Try reading it again. The article covers exactly that point.
No. Any large planet would do. Any of the gas giants in our own solar system.
Read the article. It covers precisely that.
No. These bodies are, as I noted already, inside the fluid Roche limit, but outside the rigid Roche limit. The are quite small bodies - and markedly non-spherical, which means that the electromagnetic force signifies at that scale, a few tens of kilometres.
Larger celestial bodies - larger moons and the largest asteroids, and of course all the planets - are approximately spherical precisely because gravity has won out over the electromagnetic force as far as tensile strength goes. The electromagnetic force is far, far stronger when it comes to compression, which, as I said, is why planets don't collapse into tiny neutron stars.
The bonds in molecules are irrelevant on a planetary scale. The repulsive forces between attoms are what matter.
Not really, no. That's why the world is round.
Of course the electromagnetic force is repulsive. What do you think it is that stops the Earth from collapsing under its own gravity?
Originally Posted by kenkoskinen
PixyMesa Your
http://en.wikipedia.org/wiki/Roche_limit reference is not about any comparisons in the gravity vs. EM discussion.
Try reading it again. The article covers exactly that point.
My answer: Well not specifically ... the article doesn't mention EM anywhere.
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PixyMisa in your earlier post you claimed EM can signify on the planetary scale but repulsively. This can't be, but on Earth EM does hold molecules together and they are here on the large scale. To test the Roche limit on an Earth-like planet, it would have to be in close orbit around a massive star, a neutron star or black hole.
No. Any large planet would do. Any of the gas giants in our own solar system.
My answer: But the gas giants are held together primarily by gravity. They aren't a good test for whether EM and gravity share the role of holding our planet together.
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Such a scenario has never been observed but I'm saying the case involves tensile strength via EM and gravity. Their standard math doesn't compute the case. I wouldn't get too high on the Roche limit stuff as it stands.
Read the article. It covers precisely that.
My answer: No it doesn't. The EM force isn't even mentioned.
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The article states: "Some real satellites, both natural and artificial, can orbit within their Roche limits because they are held together by forces other than gravitation. Jupiter's moon Metis and Saturn's moon Pan are examples of such satellites, which hold together because of their tensile strength."
I wrote: Although there is an estimate on the mass of Pan, Metis' is unknown (see Wikipedia articles Pan (moon) & Metis (moon). It's hard to imagine that gravity isn't a factor in their cohesion. Maybe we already have an example (perhaps two) where EM & gravity working together have out foxed the Roche limit estimate.
No. These bodies are, as I noted already, inside the fluid Roche limit, but outside the rigid Roche limit.
My Answer: Not so, the article doesn't clearly state which limits were implied. It simply states both are within Roche limits. Later it delineates the moons in a table and gives both as percentage values. Metis is in italics to signify uncertainty. Can you cite a more definitive source?
The are quite small bodies - and markedly non-spherical, which means that the electromagnetic force signifies at that scale, a few tens of kilometres.
My Answer: Not necessarily, especially if these moons are on the edge of Roche limits, accounting for non-spherical shapes. The real question is this. Is there a specific point or instant button at which gravity becomes the only force in keeping a celestial object together? Judging from your answers ... I assume you'd say yes. I'd say that mass always gravitates and EM can also play a role in holding things together. The shape of a celestial object also depends much on its accretion or build up & continual history, (including on-going collisions that break away pieces and some that can add mass) as well as the effect of gravity. Also, even if these moons are within any Roche limit, gravity could still be playing a role in adhesion. Yes ... I don't think the universe is based on a one and only place button.
Larger celestial bodies - larger moons and the largest asteroids, and of course all the planets - are approximately spherical precisely because gravity has won out over the electromagnetic force as far as tensile strength goes. The electromagnetic force is far, far stronger when it comes to compression, which, as I said, is why planets don't collapse into tiny neutron stars.
My answer: What is stopping Jupiter from collapsing? Even if it did, it couldn't even generate a star like our Sun, never mind any size of neutron star. Can you cite a reference to this strange tale?
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I wrote: "You claim that gravity holds planets together and the electromagnetic force keeps them from collapsing into tiny little neutron stars. Do you have any idea what it takes to create a neutron star? You need a large star to go supernova and a neutron star is left in the aftermath. If the star were larger yet, it would form a stellar black hole. A planet can't become even a tiny neutron star and the EM bonds in molecules couldn't possibly prevent it.
The bonds in molecules are irrelevant on a planetary scale. The repulsive forces between attoms are what matter.
My Answer: Well atoms combined are molecules. When I walk on a floor, I don't sink into it. You could also call that a repulsive force but I can't find any references that agree. It's more like neutral molecules in the solids of my floor (for our example) don't allow the neutral molecules of my feet to penetrate. The force of friction includes a surface exchange of electrons allowing me to walk. Atomic bonds don't cancel gravity. Albeit EM creates structure that resist collapse by other EM structures. All of these structures also gravitate. A planet can't ever collapse onto itself to become any size of neutron star since if it did, it would no longer be a planet. It would have to at least be as massive as a large star.
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On earth there are lots of molecules. EM force binds atoms and molecules together. It acts like an atomic glue holding things together. It works along with gravity in holding our world together.
http://videolectures.net/mit802s02_lewin_lec01/
Not really, no. That's why the world is round.
My answer: It's round because of its accretion history and due to gravity. It wasn't always so, especially during parts of major impact events. It takes time for gravity to re-capture matter from a field, and then reform into a sphere. The moons in our discussion are in fields with much debris that impacts them constantly and they are also close to or within Roche factors. Therefore they may not be gravitating sufficiently to create a rounded spheres but gravitational cohesion and EM factors may both be operative. Don't be over taken by a mathematical model. It may not be covering all of the bases.
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You wrote: "Yes. Was there a point to all that?"
The point is on Earth EM works with gravity to hold our world together. How could you have missed it ... again? View the video, EM is worldwide on Earth and not repulsive.
Of course the electromagnetic force is repulsive. What do you think it is that stops the Earth from collapsing under its own gravity?
My answer: EM structures have tensile strength but these also gravitate. So it's not simply an issue of EM preventing the earth from collapsing. The Earth could shrink depending on the quantity of accumulated mass but it isn't going to become neutron star any time soon.
