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No gravity at center of earth= no pressure?

  1. Sep 14, 2010 #1
    Hey, Just a general question.

    So, I've just learned that the force of Net gravity anywhere in a shell is 0. And the earth can be thought of as a continuation of shells- like an onion or something. And at the center the net gravity is 0.

    But I thought the immense force of gravity is what caused nuclear fusion in the center of stars? Does no force of gravity mean no pressure? After all P= F/A. Could someone help me understand this?

    Thanks
     
  2. jcsd
  3. Sep 14, 2010 #2

    Doc Al

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    No, far from it. While the gravitational force at the center might be zero, you still have the enormous weight of the layers of earth above you crushing down.
     
  4. Sep 14, 2010 #3
    That's what I thought intuitively. Pressure in a liquid is dependent on height. But it's also dependent on g, and if g is 0....

    Sorry if my question sounds dumb, but I don't quite understand.
    You say you have the "enormous weight of the layers of earth above you crushing down"
    But crushing down with what force?? It can't be the force of gravity because it's 0. at the center.

    Thanks for replying so fast
     
  5. Sep 14, 2010 #4

    Doc Al

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    But the value of g is only zero at the center of the earth. As you move away from the center, the value of g increases until it reaches its maximum value at the earth's surface.

    Each layer of earth must support the layers above it. Only the very center is 'weightless'.
     
  6. Sep 14, 2010 #5
    Hmm, that's a good way of looking at it. If the sun is in static equilibrium (at least radialy, not shrinking or growing) then atoms near the center must supply the normal force to counteract that massive column of weight above it.

    And perhaps only one lucky atom, (or maybe better electron if it's point like) at the dead center of the sun will feel no pressure at all.

    Do I have the right Idea now?
     
  7. Sep 14, 2010 #6

    Doc Al

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    Exactly.

    I'd think that anything at the dead center would feel maximum pressure, all things being equal. (Not sure it's productive to talk about an individual atom or electron, though.)

    I think you've got it.
     
  8. Sep 14, 2010 #7


    Yeah I see it now.
    Cool, thanks so much for helping me figure that out!
     
  9. Sep 15, 2010 #8
    I have a hard time wrapping my head around it, too.

    Imagine putting a huge, heavy iron block on top of a table. That table has to support the weight of this block.

    What if we made the table out very light material? Let's make it out of unobtanium, which has a weight of 0. Then we have a weightless table, which is awesome. Unfortunately, that iron block on top of it still weighs the same...and the table still has to support it.

    Same with that single atom in the very center of the earth. It might not weigh anything, but everything on top of it still does, and everything on top of it still wants to go where that single atom is.
     
  10. Sep 15, 2010 #9
    Think of it this way: Imagine the earth and moon were 1 foot apart from eachother, and you were trapped between the two. Same effect as being at the center of the earth, just the "earth and moon" you're trapped between happens to be umpteen billions of tons of earth on "either side" of you.
     
  11. Sep 15, 2010 #10
    My understanding is that the reason gravity is zero at the center of a sphere is because the mass cancels its own gravity out in the various directions. This would mean that halfway to the center of the Earth, the mass above you would cancel out the gravity of half the mass below you. Therefore, I would think the gravity would keep decreasing until you get to the center. So if net gravity keeps decreasing as you go deeper, why would pressure continue to increase until the center? If anything I would think there would be a hollow void due to the centrifuge effect caused by rotation.
     
  12. Sep 15, 2010 #11
    Gravity at the center of a sphere is zero, but its pressure is not. Pressure adds from everything above it. Otherwise the pressure of air at sea level would be zero instead of 15 lbs per square inch.
     
  13. Sep 15, 2010 #12
    no... no force =/= no pressure....

    think of going at center of earth.. all the mass on earth is concentrated at center.. so there will be pressure
     
  14. Sep 16, 2010 #13

    cjl

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    Nope.

    Halfway to the center of the earth, the mass above you cancels out none of the gravity of the mass below you. Instead, it just cancels out the mass that is farther from the center than you are, but on the opposite side of the earth. Gravity pulls downward (albeit with decreasing strength) all the way to the core, and because of this, everything is trying to get to the core. Even though there is no gravity at the core itself, the entire earth is trying to squish itself towards this point, so the pressure is enormous to counter this force.
     
  15. Sep 16, 2010 #14

    Drakkith

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    This is the way i understand it. I could be wrong.

    If i were 1 mile under the earth, all the mass above me would be exerting a gravitational pull on me just like the rest of the mass below me and to the sides. Now, the gravity of that 1 mile of material would make me "lighter" in weight than i would be standing closer to the center of the earth, because it is no longer pulling me down with the rest of the earth. However, that 1 mile of earth above me, IS being pulled down by the rest of the earth, and since it is being pulled down it will exert pressure and i would be crushed.

    If i were to be at the center of the earth, i would "weigh" nothing, because there is nothing to pull me predominately in one direction like what happens to us here at the surface. The gravity exerted by all the mass surrounding me effectively cancels out each other in sense. But again, all the mass is attracting itself, being pulled inwards and exerting enormous pressure at the center.
     
  16. Sep 16, 2010 #15
    A car in a car crusher has a net force on it of 0, because the forces cancel out. It will nevertheless get crushed.
     
  17. Sep 16, 2010 #16
    Let there be no mistake: Gravity DOES NOT cease to exist just because you are in a hollow center area of a massive sphere.
    Rather, instead of being pulled down as on the earths surface, you are being pulled up in all directions.
    It is only the "effect" of gravity that is altered in different scenarios. Gravity itself continues to exist.
    To my knowledge, "destructive interference" of gravity has not yet been proven.
     
  18. Sep 16, 2010 #17

    Doc Al

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    When you are inside a uniform spherical shell of mass the gravitational attraction on you from that shell is exactly zero. The net effect is that you feel no gravitational force at all. There is zero gravitational field (at least from the mass of the shell) at all points within the shell.
     
  19. Sep 16, 2010 #18
    What about rising and falling tides?
     
  20. Sep 18, 2010 #19
    Good analogy.


    Yeah thanks everyone for your replies. I understand it now and it totally makes sense. I just didn't really have the right way to look at it in the beginning.
     
  21. Sep 18, 2010 #20

    Drakkith

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    Are you saying that there is no gravitational force on you at all, or is it that it all balances out and because of that you feel no force overall?
     
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