# How strong is gravity in the center of the earth?

• May 11th, 2011, 07:07 PM
Quantime
How strong is gravity in the center of the earth?
Is it greater or less?
• May 11th, 2011, 08:15 PM
GiantEvil
We were just discussing that here; http://www.thescienceforum.com/viewt...30706&start=15.
• May 11th, 2011, 08:46 PM
kojax
There isn't any gravity at the center of the Earth, at least not the dead center. If you stood at the center of the Earth the mass on every side of you would be evenly distributed, leading to a zero net force.

(edited twice, once to restore it to the original)
• May 11th, 2011, 08:50 PM
inow
Quote:

Originally Posted by kojax
There isn't any gravity at the center of the Earth, at least not the dead center. If you stood at the center of the Earth the mass on every side of you would be evenly distributed, leading to a zero net force.

If you want to get pedantic, you actually WOULD still be impacted by the gravity of our sun, the center of the galaxy, and other objects like that... even at the center of the earth.

Also, I suspect even if you disregard all that, and only consider the earth system, you'd ultimately be pulled in at least one direction or another since the earth is lumpy and not perfectly spherical... there is no perfect center, really. Essentially, you'd get pulled toward the direction with more mass, or the side with the taller mountain or something.

To the OP, here's a pic from the wiki page on earth gravity (as you can see, gravity seems strongest between the outer core and lower mantle, and is most certainly higher at the surface than the core):

Quote:

Earth's gravity according to the Preliminary Reference Earth Model (PREM).[7] Two models for a spherically symmetric Earth are included for comparison. The straight dashed line is for a constant density equal to the Earth's average density. The curved dotted line is for a density that decreases linearly from center to surface. The density at the centre is the same as in the PREM, but the surface density is chosen so that the mass of the sphere equals the mass of the real Earth.
• May 11th, 2011, 08:53 PM
kojax
That's true. It would have been more accurate for me to say that gravity is just less, as we get closer to the core, because of the effect of having large amounts of mass on all sides of you, pulling you in all directions (which causes the effect to cancel out.)
• February 11th, 2013, 06:29 PM
deek3117
hey guys, im a freshman in highschool and i have not even taken a physics class, but i can see you're missing something. There would be a gravtation force in which there would be a push in all directions inward to a point a singularity, which is a point so small it is unacheivable unless it is in a black/white hole. so may i inquire what force would be that of in all directions of a perfect sphere bigger than the point of singularity?(assuming the earth is perfectly spherical)
• February 11th, 2013, 06:35 PM
Dywyddyr
Quote:

Originally Posted by deek3117
hey guys, im a freshman in highschool and i have not even taken a physics class

That's enough.

Quote:

but i can see you're missing something. There would be a gravtation force in which there would be a push in all directions inward to a point a singularity, which is a point so small it is unacheivable unless it is in a black/white hole. so may i inquire what force would be that of in all directions of a perfect sphere bigger than the point of singularity?(assuming the earth is perfectly spherical)
Have you taken any English classes?
Where's the "push" come from?
• February 11th, 2013, 06:41 PM
Strange
Quote:

Originally Posted by deek3117
hey guys, im a freshman in highschool and i have not even taken a physics class, but i can see you're missing something. There would be a gravtation force in which there would be a push in all directions inward to a point a singularity, which is a point so small it is unacheivable unless it is in a black/white hole. so may i inquire what force would be that of in all directions of a perfect sphere bigger than the point of singularity?(assuming the earth is perfectly spherical)

I'm afraid I have no idea what you are trying to ask. Let me take a guess. Are you trying to compare the center of a black hole (where all mass is compressed to a point) with the center of the earth (where the mass is evenly distributed in a sphere around you)?
• February 11th, 2013, 11:27 PM
WaterWalker
Quote:

Originally Posted by deek3117
hey guys, im a freshman in highschool and i have not even taken a physics class, but i can see you're missing something. There would be a gravtation force in which there would be a push in all directions inward to a point a singularity, which is a point so small it is unacheivable unless it is in a black/white hole. so may i inquire what force would be that of in all directions of a perfect sphere bigger than the point of singularity?(assuming the earth is perfectly spherical)

I don't quite get what you are trying to ask. But, just to clear a few points. Gravity doesn't push, it pulls. It's a force between two objects, not outside of them.

As far as I know, a white hole doesn't have a singularity. (Anybody, correct me if my understanding is incorrect). As I understand it, that singularity can only be achieved under immense gravity pull. A black hole pulls matter into it, while a white hole spews matter out. And as such, it can't have a singularity in the same form as a black hole.

(I think this might help with the answer you seek). The Earth doesn't have enough matter to form a singularity. And because of this, and the rotation (maybe), the gravitation between all the atoms tend to keep us from being squashed into a singularity.

On a side note, the density of the object has to be taken into consideration.
• February 11th, 2013, 11:38 PM
markashley
Quote:

Originally Posted by inow
you'd ultimately be pulled in at least one direction or another since the earth is lumpy and not perfectly spherical... there is no perfect center, really. Essentially, you'd get pulled toward the direction with more mass, or the side with the taller mountain or something.

I may be getting a bit picky here but... even though the Earth is not a perfect sphere, I believe the Earth still has a point of center of mass where you would be pulled in all directions by the Earth equally.
• February 12th, 2013, 07:00 AM
RedPanda
Quote:

Originally Posted by markashley
I may be getting a bit picky here but... even though the Earth is not a perfect sphere, I believe the Earth still has a point of center of mass where you would be pulled in all directions by the Earth equally.

Just to add to the complexity: tides will cause the centre to be constantly moving.
Also, each life-form will change the centre point as it moves about on the Earth's surface.

I guess there must be an actual centre - but it would be constantly changing its position as life moves around a sea which is moving around a planet which is moving around a solar system which is moving around a galaxy.....and so on.
• February 12th, 2013, 07:15 AM
Strange
Quote:

Originally Posted by RedPanda
Just to add to the complexity: tides will cause the centre to be constantly moving.

That would probably be a measurable effect.

Quote:

Also, each life-form will change the centre point as it moves about on the Earth's surface.
I doubt that needs to be taken into account. But its an interesting question!

I doubt the total mass of animals on earth is much more than 2 or 3 billion tons. If you could persuade every animal to swim, crawl or fly from one side of the planet to the other, it would only make a difference to the center of mass of a few microns. Normal movement of animals will largely be random and cancel out. Any net effect is probably less than the width of an atom.
• February 12th, 2013, 11:25 AM
forrest noble
Quote:

Originally Posted by Quantime
How strong is gravity in the center of the earth? Is it greater or less?

The behavior of gravity would not change but our perception/experience of gravity would be very different. From the Newtonian perspective one would be pulled in all directions about equally so that you would be weightless, like being in outer space you would generally "float." :) The gravity toward the sun would also be counteracted by your orbital momentum around it.
• February 13th, 2013, 06:41 AM
RedPanda
Quote:

Originally Posted by Strange
If you could persuade every animal to swim, crawl or fly from one side of the planet to the other, it would only make a difference to the center of mass of a few microns.

So - all we need is fatter animals and better coordination and we could steer this crazy planet through the cosmos!!

But, yes, it is definitely in the "negligible difference" category.
• February 13th, 2013, 08:16 AM
SpeedFreek
The gravity of the Earth has its LARGEST effect at the centre of the Earth - the gravitational time-dilation due to the Earths gravity is at its greatest at the centre of the Earth.

Just because you would feel no fictitious forces acting upon you at the centre of the Earth, due to the gravity in each direction above you cancelling out, does not mean that there is no gravity there - there IS gravity there, and in fact there is more gravity there than anywhere else on Earth!

:)
• February 13th, 2013, 10:42 AM
Ascended
Perhaps some background on Newtonian shell theorem may prove helpful here,
Shell theorem helps us to understand how in classical mechanics gravity behaves
in regard to spherical bodies such as planets and moons and thus allowing for the
gravitational caluculations to be simplified. It states that external objects are affected
gravitationally by a spherically symmetric body as though all of its mass were
concentrated at a point at its centre.

The simple part to understand and this is the 'shell' part, is that a spherically symmetric shell,
think of a hollow planet, moon or even ball, exerts no net gravitational force on anything
inside, so you can be anywhere inside the shell and feel no such gravitational effects.
• February 14th, 2013, 07:42 AM
SpeedFreek
Quote:

Originally Posted by Ascended
The simple part to understand and this is the 'shell' part, is that a spherically symmetric shell,
think of a hollow planet, moon or even ball, exerts no net gravitational force on anything
inside, so you can be anywhere inside the shell and feel no such gravitational effects.

Indeed, but you would stil be gravitationally time dilated (your clock would run slow) in comparison to an observer outside the shell.