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Orbit of Cube planet

  1. Dec 15, 2015 #1
    What is the physics behind a orbit a cube planet. Does the convential physics including keplars laws and circular motion still apply. Also The cube having a centre of mass posited in the middle of the cude does this mean it can be consider point mass and the same as spherical planet orbit?

    any relevent physics at all levels appriciated
    can you point tell me about this or the relevant physics page thanks.
    there doesn't seem to anything on the web
     
  2. jcsd
  3. Dec 15, 2015 #2

    phinds

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    There's probably nothing on the web because "cube planet" does not make any sense. By definition, a planet is a cosmological body that has enough mass to pull itself into a sphere.
     
  4. Dec 15, 2015 #3

    DrClaude

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    Yes.

    The position of the center of mass with respect to the entire object is not important. When you are calculating the orbit of a planet, you are actually calculating the motion of the center of mass.

    Any good book on classical mechanics should be of help. You simply need to understand how to separate the center of mass motion of a body and its internal motion (essentially rotation in the case of a solid body).

    We don't usually need to care about cube planets :wink:
     
  5. Dec 15, 2015 #4

    jbriggs444

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    At typical astronomical scales, the shape of a planet is irrelevant. It is far enough from the primary that its center of mass and center of gravity will be at approximately the same point. However, this is not exact. The center of mass of an object is essentially its average position -- where the average is mass-weighted over the volume of the object. The center of gravity of an object is its average position -- where the average is weighted by gravitational force over the volume of the object. For a spherical object, the spherical shell theorem says that the two are identical. For a cube-shaped object, they need not be identical.

    But again, the discrepancy will be way to small to worry about.
     
  6. Dec 15, 2015 #5
    You could try the gravity simulator at www.testtubegames.com.
    It doesn't support cube planets but you can place several "fixed stars" close to each other and then make a planet orbit around them to achieve a similar effect. If the distances are large enough there is no noticeable difference from a planet going around a single spherical star. But if the distance is small you get an orbit that is not elliptical and doesn't close. Here is a screenshot.
    jp9d8w.jpg
     
  7. Dec 15, 2015 #6
    Wait, what orbital physics are you referring to? The cube orbiting its star or a satellite orbiting the cube itself?
     
  8. Dec 15, 2015 #7

    jbriggs444

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    Newton's third law -- the two are both aspects of the same thing.
     
  9. Dec 15, 2015 #8

    SteamKing

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    And the people from the Bizarro World get neglected yet again:

    http://drlenhardt.com/wp-content/uploads/2015/08/bizzaro-world.jpg [Broken]
    https://en.wikipedia.org/wiki/Bizarro_World
     
    Last edited by a moderator: May 7, 2017
  10. Dec 15, 2015 #9
    Not true, it makes a big difference if the cube is a million times smaller than the object it's orbiting or if the cube is a million times larger than the object that's orbiting it?

    If you have a small object orbiting a large cube, because the corners jut out above the flat surface, when the orbiter passes over a corner, it's much closer to more mass so it should be pulled harder than when it's at the same point in it's orbit above the face.
     
  11. Dec 15, 2015 #10

    Janus

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    http://arxiv.org/pdf/1206.3857.pdf
     
  12. Dec 15, 2015 #11

    jbriggs444

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    What, exactly is not true? That Newton's third law holds? It does.
     
  13. Dec 16, 2015 #12

    jtbell

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    As I hope you can see from the replies that you've gotten so far, you need to clarify the situation that you're thinking of. It makes a difference:
    • The orbit of a cubical planet the size of the Earth, around the sun, at the Earth's distance from the sun
    • The orbit of a satellite around a cubical planet the size of the Earth, at a typical altitude for an Earth-orbiting satellite (say 100 to 200 miles)
    • The orbit of a satellite around a cubical planet the size of the Earth, at a much greater distance than above (say 500,000 miles?)
     
    Last edited: Dec 16, 2015
  14. Dec 16, 2015 #13
    Newton's law would be the same, the shape of the gravitational field is not uniform for a cube, so it makes a difference which object is the orbiter and which is the orbitee.
    Source: http://arxiv.org/abs/1206.3857
     
  15. Dec 16, 2015 #14

    jbriggs444

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    You do realize that the distinction between orbiter and orbitee is arbitrary, right? Both orbit each other.
     
  16. Dec 16, 2015 #15
    Yes, I do, but orbiter is usually considered the less massive object, that's why when we landed on the moon, we called the object that orbited the moon the lunar orbiter. Anyway, it doesn't change the fact that a small object orbiting a large cube will have very different dynamics than a small cube orbiting a spherical object. (Assuming that the two objects are close to each other.) See figure 3 in this document: http://arxiv.org/pdf/1206.3857.pdf In fact, that document should help the OP too, it's literally about the dynamics of orbiting a planet-sized cube.
     
  17. Dec 17, 2015 #16
    It is great that you, are thinking outside our box.
    Every different shape or density of mass affects, space displacement, gravity, momentum or force.. Just as if our universe, was the shape of a pancake, or a cube.. This would affect us and everything else in our universe. If space displacement is affected by mass, density, or velocity, it will also be affected by the shape or uniform density of the mass, if different from a sphere. All large masses are spherical ,due to pressure or gravity applied to all angles, onto to the mass. From space displacement.
    I hope you agree.
     
    Last edited: Dec 17, 2015
  18. Dec 21, 2015 #17
    The orbit of a cuboid planet around a relatively massive sun (where the shape of the planet and therefore the gravitational field in the region of the sun can be ignored) is NOT the same thing as the orbit of a satellite around a relatively massive cuboid planet where the shape of the gravitational field in the region of the satellite may be significant.
     
  19. Dec 21, 2015 #18

    jbriggs444

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    It is the same thing. It is just a matter of relative scale that makes the effect significant in the one case and insignificant in the other.
     
  20. Dec 25, 2015 #19

    mfb

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    And the relative scale does matter. That's why we landed on the moon, instead of having moon land on the Apollo descent stages.
     
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