Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Planetary formation and effects of gravity

  1. Feb 27, 2007 #1
    I was curious as to how planetary type spherical bodies form. Specifically, I was curious as to how the gravity affects the nebulous particles moving within themselves just prior to a solid planetary body forming.

    Is it not true that gravity is essentially zero in the center of a planetary body, since there is just as much mass in all directions?

    Prior to the creation of a planetary body, does the circular movement of the particles not create a centripetal force that pulls the particles further from the center at a greater force as one moves towards the outside, where the particles are spinning more quickly? So if the center of the pre-spherical body is essentially zero in aggregate gravity but has a stronger pull from a centripetal force, would it not reason to think that this centripetal force would hollow out the particles just outside the zero-gravity center until a critical maximum is reached where the force of gravity pulling the outside particle on themselves is cancelled perfectly with the centripetal force created from the spin. Essentially the end result may be a more hollow spherical form that does not collapse due to the same principle that a roman arch does not collapse. :surprised

    If anyone has any input to these questions, I would greatly appreciate it, since this information is not easily accessible.
  2. jcsd
  3. Feb 27, 2007 #2


    User Avatar

    Staff: Mentor

    Your argument is not internally consistent and contains some major conceptual errors.
    Yes, but that has nothing to do with this issue.
    With a greater force? What makes you think the particles that formed the earth were rotating that fast around their center of gravity?

    The earth formed from the protoplanetary disk which was orbiting the sun, but contained very little additional rotation in parts where the planets formed. How could it? - particles that would have been moving much greater or less than orbital velocity would not have been in stable orbits around the sun.

    Consider the case of a figure skater spinning with her arms outstretched. As she brings her arms in, the little angular momentum she started with leads to fast rotation as her moment of inertia decreases. So too with the cloud of debris that created earth. There was very little initial angular velocity around the center of gravity.
    OMG, NO! Since the center has no distance from the center ( :uhh: ), there is no associated centripedal acceleration. Just look at the equation and see that it depends on distance! Zero distance means zero centripedal acceleration.
    No, it wouldn't. Your logic and understanding of gravity and centripedal acceleration is horribly, horribly wrong. The concepts are simple and the math is simple. I strongly recommend learning them.

    Or try this: consider the closest things we have to "shells" in the solar system: the rings of Saturn and the asteroid belt. You tell me why they are disk-shaped and not spherical (as is the plane all the planets lie on). Then tell me why in most other orbits (say....where the 8 planets are), the debris didn't just stay evenly spread-out around the sun. Using that analagous situation, you tell me what a uniform and quickly rotating sphere of debris turns into after being released to its formative forces....

    I'll give you the benefit of the doubt for your first post here, but please watch your step: what you are saying sounds like the old hollow earth crackpot "theory". Anyone with the most basic, junior-high level understanding of physics will be able to see just how absurd it is. We don't allow crackpottery here and we expect people to have a real desire to learn.

    edit: BTW, if the rotation of the earth were fast enough that the force of gravity were exactly cancelled by the centripedal acceleration at your location, how much would you weigh?
    Last edited: Feb 27, 2007
  4. Feb 27, 2007 #3
    Thanks for taking time replying to my post.

    As to the asteroid belt and saturn ring example. The objects in the asteroid belt: does the evidence not point to it being a moon size body whose debris resulted from an ancient solar system collision. Perhaps the collision distributed the debris far enough that the gravity was not centralized in a specific sphere, so the debris permeated, with some time and at slightly different rates of speed the entire large orbital area that would have otherwise been the orbit of a planet.

    I didn't mean to imply that the exact center of the sphere is the matter that is being pulled outwardly. What I meant to ask about is the mass that is just outside the center, which would have a gravity induced directional pull towards the center of the sphere. Is the gravity pull at a radius distance of about 500 miles enough to overcome a centripetal acceleration that is close to earth's spin around its axis?

    I guess what I am asking is mathematically what kinds of speeds would be required to have such centripetal force overcome gravity that is within 500 miles of the center of earth?

    I don't know what mechanism would create such spin, but is the earth's current spin around its axis not evidence for it's creation having come about from spinning particles?

    edit: BTW, if the rotation of the earth were fast enough that the force of gravity were exactly cancelled by the centripedal acceleration at your location, how much would you weigh?

    You would weigh nothing, but all the particles around each other would pull on their own gravity, essentially creating a denser concetration of particles.
    Last edited: Feb 27, 2007
  5. Feb 27, 2007 #4


    User Avatar

    Staff: Mentor

    Please reread my post very carefully. You missed quite a lot and didn't answer a lot of questions that will lead you to a better understanding of the issue.
    Yes, or perhaps kept from forming by tidal forces. In any case, the rings of saturn, the asteroid belt, and the planets orbits themselves are planar. This is not a coincidence, it is a consequence of the very phenomena you are asking about. Can you tell me why?
    Ok.... How about a piece of matter above the center, meaning on a line with the pole. How fast is it moving and how much centripedal acceleration is there? This is very important.
    Clearly, yes, but you are forgetting something very important: an object's own weight is not the only thing pushing it toward the center of the earth. It also has to deal with the weight of everything above it. In any case, try playing with the equation: a=v^2/r.
    You tell me. The equation is simple to use. But how is that helpful, anyway? If an object at 500 miles from earth's core is weightless, how could an object on the earth's surface not be thrown off into space?
    Reread my explanation of conservation of angular momentum. The angular momentum of Earth now is the same as the angular momentum of the cloud of debris that formed it. And because of that, the rotation rate of that cloud was much, much slower than the rotation rate of the Earth.
    Yes!! So why are you still asking these questions!?!? Clearly, it is wrong because clearly, you are not weightless.

    Follow-up question: How much centripedal acceleration does an object at the North Pole have? The answer to this question is why you can't have a spherical shell (or spherical solar system) held-up by its own centripetal acceleration.

    BTW, the earth is not a perfect sphere, but an oblate (flattened) spheroid. It bulges very slightly at the equator. What does that tell you about this idea of yours?
    Last edited: Feb 27, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook