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Gravity inside the earth

  1. May 7, 2008 #1
    If one pass the earth's surface and move inward to the center. What would happens?

    1/ would he get stretch
    2/ would he get compress
    3/ or would he feel nothing at the center?
  2. jcsd
  3. May 7, 2008 #2


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    There's quite an extensive discussion of this around here somewhere, if you want to do a forums search.
    Basically, the gravity pulling from all sides at the centre would render one effectively weightless.
  4. May 7, 2008 #3
    If one could fall from the Earth's surface to the center, you would accelerate downward at a decreasing rate, until you reached the center at which point the would be no force on you.
  5. May 7, 2008 #4
    sorry to bring back the old tocpic. I couldn't find the search topic fuction. Anyway, my wonder for for this question is if this is our assumption. how can we explain collapse of stars under its own gravity? Not to mention hard metals like rocks and diamond, which form under extreme temperature and pressure. However I think I see it. Though gravity is zero inside, a pressure with value of integral from 0-r of density x gravity dr where gravity is a function of radius .btw, how do you put in math sympbol and expression. Thx
  6. May 9, 2008 #5

    Shooting Star

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    As you know, matter tends to clump together due to gravitation. But other forces resist this clumping once the matter particles come close enough to each other. This gives rise to pressure at every self-gravitating body like earth or a star, though the forces resisting gravity are different in the earth and in a star. The pressure is highest at the centre because the weight of all the outer layers are acting on it, and decreases as you go toward the surface, as you have rightly pointed out.

    In a star, this force against gravity is provided by hot gases which are kept hot due to nuclear reactions. When all the nuclear reactions stop because the fuel has run out, the star may tend to shrink due to self gravity, until it can shrink no more due to some other force. For example, in a white dwarf, it is the electron degeneracy pressure which resists gravity. If the mass is too much, then no force may be able to stop it from shrinking, and a black hole may form.

    Inside the earth, the pressure is very high compared to our standards, and is enough to form rocks or diamonds or fossil fuels. If you were put inside, you would get really compressed and burnt to a crisp.

    If a small tunnel was drilled through a solid sphere passing through the centre, and you fell into it, then the motion would be an SHM, but I don’t think that was your question.

    For math symbols, you can use latex. Click on the ∑ sign above.
    Last edited: May 9, 2008
  7. May 9, 2008 #6


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    Just to clarify- you would not stop at the center of the earth: once you had reached the center of the earth, you would have built up a pretty good speed and would continue through, having just enough energy to (neglecting friction) reach the other side of the earth. As Feldoh says, it is the acceleration that would be decreasing, not the speed.
  8. May 9, 2008 #7
    Yeah, to add a bit more, if we pretend there is a tunnel straight through the center of the earth and you jump into it, you would actually oscillate back-and-forth through the earth. (assuming no air resistance :rolleyes:)

    But that's not really what the OP is asking
  9. May 9, 2008 #8
    This... is definitely a good project the world should invest in! $ 200 USD for each oscillation!
  10. May 9, 2008 #9


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    The pressure at the center of the Earth is estimated to be about 350-360 GPa (or about 3.6 million atm) from the gravity pulling on the matter of the Earth.
  11. May 9, 2008 #10


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    Hmmm.... you'd have to pop your ears a couple of times on the way down.
  12. May 9, 2008 #11
    Dang, some good pressure. I'm trying to make some synthetic diamond so I can get marry with less cost. I'm encountering problem with heat source. Where can I get 4000K ? no metal would hold that temp even ceramic. I'm thinking concentrate sunlight intersecting beams +conventional fire. Crap, it seems easier buy that stone.
  13. May 9, 2008 #12


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    Keep in mind, Atom, that the concept of engagement rings was instigated by the diamond industry in the Great Depression. They are not something that has ever been either socially or religiously sanctioned. Not diamond-wise, that is. My mother's wedding band was a pasitc coil formerly used as a leg-band on a chicken. A couple of years later, she put on her late mother's gold band, which she is still wearing at the age of 95.
  14. May 9, 2008 #13
    Haha it's useless if we're the only one get this message. Oh well, I guess it's a challenge to see if we can concentrate energy. I think that's science never ending break through point; concentrate the forces of nature into a fine point and control it.
  15. May 10, 2008 #14
    If all the earth's matter was concentrated on it's surface, and distributed evenly, we wouldn't have a gravitational field inside the earth.
  16. May 11, 2008 #15
    Okay, but it's a fun question. If you drill your tunnel through the poles I think you might just oscillate, but what if you do it at the equator? I think on the way down you'd get whomped to one side, and then on the other side on the way out. So you'd need a lot of lubricant. Oh, and is your tunnel a sealed vacuum? If not, what does the pressure profile look like? Just in case we ever build this thing ;).
  17. May 11, 2008 #16
    You're worry too much. The worst case is we forget about dampen and leave somone hanging at the center. Just get a big rope and pull him up. lol
  18. May 13, 2008 #17
    Okay, gotcha .... but I'm still worried that he might be a bloody pulp being squished by a gazillion atmospheres. But you're right, we should just give it a try .... ummm ... you first! ;).
  19. May 13, 2008 #18

    So at the centre of the earth, despite zero gravity, there is still a huge pressure caused by the fact that everything around the centre indeed is accelerated towards it?

    Can we say that in practice what really crushes you is not so much what is immediately around you, but rather what is far away from the centre? That the soil (or whatever) a few meters around you would not really want to hurt you, but is pushed down by what is above which pushes a little more due to higher G, and is in turned pushed even more by the next layer above etcetera?
  20. May 13, 2008 #19
    If you could build a tube-like structure that would negate the temperature and pressure at the core, and jumped in, would you then oscillate back and forth continuously, or would you eventually come to rest at the center?
  21. May 13, 2008 #20
    In ideal conditions, you would continue to oscillate forever.That is because force of gravity increases as you move away from the center and so the force pulling you towards the center would increase with distance from the center.Hence you would get a condition like F=-kx(k is a constant and x is displacement) similar to eqn for simple harmonic motion.
    g'=apparent force of gravity
    r=radius of earth
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