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Spiraling of Space and Gravity

  1. Jul 8, 2008 #1
    Why is everything in space spiraling around larger (more massive) objects? Does gravity make objects want to move/orbit in a specific direction, at a specific speed? Over time will an orbiting object increase speed?
    Do objects sometimes orbit against the 'flow'?

    Thanks in advance.
     
  2. jcsd
  3. Jul 8, 2008 #2

    DaveC426913

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    Note: They are not on spiral paths, they are on elliptical paths.


    Gravity does not set objects moving, gravity simply guides their movement based on the speed the object already has.

    Let's think about it starting from the beginning. Let's say there are only two objects in our sample space out in the middle of nowhere. They happen to be somewhat far apart. These two objects, because of the random nature of the universe, happen to have some motion relative to each other << this is key. Three possible things can happen:

    1] If the relative motion happens to be away from each other, then, well, they don't interact much. They continue to move apart, eventually leaving our sample area.

    2] If the relative motion happens to put them on a collision course, then they approach each other and collide - this is an extremely unlikely scenario due to the randomness of their positions and velocities and the vastness of our sample area. (Try throwing a ball at a small target 200 yards away - with your eyes closed.)

    3] They approach each other but pass by each other. This is by far the most likely scenario campared to 2]. When they approach each other, their mutual gravity bends their paths, bringing them even closer. But as they get closer, they also get faster. As they get faster they're more able to shoot past each other and escape again. Sometimes this happens. It is called a hyperbolic trajectory. However, often the two have just the right speeds and trajectories that that they balance out and end of in a stable orbit around each other.

    The lesson here is that the motion is inhernet in the initial velocities the two bodies before they came into each others' influence, which is then modified by gravity.

    Actually, they do - on every orbit. This increased speed sends them farther from their parent. As they move away they lose the speed, falling back again, causing them to pick up speed. This is cyclical and stable. Read about Kepler's 2nd Law:

    "A line joining a planet and the sun sweeps out equal areas during equal intervals of time as the planet travels along its orbit. This means that the planet travels faster while close to the sun and slows down when it is farther from the sun. With his law, Kepler rejected the Aristotelean astronomical theory that planets have uniform speed."
    http://en.wikipedia.org/wiki/Kepler's_laws_of_planetary_motion

    Yes. Some objects will go into orbit in the opposite direction of the prevailing direction of other natural satellites around a body. This is rare though because
    1] it is a narrower "window" of position vs. velocity to get into an orbit, and
    2] if it is achieved, it is unstable. The other bodies will tug and push and pull on the rogue body until it either flies off, falls inward or is teased into an orbit that matches its orbity brethren. There were likely more of these early in our solar system's history.
     
    Last edited: Jul 8, 2008
  4. Jul 8, 2008 #3

    russ_watters

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    Good explanation, but one key word I didn't see in there: barycenter. A small object does not orbit a larger object, they both orbit their common center of gravity, AKA, the barycenter of the system: http://en.wikipedia.org/wiki/Center_of_mass
     
  5. Jul 8, 2008 #4

    DaveC426913

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    Yah, thought about it but I figured that'd be for a subsequent lesson.
     
  6. Jul 8, 2008 #5
    Thanks guys.

    But why do most planets orbit in the same direction, and their solar system orbits another system, the same direction (?), all the way up to a blackhole, or larger object. (?) Where everything is swirling around like water down a massive drain? It seems this order needs some sort of force, or mechanism other than "follow the leader's gravity", I guess you can call it. Do other solar systems with planets all the in the opposite direction would be proof of randomness. Or are they all mostly clockwise or counterclockwise orbits?

    Sorry if I missed a point you already mentioned.

    Thanks.
     
  7. Jul 8, 2008 #6

    DaveC426913

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    All planets in a given solar system usually orbit in the same direction. Reread post #2.


    But what makes you think other solar systems orbit in the same direction? If you look at the system upside down the planets will be going in the opposite direction. There is no up or down in space. If you examined the orbital planes of a random collection of stars, you would no pattern to their orbital planes; each system would be at a different angle.

    What you may be experiencing is they are usually depicted in the same orientation merely by convention.
     
    Last edited: Jul 8, 2008
  8. Jul 8, 2008 #7
    Thanks. I see your point about up/down not existing in all the solar systems and galaxies out there. But.. if you lined up all the N/S poles of all the planets, and suns, in a specific orientation, would there be a standard direction of rotation? I don't know if this can be figured out, I'm just curious.

    All the swirling images of the universe, made me think there was some force causing the swirling symmetry (maybe just gravity).
     
  9. Jul 9, 2008 #8

    russ_watters

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    Since the solar system started from one rotating cloud of gas and dust, everything in it will tend to rotate and revolve in that direction.
     
  10. Jul 9, 2008 #9

    Chronos

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    Assuming the solar system arose from an inspiraling gas cloud, orbital motions make sense. Exceptions can be attributed to collisions. None of the 8 major planets orbit in the 'wrong' direction, to my knowledge. Some may spin in the 'wrong' direction, but, that is a separate issue - think collisions.
     
  11. Jul 9, 2008 #10

    DaveC426913

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    Ay, there's the rub. How do you determine which is the North pole versus which is the South pole?
     
  12. Jul 9, 2008 #11
    A compass? Why wouldn't that work.
     
  13. Jul 9, 2008 #12

    DaveC426913

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    That would simply determine which is the North vs. South magnetic pole. While we could certainly display all star systems with the north magnetic pole at the top, what good would that do? Usually we're trying to copmare other systems to our own - a more relevant mapping is to ensure that the system revolves in the same direction in comparisons.

    So, what you're seeing is an artifact of convention in how the data is represented, not any significant physical phenom.
     
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