most "gravity", inside or out?

[James R]

nebel:

I still can't see the image you're referring to: nothing in post #265, and nothing in post #280.

I'll ask you again:
It is possible that all you are saying here is that as a spherical mass is compressed, the acceleration due to gravity at its surface increases. That is correct. Is that all you're saying?

 

[nebel]

That's false. The gravitational field from a mass projects in all directions. Every bit of mass attracts every other bit of mass, at the fundamental level.

of course, but if you think of a particle in the interior of an entity, say at 1/2 R above, it's gravitational field pulls both inward and out. Only because the inward pulling vectors? are cancelling on the other side, is surface gravity present and adding up.

Irregularly-shaped masses are seldom close approximations to spheres.

true, shapes can be extreme, but somewhere there is that centre of gravity, where all the pulls come to zero. Treat odd shapes as distorted shells.

"

James R said: "I don't know how you know this. Assuming approximately uniform density, that looks reasonable, though."
January 2019. <www.sciencedaily.com/releases/2019/01/190101130547.htm>. here is another image of that zero gravity center of rotation, good guesstimate.

 

[James R]

Haven't we already covered both of the points you raised in #282?

We agree that the net gravitational field at a point inside or outside the object is the vector sum of the fields from the individual mass elements making up the object, wherever they happen to be.

We also agree as to the location of the centre of mass of Optima Thule and that there would be zero acceleration due to gravity at that point.

Is there anything else that needs clearing up?

 

[nebel]

Overall, though, all the fields from the individual mass elements add up (as vectors) to create zero net acceleration due to gravity at the centre of mass.

thank you for the correction I meant, of course and should have said mass effect.

The net gravitational field of the joined object is simply the superposition of the individual fields of the two fields

here I come to the conundrum I haggled about with origin in post #278 namely:
are the two gravity fields of the 2 halves of a globe, or our THULE and ULTIMA fragments intact and superimposed in the merger, or do the interior overlapping fields collapse because cancel out? in other words: There is clearly zero gravity at the center of rotation, but:
Is it the result of 2 superimposed and intact but cancelling fields, or one truly single field with all traces of the former individual gravity gone? It would be great to see a computer simulation of the field re-configurating itself with the new zero point to the one halo -like outside.
Do you really see two overlapping gravity fields in the CG?

Surface tension is not the same as gravity. It is, as the label says, a surface or boundary phenomenon. Gravity is a bulk phenomenon.

I used the soap bubble picture only as a rough illustration how gravity would reshape it self around Ultima-Thule. of course gravity reaches deeper into space than a film.
Gravity is a bulk phenomenon, the bulk of it is on the outside.

This is quite wrong, I'm afraid.

re: surface gravity: well, consider that you said: "The gravitational field from a mass projects in all directions." . Knowing that macro objects are really made up of many small masses that project gravity in all directions, but that all the inward vectors cancel, from the masses on the opposite side, that projected their gravity inward too,
gravity turns out to be really a collection of all the remaining surface fields projecting outward only, because all inward components of the gravity has been cancelled or overlapped.

Like I said, it's a simple matter of adding the effects of individual mass elements as vectors. Gravitational fields obey the principle of superposition, at least in the Newtonian picture.

Yes, and in that superposition, all the inward components cancel, leaving us with the resultant, surface gravity, projected outward. imho. thank. you.

.

 

[Q-reeus]

[QUOTE="James R, post: 3558121, member: 4402"
...Gravity is a force (let's go with the Newtonian picture unless we find we need General Relativity)...[/QUOTE]
No. Gravity as a phenomenon is embodied as a field, in both Newtonian and GR picture. A gravitational force in Newtonian picture has the form F = mg, where g is the gravitational field strength vector locally acting on a mass m.
Just why anyone wants to continue to discuss anything to do with gravity with nebel after the previous 14+ tortuous pages is beyond me. It's clear this will just keep going in meandering circles or spirals etc.

 

[nebel]

No. Gravity as a phenomenon ---- locally acting on a mass m.

The mass m being nebel, now in his 89st year, locally in his daily down hill run on his skate board, powered by gravity. I can feel gravity in my bones, love the effects, discussing them with all comers, thank you.

 

[Q-reeus]

The mass m being nebel, now in his 89st year, locally in his daily down hill run on his skate board, powered by gravity. I can feel gravity in my bones, love the effects, discussing them with all comers, thank you.

Right - gravity assist can be fun. Look I wasn't trying to be nasty last post, just pointing out that you don't seem to be able to retain the basic principles covered ever since page 1 here. Once the basics are internalized, there should imo be no reason to continue pushing a succession of particular scenarios that don't really add anything new at a conceptual level. It might pay to carefully review all the useful posts beginning at the beginning. Taking notes along the way!

 

[nebel]

Right - gravity assist can be fun. Look I wasn't trying to be nasty last post, just pointing out that you don't seem to be able to retain the basic principles covered ever since page 1 here

Of course , I accept all posts as learning material. (my greatest fun gravity assists were flying gravity powered planes, aka soaring). I, and perhaps all viewers have learned from day one.
For example only in the last exchange with James R, did it become clear to me how the sum of all the gravitational fields, that are projected in all directions from small interior entities can create the peak surface gravity.
thanks for showing an equation in my flight of fancy, foregoing formulae.

 

[James R]

nebel:

here I come to the conundrum I haggled about with origin in post #278 namely:
are the two gravity fields of the 2 halves of a globe, or our THULE and ULTIMA fragments intact and superimposed in the merger, or do the interior overlapping fields collapse because cancel out?

The field after the merger is a vector sum of the two fields we would have if we considered the fragments individually in the same place.

It makes no difference whether you choose to conceptualise the merger as a single field "reconfiguring" or as two fields interacting. The end result is the same.

in other words: There is clearly zero gravity at the center of rotation, but:
Is it the result of 2 superimposed and intact but cancelling fields, or one truly single field with all traces of the former individual gravity gone? It would be great to see a computer simulation of the field re-configurating itself with the new zero point to the one halo -like outside.

It wouldn't be hard to create simulation showing some field lines. It's simple vector addition, like I said.

It might be best if you think of the field not as a "thing" that can be intact or whatever, but as a calculational abstraction.

Do you really see two overlapping gravity fields in the CG?

All you can "see" is the result of an experiment. You can never "see" a gravitational field directly. The best you can do is put a mass in the field and observe how it moves.

Gravity is a bulk phenomenon, the bulk of it is on the outside.

You still haven't explained what you could possibly mean using that kind of language. I've asked you two or three times now to explain concisely, but you haven't done it yet. Do you know what you mean, or not?

re: surface gravity: well, consider that you said: "The gravitational field from a mass projects in all directions." .

I said that? It doesn't seem like something I'd say.

Knowing that macro objects are really made up of many small masses that project gravity in all directions, but that all the inward vectors cancel, from the masses on the opposite side, that projected their gravity inward too,
gravity turns out to be really a collection of all the remaining surface fields projecting outward only, because all inward components of the gravity has been cancelled or overlapped.

First, it is not true for an object of irregular shape that "all inward vectors cancel". The shell theorem only applies to spherical shells, as I have told you two or three times previously.

Second, the net external gravitational field of an object is not due just to the mass on the surface, but to all the mass in the object - something I have also told you previously. This stuff about "surface fields projecting outwards" is nonsense.

I hope this helps.

 

[James R]

[QUOTE="James R, post: 3558121, member: 4402"
...Gravity is a force (let's go with the Newtonian picture unless we find we need General Relativity)...

No. Gravity as a phenomenon is embodied as a field, in both Newtonian and GR picture.

Well, if you feel the need to pick nits, fine. I take your point. Although, my focus there was on gravity as a Newtonian force rather than as a curvature on a manifold.

Just why anyone wants to continue to discuss anything to do with gravity with nebel after the previous 14+ tortuous pages is beyond me. It's clear this will just keep going in meandering circles or spirals etc.

Why are you still here posting? Is somebody keeping you here?

 

[Q-reeus]

Of course , I accept all posts as learning material. (my greatest fun gravity assists were flying gravity powered planes, aka soaring). I, and perhaps all viewers have learned from day one.
For example only in the last exchange with James R, did it become clear to me how the sum of all the gravitational fields, that are projected in all directions from small interior entities can create the peak surface gravity.
thanks for showing an equation in my flight of fancy, foregoing formulae.

Well amazingly on tediously checking back through, the first statement more or less directly implying superposition of elemental mass 1/r^2 g fields seems to be in p8 #142. You mentioned shell theorem on frequent occasions, but evidently without ever having followed or apprehended the straightforward derivation e.g.: https://www.math.ksu.edu/~dbski/writings/shell.pdf
'Cancelling' as term comes up very often too, but that being simply one aspect of superposition principle (strictly true only in Newtonian gravity), seems to have eluded you till now. Wow.

 

[Q-reeus]

Well, if you feel the need to pick nits, fine. I take your point. Although, my focus there was on gravity as a Newtonian force rather than as a curvature on a manifold.


Why are you still here posting? Is somebody keeping you here?

Because you made a non-trivial wrong statement ('force of gravity' is not synonymous with incorrect 'gravity is a force') and I felt it proper to correct it. A forum duty imo.

 

[NotEinstein]

As an amateur (from the french "aimer" like or love)

And once again, you are wrong about the origin of a word, although you got it almost right this time. The English word "amateur" is borrowed from the French word "amateur", not "aimer". See: https://en.wiktionary.org/wiki/amateur#Etymology

It's funny that you keep making the same mistake(s) over and over again.

 

[James R]

Because you made a non-trivial wrong statement ('force of gravity' is not synonymous with incorrect 'gravity is a force') and I felt it proper to correct it. A forum duty imo.

I guess you'd better correct all the physicists who refer to the four fundamental forces of nature, etc., while you're doing your duty. Good luck with that.

 

[Q-reeus]

I guess you'd better correct all the physicists who refer to the four fundamental forces of nature, etc., while you're doing your duty. Good luck with that.

So there is a lot of loose reference to 'the four fundamental forces'. Some are more careful and instead refer to 'the four fundamental interactions'. But that's apt in the modern context where gravity is described by GR or similar geometric theories. We were really in the Newtonian level of gravity and it's still incorrect there to state 'gravity is a force'.
Just as it would be wrong to state 'momentum is mass', even though mass is an integral aspect of momentum.

 

[nebel]

And once again, you are wrong about the origin of a word, although you got it almost right this time. The English word "amateur" is borrowed from the French word "amateur", not "aimer

"Etymology[edit]
Borrowed from Latin amātor (“lover”), from amō (“to love”). Compare Old French ameor, which was inherited from the same source but disappeared by the 15th century."
Not Einstein, I know you want to help, but you dont know how deep my thinking is. I am fluent in French, and so yes, amateur is a french word that came across the channel, the deeper root though is "love" (see quote from Wiki above.
Hope my amateurish thoughts on gravity, time, will turn out to be equally profound. (deeply rooted). thanks.

 

[nebel]

Well, if you feel the need to pick nits, fine. I take your point. Although, my focus there was on gravity as a Newtonian force rather than as a curvature on a manifold.

really sorry. I was referring to the lines in the graph, not warping of space, In the image, the blue and red lines both have a curved and a straight section, the outside gravity shown curved, and the straight down to the zero / corner, marking the inner gravity. and, agreed, my thinking is strictly newtonian and keep posting because there still is pseudo stuff to come.

 

[NotEinstein]

Etymology[edit]
Borrowed from Latin amātor (“lover”), from amō (“to love”). Compare Old French ameor, which was inherited from the same source but disappeared by the 15th century.

You are looking at the wrong section. Again; because you did that the last time I pointed out your linguistic fabrications. That section is about the French word "amateur", not the English word "amateur".

Also, there's no mention of the word "aimer" in your quote, so you are proving yourself wrong.

Not Einstein, I know you want to help, but you dont know how deep my thinking is.

You're right, I don't. I do however know that you were wrong about this, and I have provided evidence for that claim. You have also provided evidence for that claim. Perhaps you should try "accurate thinking" instead of "deep thinking"?

I am fluent in French, and so yes, amateur is a french word that came across the channel, the deeper root is "love" (see quore from aiki above.

Sure, and I'm not disputing that, but that's not what you said. You said that the English word "amateur" came from the French word "aimer", which it clearly doesn't, and you just agreed with me that it doesn't. You are therefore wrong, and you now know it.

Hope my amateurish thoughts on gravity, time, will turn out to be equally profound. (deeply rooted).

Your "amateurish thoughts on gravity, time" are as profound as your claim that the English word "amateur" comes from the French word "aimer" is correct, yes.

 

[nebel]

Your "amateurish thoughts on gravity, time" are as profound as your claim that the English word "amateur" comes from the French word "aimer" is correct, yes.

well that is a good prognosis, because as this critique of yours shows, my wording is terrible,( as James R can attest to), but in the end, when all is said and done, the french word for love, "aimer", wiggled its way into our amateur. In the long run I might be right.

 

[James R]

really sorry. I was referring to the lines in the graph, not warping of space

I know that. I was trying to avoid some smart-alek jumping in when I said "gravity is a force", with a comment like "Einstein said it isn't a force". Instead, some smart-alek jumped in to nit-pick the difference between a force and a field, as if I was unaware of that. Sometimes you just can't win.