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The Center of the Earth and Black holes and Gravity

  1. Dec 21, 2014 #1
    I was thinking of the conditions exerted at the center of our Earth. If a man let's say in America falls toward the Center of the Earth and a man on the opposite side of the Earth falls, where would they meet? I read that the gravitation force at the center of the Earth is at 0. If they both fall towards the center, what would be the force that keeps them at the center? Would they even be kept at the center radius? This is excluding external gravitational forces including orbiting planets and satellites.

    Why would the conditions reach to 0 and defunct freefall speed? After all an object that stays in motion stays in in corresponding motion. There would have to be another force to repel the gravitation G that is lost to reach a state of 0. Hence, mathematically for the scenario we seem to run into a problem. You cannot take away 1G from a force without losing it in another place. I keep concluding an opposite reaction or negative reaction to gravity. Maybe gravity has an opposite. Can that be possible? If gravity cannot have a negative connotation, can Stephen Hawking's theory of microscopic wormholes have anything to do with it stating that another gravitational force can be exerted through a wormhole like phenomena at the center of any gravitational body.

    I also read in astronomy, that matter that is destroyed into a black hole is refracted back into the universe. In other words there is no energy lost into the black hole itself. The black hole just dumps back all the matter and energy back into the universe it doesn't absorb it. That defers any theory about black holes leading to parallel universes and dimensions and making black holes volatile. I also run into a problem. If black holes are volatile that would make gravity volatile. But that cannot be if gravity seems to have set rates known as freefall. If anyone is considering why I consider black holes as volatile, that is because black holes would break apart a star and spit the energy back out. To spit back the energy and matter of the star, the energy must have hit a snag in the black hole maybe a repelling force or the gravity in the black hole itself must be shifting to return the energy similar to what the stomach does when it's upset. Just like the stomach would be volatile, the black hole is as well. But just like the stomach's acid or the body's refusal to gorge, there must be a reason why the black hole refuses to absorb the energy or matter. I also run into a problem. There are theories stating that a black hole would absorb another black hole making the black hole massive or more massive. The black hole absorbs and repulse. But why? These are just theories but gravitational bodies seem to mysterious.
     
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  3. Dec 21, 2014 #2

    russ_watters

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    Welcome to PF!

    Your instinct was leading you int he right direction, but you didn't quite get there: Since there is no net gravitational force at the center of the earth, a person falling through a hypothetical tunnel would just keep going past the center and out the other side (where, if no one caught him, he'd just fall back into the tunnel again!).

    For the rest, it seems you are mostly just letting your imagination run free... better to get a book or a good source on the net and read up on it.
     
  4. Dec 21, 2014 #3
    I get what you are saying he would become like a yoyo. But that would mean that the gravity would be pushing him away and pulling him. Opposite forces. Or the same force just positive and negative. That would still point out to a source for a singularity or single point if there is a center of gravity. But that wouldn't explain why the object would fall back down. But the tunnel itself would point out that it can't be 0. If it was at 0 eventually the object would reach a state of rest. If the net 0 wasn't 0 actually maybe even a negative equal to the opposite then that would conclude that the object can become like a yoyo actually like a pendulum. The pendulum becomes free swinging because the gravity isn't 0 it constantly pulls it down. If it was 0 then the pendulum would reach a state of equilibrium. But it's terminal velocity breaks the gravitational pull thus swinging it back again. But also a pendulum reaches terminal velocity at the center. That is not the case when you free fall you reach terminal velocity before you reach the center and if the net gravity is 0 at the center, then you might actually slow down because the force of gravity is less and gets to nil the closer you get to the center. Considering there is a time that an object reaches terminal velocity, maybe that is the same rate to decelerate to a state of 0.
     
  5. Dec 21, 2014 #4
    Do you have any links?
     
  6. Dec 21, 2014 #5

    russ_watters

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    No, it wouldn't. Gravity is (at least on Earth) only attractive. When he's going out the other side, his momentum keeps him going and gravity (still attracting toward the center) slows him down, to a stop, as he gets back to the surface.
    I was assuming for the sake of the hypothetical that the tunnel is evacuated, so there is no terminal velocity.
    To what, exactly? Have you read the wiki on gravity?
     
  7. Dec 22, 2014 #6
    I was actually switching back from the tunnel back to free falling on Earth you might have gotten confused. Momentum would keep the object moving back and forth at roughly the same speed indefinitely but I cannot understand how gravitational pull would be a 0 at the center of gravity. Acceleration reaches 0 but why would the gravitation pull decrease. If gravity is attractive what would the object be attracted to at the center?
     
  8. Dec 22, 2014 #7

    Bandersnatch

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    This is pretty much the same thing in this context. The force of gravity divided by the mass of the infalling object nets the gravitational acceleration. If one reaches zero so does the other.

    As for the why - force of gravity at the centre is a zero net force. That is, attraction from all the mass around the centre is equal, and for any direction you choose there is an equal force in the opposite direction that cancels it out.

    Another way (actually it's the same) of putting it is by using the shell theorem (http://en.wikipedia.org/wiki/Shell_theorem).
    The theorem shows that for a uniform sphere of mass, only the mass concentrated in a sphere below the position of a test particle is attractive, all the remaining mass in the shell (hence the name) above that radius nets zero force (it all cancels out).
    For R=0 you've got all the mass in the shell above (r>R) the test particle in the centre and no mass below (can't get lower than R=0). The attraction from the mass in the shell cancels out and there is nothing to pull you in any direction.
     
  9. Dec 22, 2014 #8

    A.T.

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    How do you quantify "gravitation pull", if not as acceleration in free fall, relative to the Earth?

    That is a question that you have to answer, if you claim that gravity is not zero at the center.
     
  10. Dec 22, 2014 #9
    That is the thing, If acceleration reaches the state of 0 the same as gravitation pull decreasing. What would cause the gravitation force to decrease? On Earth friction would cause an object to decrease velocity. But if an object freefalls in a vacuum through the center of the Earth it should continue to accelerate. I understand momentum would keep the object trajecting past the center but then at a certain point it decreases in velocity thus falling back through the center again. Technically it should just continue on with the same velocity. I keep running to a conclusion that it will somehow stop at the center of the gravitational well. because at some point it decelerates. That means there must be a force that keeps drawing it in. How do we know that it will continue to bounce back and forth indefinitely? Also if the force of gravity was a net of 0 using Shell theorem, at the center of the Earth or gravity on Earth should be hollow and attracted to the outward masses. This also leads to the conclusion that maybe every atom exerts a gravitational pull.
     
  11. Dec 22, 2014 #10

    Bandersnatch

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    Can you tell us what's your background knowledge? Do you understand vectors, equations of motion, Newton's gravity? Can you analyze the motion of a pendulum?
     
  12. Dec 22, 2014 #11
    I was just theoretically speaking. I did physics in high school. I understand motion but just confused with gravity.
     
  13. Dec 22, 2014 #12

    Bandersnatch

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    I'm asking so that we may know where to begin with explaining. You say you understand motion, but if I were to say the problem is the same as that of a simple harmonic oscillator (a pendulum), would that suffice?
     
  14. Dec 22, 2014 #13
    but the problem with a pendulum is that a pendulum doesn't have a net gravity of a zero at equilibrium state. Gravity would still exert 1G on the pendulum at equilibrium state.
     
  15. Dec 22, 2014 #14

    Bandersnatch

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    What do you mean by equilibrium state?
    The acceleration tangent to the path of the bob of a pendulum varies from maximum at the sides (points of maximum extenstion), to 0 at the bottom. The direction of acceleration tangent to the path of the bob changes halfway though. At the same time velocity varies from 0 at the sides to maximum at the bottom. Velocity changes direction at the points of maximum extension.
    With a person falling down the tunnel through a planet the acceleration is maximum at the surface, 0 at the centre, and its direction changes halfway through. Velocity goes from 0 at the surface to maximum in the centre and it changes direction at the surface.

    (all disregarding air resistance etc.)
     
  16. Dec 22, 2014 #15
    So you are saying that gravitational force (Gs) will not have any interactions with the a person oscillating back and forth. But wouldn't an object oscillating back and forth on a pendulum use it's own velocity to reach back to maximum height?
     
  17. Dec 22, 2014 #16

    Bandersnatch

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    No, I'm not saying that. The acceleration changes continuously throughout the motion.
     
  18. Dec 22, 2014 #17
    I didn't mean force I mean Gs. I don't even know if force and Gs would be the same. But I also read that a person would slow down as it reaches the center of the Earth what would cause that phenomena. A pendulum doesn't slow down until it reaches maximum height.
     
  19. Dec 22, 2014 #18

    Bandersnatch

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    Force and acceleration change at exactly the same rate.
     
  20. Dec 22, 2014 #19
    I edited my last post do you have any insight?
     
  21. Dec 22, 2014 #20

    Bandersnatch

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    What you read is not correct then. Your intuition about pendulum is, though. A person falling through a (evacuated) tunnel down the planet accelerates all the way to the centre, with acceleration constantly diminishing. The slowing down begins only past the centre, and increases in strength until it reaches maximum at the surface on the other side of the Earth.
     
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