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LostInParadise's avatar

What would if feel like gravitationally at the center of the Earth?

Asked by LostInParadise (17174 points ) July 18th, 2011

This is just a thought experiment. I know that it is very hot at the core. The net gravitation would be zero, but there is a gravitational pull in all directions. Would it feel like being torn apart? How strong would the force be in a given direction? I did a Google search on this, but all I can find is that the net force is zero.

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31 Answers

roundsquare's avatar

Yeah, I think you would feel like you’re being pulled in every direction. I suppose the net force would be about half a g in direction (perhaps more since I think the earth gets denser in the center). Thus, if you weight 200 lbs, you’d feel 100lbs pulling up, down, left, right, forwards, backwards and in every other direction.

DrBill's avatar

You would feel weightless, you will not feel pulled in every direction because the force would be pulling you towards the center.

roundsquare's avatar

@DrBill The total force would be zero but I don’t think you would feel no pull. Here is a thought experiment I was using to think about this:

1) Pretend the earth is a perfect sphere with a hollow center just big enough for me.
1a) Thus, standing on the surface, the force is 1g. (A tiny tiny bit less wit the hollow center, but I think we can ignore that for now).
2) Cut the sphere in half and move the two halves apart to the “left” and “right” very far away.
2a) If I’m in the middle, I should feel force pulling me left and right but the force would be weak in each direction. The two forces will be pulling me in each direction.
3) Start moving the two half spheres towards me.
3a) The force to my left and the force on my right will get stronger and stronger.
4) Eventually the two halves meet.
4a) As you and Wikipeida say, the force will be zero.

How can this be? All of a sudden, they meet, and the force drops to zero? Am I making a mistake somewhere else in this thought experiment?

AstroChuck's avatar

@DrBill is right if we are talking the exact center. As you move away from the center, the gravity increases until it reaches maximum G at the surface. But even though you would be weightless in the center of the Earth you would feel the enormous pressure of the Earth pressing on you.

LostInParadise's avatar

@roundsquare‘s argument makes good sense, but then shouldn’t we feel lateral gravitational forces standing on the surface of the Earth? The net force is downward, but there is a lot of Earth in other directions. Why don’t we feel opposing forces in these other directions?

roundsquare's avatar

@LostInParadise That’s a good point. My only explanation is that our bodies do feel it but we don’t register it. In the same way that we don’t feel our clothing after a few minutes or deep ocean fish don’t feel the water pressure that we would feel if we were that deep. I’m not sure if this explanation holds water though.

AstroChuck's avatar

The reason we get zero gravity at the center of the Earth is because we have an equal attraction from the bodies of mass on all sides.

roundsquare's avatar

@AstroChuck isn’t that zero net gravity?

Vortico's avatar

Being torn apart would require that different parts of the body are experiencing different directions of force. However, each particle in the body sums the forces between it and every particle in the earth (a process known as superposition), so when we “feel” the force of all the earth particles, we feel no force and thus no tearing.

roundsquare's avatar

@Vortico Ahh, I see. Very good point. Somehow I thought about the earth on a particle by particle basis but not the person in the center. This also answers @LostInParadise‘s question quite nicely. Thanks!

gasman's avatar

I agree that at the Earth’s center the gravitational force is exactly zero but the pressure is maximum. You would be weightless but crushed to the max.

As to whether you would be “torn apart” by gravity, you’re talking about tidal forces that arise from differential gravitational forces. As at the Earth’s surface, the tidal forces are so tiny you wouldn’t feel it.

It’s well known, however, that falling into a black hole you experience enormous tidal forces, eventually stretching you apart— even your atoms and molecules are pulled apart by the tidal forces. Stephen Hawking calls this “spaghettification.”

roundsquare's avatar

@gasman @AstroChuck Why pressure? What is pushing in?

gasman's avatar

The pressure arises in the same way as going underwater. As you descend below the Earth’s surface the weight of everything above you is pressing down. The deeper you go the greater the weight of material above you. At the center you’ve got the weight of the core, mantle, and crust above you, whose total thickness is the Earth’s radius, all pushing toward the center from every direction.

This is true for any spherically symmetrical body: At the center the gravitational force itself is zero but the pressure due to gravity is maximum.

LostInParadise's avatar

@Vortico , Suppose two people are pulling on your arms with equal force. The net force is zero, but you will experience the sensation of being torn apart.

cockswain's avatar

@gasman Great point. Initially I read this and thought “you’d be weightless.” But if you drilled a hole all the way through the diameter of the Earth and were in the center, you would have way more atmospheric pressure exerted on you from each side. If it’s enough to atmospheric pressure to kill, I don’t know.

AstroChuck's avatar

Think of the center of the Earth as a Lagrangian point.

Vortico's avatar

In your scenario, @LostInParadise, different directions of forces are being exerted on different parts of the body. Gravitational weightlessness means that the net force of gravity is zero for all particles. In addition, the particles themselves do not experience tearing because they “calculate” the net force before acting upon it. (That’s one way to look at it.)

gondwanalon's avatar

The entire Earth displaces space-time continuum around it which causes it’s gravity. The focal point of Earth’s distortion of the space-time continuum would be it’s core. That is one reason why the Earth’s core is made up of dense solid sphere of mostly iron. Therefore the most logical result of being at the center of the Earth would be a greater gravitational force.

LostInParadise's avatar

Okay, I get it, finally.! Every part of the body feels zero gravity so there is no net force.

augustlan's avatar

[mod says] This is our Question of the Day!

gorillapaws's avatar

This was fascinating, GQ, and many interesting answers.

roundsquare's avatar

@gasman Does what you’re saying assume a hole such as what @cockswain said? I sort of assumed you were “magically” put into the center of the earth and thus there were no “holes.” Or does this pressure come for another reason?

Btw, this question, right here, is one of the reasons I love fluther.

cockswain's avatar

I’d really like to know if atmospheric pressure could crush someone at the center of the Earth. I know water pressure would after a mile or so.

gorillapaws's avatar

@cockswain I’m pretty certain it could. Just picture entering a hyperbaric chamber and having someone crank it to max. I’d imagine it’s the opposite of being decompressed.

cockswain's avatar

I wonder if the pressure increase is linear. At sea level, it’s 14.7 psi if I remember right. I don’t know how many miles thick the atmosphere is, but let’s say it’s 10 miles. Would that mean then that 90 miles below the surface it would be 147 psi, 990 miles would be 1470psi, etc…

I don’t know the radius of the Earth offhand, but I’m thinking the atmospheric pressure would be sufficient to squish any living thing.

LostInParadise's avatar

Wouldn’t the zero gravity argument apply to air pressure as well? Air pressure results from the weight of the air.

roundsquare's avatar

@LostInParadise Nope. Gravity is a force so it cancels out if its in opposite directions. Air pressure is from the air molecules hitting you so they don’t cancel out like that.

gasman's avatar

I think a lot of participants in this thread are confused—I hope I’m not one of them! The gravitational force on an object placed exactly at the center of a spherical body is zero. If you could somehow enclose yourself in a protective bubble at the Earth’s center then you would float, weightless, just as you would in outer space.

One way to explain this is to consider the Earth to be a nested series of concentric spherical shells. It’s well known that the gravitational force inside a spherical shell of mass is exactly zero. Thus as you descend below the surface you only feel the gravitation of what lies below you (toward the center) but not above you (toward the surface). When you reach the center there is nothing below you so your weight becomes zero.

Indeed if you bored a hole straight through the earth’s center and jumped in, your weight would decrease linearly until you passed through the center, then it would increase linearly again until you emerged on the opposite side.

One point of confusion in this discussion is the notion that at the center you are somehow pulled outwardly in all directions, being stretched apart, even if weightless. This is false. It would be as if there were no gravity at all—no falling, no stretching, no gravitational force at all.

(My earlier mention of black holes was in connection with very large tidal forces that stretch you out—in sharp contrast with the situation at the Earth’s center.)

Another point of confusion has to do with pressure. Gravity is zero at Earth’s center, yet the pressure is at maximum—in fact it’s calculated to be millions of atmospheres (ref). That’s why the inner core is solid iron despite being way hotter than iron’s normal melting point. You’d be instantly crushed into a greasy lump. I wouldn’t describe this merely as “atmospheric pressure,” however, since it arises from the combined weight of atmosphere, crust, mantle, and core.

It might seem counter-intuitive that gravity could be zero while the pressure is enormous, but really one has nothing to do with the other.

cockswain's avatar

Thanks for the clarifying post.

I wouldn’t describe this merely as “atmospheric pressure,” however, since it arises from the combined weight of atmosphere, crust, mantle, and core.

No, but I was just musing that if there was a hypothetical tube running through the diameter of the Earth, how many atms is that. And your reference addresses that, thanks for looking it up.

Hypothetically I was also leaving out the temperature of the inner Earth, since obviously we know lava can kill us.

It is interesting to note that the pressures of everything above the core are great enough to force the iron into a solid state far above melting temperatures we observe on the surface. It’s the same reason why water boils at a lower temp in the mountains vs. sea level.

mattbrowne's avatar

If we just consider gravitation it’s actually not zero, because both the sun and the moon are pulling. And so is the rest of the solar system and the entire universe. The sun’s pull is countered by the Earth’s movement circling the sun.

gondwanalon's avatar

@mattbrowne is on the right track. You must look at the big picture here. As I attempted to say above, just because you happen to be located at the center of the Earth, it does not separated you from the huge distortion that the Earth creates in the space time continuum. It seems logical to me that such a location as the center of the Earth would be of crushing g-forces, Because that is the focal point of of Earth’s disruption of the space-time continuum.

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