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B Net force on 'ground' with object on top of it...?

  1. Oct 20, 2016 #1
    A stationary object resting upon the ground experiences two forces, the weight force as well as the normal reaction force of the ground. These forces sum to zero. The ground however experiences the weight force acting downwards upon it. What other force acts upon the ground to prevent it from experience a net force downwards? Is it the normal force of the ground immediate below? If we continue this until we get to the centre of the earth, what is the normal reaction force of an infinitesimally thin piece of ground immediately about the centre of the Earth?
  2. jcsd
  3. Oct 20, 2016 #2


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    What is the normal force on the hub of a bicycle wheel from all of the spokes? What is it about the bicycle wheel that allows such a force to exist? Is there a similar effect that you are not accounting for within the Earth?
  4. Oct 20, 2016 #3


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    Good reasoning. And it leads to something strange, so there must be a flaw somewhere. Because if you come from the other side of the earth the forces are pointing the other way!
    One of the flaws makes things worse: the further down you go, the smaller the area of the remaining sphere that has to deliver all this normal force ! So would that force become bigger and bigger, to even go to infinity at the very center ?

    Gravity makes things with mass attract each other. But things have volume, so at some point the opposite force (opposition to compression) prevents further clumping together and disappearing into a zero volume.

    As they say: under pressure everything becomes fluid. That's the reason planets are spherical. If they were cubic, the corner tips would ecxercise so much pressure on the material underneath that the latter would give way until things even out.
  5. Oct 20, 2016 #4


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    Keep going! The line of logic you are on is working fine.
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