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So I was curious about, what if we make a tunnel from one side of the earth, to the other side of the earth? Gravity is ofcourse always negative, which makes you "fall" and not "float".

If we take this image, let's imagine that the BLACK LINE in the midle of the earth, is a tunnel from one side of the earth, to the other side of the earth.

The red DOT is the midpoint, where something in theory, should happen to gravity that is out of our forces.

What would happen, if a human were to be in the red dot? Would the human just stay there for eternity?

Notes: Imagine that the being in the center couldn't kill you, neither would it bring you in contact of any substance. Just a long tunnel with nothing but the tunnel itself.

enter image description here

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  • $\begingroup$ Your right, just couldn't find that. $\endgroup$
    – Kevin Jensen Petersen
    Commented Mar 15, 2014 at 15:16
  • $\begingroup$ Extreme pressure would kill you. $\endgroup$
    – Earth is a Spoon
    Commented Mar 15, 2014 at 15:19
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    $\begingroup$ And, if you drop a parcel in one end, it'd arrive at other end after 40 minutes. $\endgroup$
    – Earth is a Spoon
    Commented Mar 15, 2014 at 15:20
  • $\begingroup$ Note that most answers assume an spherical Earth with uniform (or linear at best) density. These two assumptions are not true. $\endgroup$
    – jinawee
    Commented Mar 15, 2014 at 15:28

2 Answers 2

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At the Earth's surface, gravity pulls on us at 9.81 meters per second squared. That means that, for each second you fall, you speed up by 9.81 meters per second -- but only near Earth's surface. Gravity is a function of mass, and mass is a property of matter. On the surface, all of Earth's matter lies below your feet but, as you fall, more and more of it surrounds you, exerting its own gravity. These horizontal tugs counterbalance each other and cancel out, but the increasing proportion of mass above your head exerts a growing counterforce to the proportionately decreasing mass below, so your acceleration slows as you near the core. At the planet's centre, your acceleration due to gravity is zero -- Earth's mass surrounds you, gravity cancels out and you are weightless. But still you are moving at 24000 kmph speed at the centre and due to inertia forces you continue to move down and will move ideally to the other pole where your velocity will gradually become slow and you will stop only to find yourself going back in the direction you started. So basically it would be a simple harmonic motion. But from your question if by magical means somehow you put a man in that red spot, he wont experience any gravity.

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Assuming a perfectly spherically symmetric earth, you would just float there. Remember that the total force acting on you is given by the vector sum of all partial forces. Now if you are at the center of a spherically symmetric mass distribution, each force that would pull you in a certain direction is exactly cancelled by one pulling you in the other direction. Hence, you would not move anywhere.

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  • $\begingroup$ Interesting. Your answer has given me a pretty good perspective on it.. Thanks man! $\endgroup$
    – Kevin Jensen Petersen
    Commented Mar 15, 2014 at 15:18

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