Gravity in The Core of The Planet

So uhh, yea, earlier on today, I was wondering about what happens to the size of the force of gravity as you get closer to the core.

Apparently it is accepted that the force grows stronger as you get closer to the core.

But, is it not true that gravitational force is dependant on the size of mass?

If that is the case, then how come as you get to the core, the gravitational field increases?


Take this for example (Hypothetically, of course):
You make an elevator which brings you right down to the core.
As you near the bottom, there will be less mass beneath you, therefore the gravitational force pulling from below will be weaker, and the force pulling from above will get stronger (Which means of course, the total force will be weaker).

Now, if you were to be right in the very middle of the planet, wouldn't you be sort of being pulled out the way in every direction? This would equal a total force of zero. Even though going by the whole "stronger as you get closer" theory, it should be stronger than it is on the surface.


So... is this not true, then?
If not, then care to explain why?

I have always been fascinated by gravity, I just haven't looked into it as much as this. I also googled for a little while and didn't really find anything about this.
So either I never googled very well, or it is just bullshit enough to not have been talked about before. I dunno. But I am very curious.

No, mathematically, gravity is 0 in the centre of a sphere, where did you get the idea that the centre of the earth would have stronger gravity?

Perhaps there would be slight gravity facing outwards.

The gravity does get lighter if I remember correctly, BUT, you will have the pressure of hundreds of miles of rock above you.

Quote:

---

Originally Posted by ninja9578

No, mathematically, gravity is 0 in the centre of a sphere, where did you get the idea that the centre of the earth would have stronger gravity?

---

Cause when I googled it, it said "It is accepted that as you go closer to the core of the earth, the gravitational force increases" or something along those lines.

No, because the more rock you have above you, the more it cancels out the rock below you. Gravity at the centre of a ring or a sphere is 0, always.

Perhaps they are getting confused by the fact that you get closer to the centre of gravity, the strength of it increases, but that doesn't apply if you are inside of the object :P

And jarhead is correct, the reason the centre of the earth is molten is because of pressure, not gravity.

Everything you said there is what I assumed was true before I googled it.

Thanks. *shakes fist at google*

...ok I probably read it wrong.

Quote:

---

Take this for example (Hypothetically, of course):
You make an elevator which brings you right down to the core.
As you near the bottom, there will be less mass beneath you, therefore the gravitational force pulling from below will be weaker, and the force pulling from above will get stronger (Which means of course, the total force will be weaker).

Now, if you were to be right in the very middle of the planet, wouldn't you be sort of being pulled out the way in every direction? This would equal a total force of zero. Even though going by the whole "stronger as you get closer" theory, it should be stronger than it is on the surface.

---

You are exactly right Slash.

At any point above the surface of the Earth, you can model the gravity using a single point at the centre of the planet, with the mass of the planet (i.e. the inverse square law applies).

However below the surface it gets a lot more complicated and you have to use various integrals to work it out (except at the centre where it's clearly 0 and you would float about as if in outer space). The best way to think about it, as you have done, is to consider in which direction each individual atom is pulling you.

Quote:

---

And jarhead is correct, the reason the centre of the earth is molten is because of pressure, not gravity.

---

Well, gravity is what causes the pressure... also, pressure actually raises the melting point of the rock. The centre of the Earth is molten because the centre of the Earth is very hot. I think this is partly due to the residual energy from the huge amount of energy created in the formation of the Earth, and also I think uranium decay is involved.

The centre of the earth is solid. The outer core, however is molten, but the inner core is still solid.

True, true. I was being sloppy, by centre I meant core.

Inner core's still solid and if I remember correctly, 10,000 - 15,000 degrees. I don't remember if that's Celsius of Fahrenheit -- probably Fahrenheit

Can't believe everything you read on the internet. You probably read it correctly, and it was just wrong. Luckily you were thinking about what you were reading, instead of just accepting everything as fact.

Because the material is denser near the centre of the earth, as you approach the core gravity does get stronger, until about 1/2 the way the core, there the gravity is strongest, but from there gravity gets weaker until it eventually becomes 0 at the core.

This diagram illustrates
http://en.wikipedia.org/wiki/File:Earth-G-force.png

I can totally understand why someone would believe that the force of gravity increases as one approaches the center of the mass. We actually have a formula in Physics that describes the force of gravity between two objects. Better yet, it's dependent on the distance between the two masses as well, where the decreased distance actually increases the force.

But, everyone else is quite right. When you're underneath the Earth, there's mass on top of you as well that pulls and cancels what pulls from below.

Are we assuming that this planet is a perfect sphere with a consistent density? In that case, if teleported to the center of the planet, one would probably feel gravity pulling the person apart in all directions. However, few, if any, planets are perfect spheres with consistent densities, though.

You wouldn't feel any force. Just consider each atom in your body. How can they be pulled apart in all directions? How would each individual atom move? The key point is that it's the sum of all the forces which counts, and at the centre of the Earth, they sum to the 0 vector. None of the atoms in your body would move anywhere, so you wouldn't feel anything. It'd just feel like falling, without the wind.

So, with no gravity, there would be no pressure, right? If you fell down the elevator shaft would you fall all the way to the center? Or would you slow down until you started floating? Would you fall back up to the halfway point? Like bungee jumping? I always assumed that it was the center of the mass that was the center of gravity. But of course there couldn't be any gravity because there is no more 'down' to fall. Just like at the north pole you can't go in any other direction except south. At the center of the Earth all directions are up.

Pressure, density, and gravity are all related on a deep level. This is inclusive of time as well. Basically, pressure can be seen as a scalar quantity dependent on the acting normal force maintained from the surface of the earth. With depth comes density comes an increase in gravity; thusly pressure, and in turn a change in the passage of time relative to an observer from some arbitrary distance with other conditions, but that's not very relevant. Anyway, the pressure one would experience at the center of the earth would be appreciable - about 3.5 megabar - however, gravity itself would diminish at some relation, as others have said, due to the surrounding mass in all directions causing a null, or zero, resultant vector. Even with a perfect cylindrical tube cut to the center of the earth, which could never happen, the air pressure alone would be enough to obliterate any human or human-made device long before it reached the center.

Interesting. It's weird to think such high pressures could exist at the centre of the Earth where there's no gravity. I suppose this is because the atmosphere above and below you is acting like a heavy cork, keeping the local atmosphere around you squashed in.

Quote:

---

Originally Posted by Dannon Oneironaut

So, with no gravity, there would be no pressure, right? If you fell down the elevator shaft would you fall all the way to the center? Or would you slow down until you started floating? Would you fall back up to the halfway point? Like bungee jumping? I always assumed that it was the center of the mass that was the center of gravity. But of course there couldn't be any gravity because there is no more 'down' to fall. Just like at the north pole you can't go in any other direction except south. At the center of the Earth all directions are up.

---

Yes, that's a good way to think about it ('by symmetry').

If there were no air in the tunnel, you would accelerate towards the centre (to a very large speed), then keep going whilst decelerating, until you came back outside the other side to ground level, and then you would fall back again.

If there were air, air resistance would mean you would only overshoot the centre a tiny bit, and then pretty quickly come to rest at the centre.

So there could be no Zion from the Matrix.

...that was underground, in the sewers. Not the centre of the Earth.

Quote:

---

Originally Posted by Xei

...that was underground, in the sewers. Not the centre of the Earth.

---

Could there be a 'The Core'? Haha, probably not.

What can you guys say about the core of the earth actually rotating at a different speed to the Earth?

Maybe there is a mini-stabilized blackhole at the center of the Earth? dun dun dun...

Quote:

---

Originally Posted by Xei

...that was underground, in the sewers. Not the centre of the Earth.

---

I just watched The Matrix again the other night and they said that Zion was near the core. And if I remember correctly, in the last movie the sentinels had to borrow for days through solid rock to reach it.

Quote:

---

Originally Posted by really

Could there be a 'The Core'? Haha, probably not.

What can you guys say about the core of the earth actually rotating at a different speed to the Earth?

---

Actually, it may. Probably depends on how dense it is. If it isn't that dense, then is probably rotates faster than the rest of the Earth to make up for it, which it creates torque as a multiplier force.

Quote:

---

Originally Posted by Dannon Oneironaut

I just watched The Matrix again the other night and they said that Zion was near the core. And if I remember correctly, in the last movie the sentinels had to borrow for days through solid rock to reach it.

---

No, it's near the mantle.

Well, of course, if it weren't fictitious, you would be right. In the movie, they say that it is near the core, which is impossible, due to the mantle, but also because of the gravity. And since the guy on the Nebuchadnezzar probably meant that it was near the Mantel, even then the gravity would make that impossible, or, extremely uncomfortable, right?

If there's no gravity in the centre of a sphere, then why would there be pressure from the rock above you? OR does that have nothing to do with gravity? But if it does, then that rock above you shouldn't matter, shouldn't it?

It's because the Earth is rigid. Imagine pushing a pencil into dough. The only force is at the top of the pencil. But because the pencil can't be compressed, that pushes the bottom of the pencil, down into the dough, even though the force isn't being caused there. It's the same at the centre of the Earth. Think of the column of dirt above your head. There's no gravity pushing the dirt at the bottom of the column down, but further up the column is being pushed down by gravity.

Quote:

---

Originally Posted by Xei

It's the same at the centre of the Earth. Think of the column of dirt above your head. There's no gravity pushing the dirt at the bottom of the column down, but further up the column is being pushed down by gravity.

---

Oh, makes sense! Thanks!