At Center of Earth: Weightless or Crushed?

[Cyrus]

I know that if you are in the center of the earth, gravity will act on you equally in all directions so you will feel weightless. But at the same time, won't you be crushed to death? Since you have the pressure of all that mass around you in all directions? It would seem that being weightless would be the least of your problems.

 

[mathman]

It depends on whether or not you can create a cavity at the center so you won't be crushed. Actually, the center of the Earth is molten iron, so you would be cooked if you didn't do anything.

 

[Nenad]

I know that if you are in the center of the earth, gravity will act on you equally in all directions so you will feel weightless. But at the same time, won't you be crushed to death? Since you have the pressure of all that mass around you in all directions? It would seem that being weightless would be the least of your problems.

Well actually, no you won't. Because think about a rock that is 10m away from the centre, it too has an environment of extremely low gravity on it, sonce there is tons of Earth pulling it in all directions. This is why the centre of the Earth is composed of a very fluid (non dence) material, because the gravity is so low.

The best way to look at gravity when digging in the Earth is to do it ilke this. Let's say you are 1000m beneath the earth, the pull of gravity on you now is:

F_g = \frac{GM_{1} M_{2}}{(r - 1000m)^2}

I am assuming that the loss of mass from that 1000m less is neglegable but you can think of it this way. So the deeper you go down, is like decreasing the radius of your planet, so if you are 10m from the centre, the force of gravity on you would be the size of a force you would experience on a 10m radius planet, or barely nothing. If you don't understand, ask away?

 

[Hurkyl]

When you're deep under the crust, it's not the gravity you need to worry about, it's the weight of all of the stuff above you.

 

[Cyrus]

Well actually, no you won't. Because think about a rock that is 10m away from the centre, it too has an environment of extremely low gravity on it, sonce there is tons of Earth pulling it in all directions. This is why the centre of the Earth is composed of a very fluid (non dence) material, because the gravity is so low.

The best way to look at gravity when digging in the Earth is to do it ilke this. Let's say you are 1000m beneath the earth, the pull of gravity on you now is:

F_g = \frac{GM_{1} M_{2}}{(r - 1000m)^2}

I am assuming that the loss of mass from that 1000m less is neglegable but you can think of it this way. So the deeper you go down, is like decreasing the radius of your planet, so if you are 10m from the centre, the force of gravity on you would be the size of a force you would experience on a 10m radius planet, or barely nothing. If you don't understand, ask away?

How would you not experience the weight of all the Earth above you. You would not feel its gravity, but you would feel its weight. If i bury you alive, you surely will feel the weight of all that soil around you. In fact, you would hardly be able to dig your way out. Just like a person in an avalanche is unable to dig himself out of a couple feet of snow. I thought the reason for the molten core is the extreeme pressure of all the earth. All that preassure equals tons and tons of heat. Heat which can thereby liqify the core, and thus the magma. Its just like hydrostatic pressure, it will increase with depth. As you go deeper, all the weight of the water above you will pile up. At the core of the earth, you have the weight not only pushing you down, but side ways too, trying to swash you in all directions, as opposed to just down when your relativly deep in the earth. Here is a neat proof of what I am talking about.
http://burro.astr.cwru.edu/Academics/Astr221/SolarSys/hydrostat.html as you can see, the pressure at the center of the Earth is clearly not zero, but very very very great. You would be hard pressed to disprove me

 

[Nenad]

read my post again, and notice the key words I am talking about 'gravity' and NOT 'pressure'. And you can't compare something like an avalanche or being burried in the Earth's crust to the centre of the earth. If you think about it, if you are 5m from the cnetre, you will not feel great weight of soil because that soil is being pulled in all directions, just like you are, cancelling out gravity, and therefore cancelling out pressure. The centre of the Earth is way different that the ouside shell.

 

[Tide]

Near the center of the Earth the gradient of the pressure will tend toward zero because the gravitational force tends toward zero. That does not mean, however, that the pressure is zero! In fact, the pressure will be proportional to the mass of the entire planet.

 

[Brad Barker]

wouldn't the gravity tend towards infinity?

 

[elas]

Surely you are overlooking the major difference between Newton's gravity and Einsteins gravity, the last time I read anything on this difference it was that the force direction was unknown and still subject to debate.

 

[Hurkyl]

Consider this, Nenad:

Get two equally strong friends, have one stand in front of you, and one stand behind you. On your mark, have them both start pushing you with all their might.

The net force you feel will be zero, since you're being pushed with equal force from in front and behind... but that doesn't stop you from being crushed in the process.

 

[Hurkyl]

wouldn't the gravity tend towards infinity?

No. Inside a uniformly dense spherical object, gravity dies off linearly... the gravity you feel is directly proportional to your distance from the center.

 

[Doc Al]

Just to add to Hurkyl's comments: The pressure on you at the center of the Earth is not due to the gravitational attraction between you and the earth--that tends to zero. The crushing pressure is due to the gravitational attraction of the Earth with itself--the force holding the Earth together. You are just crushed in the middle.

 

[GOD__AM]

Consider this, Nenad:

Get two equally strong friends, have one stand in front of you, and one stand behind you. On your mark, have them both start pushing you with all their might.

The net force you feel will be zero, since you're being pushed with equal force from in front and behind... but that doesn't stop you from being crushed in the process.

The differnece between this anology and the reality of being burried in the Earth is that the pressure is not applied evenly in your example. When pressure is exerted evenly, such as when imersed in water the human body can take a lot of crushing force without noticing at all. Granted the pressure and tempature at the center of the Earth would pulverize us.

 

[Cyrus]

read my post again, and notice the key words I am talking about 'gravity' and NOT 'pressure'. And you can't compare something like an avalanche or being burried in the Earth's crust to the centre of the earth. If you think about it, if you are 5m from the cnetre, you will not feel great weight of soil because that soil is being pulled in all directions, just like you are, cancelling out gravity, and therefore cancelling out pressure. The centre of the Earth is way different that the ouside shell.

If I bury you under 5 feet of soil, you will feel its weight, I assure you. In fact, you will feel the integral of the gravity as it starts from 9.81 m/s/s at the surface time that point mass, all the way down until you reach the center. So you will infact feel the sum of all these gravitational forces added together and you will feel a lot of pressure and force. You just won't feel any gravitational force. But I think we are debating the same point of view.

 

[mathman]

When pressure is exerted evenly, such as when imersed in water the human body can take a lot of crushing force without noticing at all.

Not true. If a submarine dives too deeply it will be crushed.

 

[GOD__AM]

Not true. If a submarine dives too deeply it will be crushed.

Read what I typed. "The human body" not a submarine is what I said. The human body is made of mostly water, as most of us know, and as a result is largely uncompressable. The submarine would never crush if the air pressure inside was equal to the water pressure outside, which is how divers breathe underwater.

 

[chroot]

read my post again, and notice the key words I am talking about 'gravity' and NOT 'pressure'. And you can't compare something like an avalanche or being burried in the Earth's crust to the centre of the earth. If you think about it, if you are 5m from the cnetre, you will not feel great weight of soil because that soil is being pulled in all directions, just like you are, cancelling out gravity, and therefore cancelling out pressure. The centre of the Earth is way different that the ouside shell.

This is pretty much entirely false. The condition of hydrostatic equilibrium requires that the pressure at the center of the Earth be higher than at the surface. In fact, the pressure is very intense there. Please refrain from making comments about physics you have yet to learn properly.

- Warren

 

[Cyrus]

Hey he was only trying to help warren. He did infact help me on another problem, so its cool if he made a simple mistake. At least we will both learn in the process, and that is the whole point of this forum anyways.

 

[geometer]

The differnece between this anology and the reality of being burried in the Earth is that the pressure is not applied evenly in your example. When pressure is exerted evenly, such as when imersed in water the human body can take a lot of crushing force without noticing at all. Granted the pressure and tempature at the center of the Earth would pulverize us.

It's not whether the pressure is applied evenly, it's actually not the pressure itself, it's the pressure differential between the inside of your body and the outside, and the material strength of your body. The reason a submarine gets crushed at a certain depth is that the pressure inside the boat is about atmospheric and the pressure outside is thousands of pounds per square inch. If the pressure inside the boat were always the same inside as outside, it would never be crushed.

 

[GOD__AM]

It's not whether the pressure is applied evenly, it's actually not the pressure itself, it's the pressure differential between the inside of your body and the outside, and the material strength of your body.

Ok, an example; The Navy conducts saturation dives at depths over 2000 feet in the ocean. This is equal to about 1000 psi of pressure (as opposed to the 14.7 psi we are exposed to typically). Applied evenly btw, and the divers surely aren't crushed or even aware of any extra pressure they feel. Take the same person, lie them down on the Earth and put a 1000 lb concrete slab on them. What happens next is the person splits at the sides where the pressure is uneven.

The reason a submarine gets crushed at a certain depth is that the pressure inside the boat is about atmospheric and the pressure outside is thousands of pounds per square inch. If the pressure inside the boat were always the same inside as outside, it would never be crushed.

This is pretty much what I said in my second post and I agree. There should be a point where bones and other non-liquid parts of the body will succumb to the pressure, but I'm not sure how much it would take.

 

[Nenad]

This is pretty much entirely false. The condition of hydrostatic equilibrium requires that the pressure at the center of the Earth be higher than at the surface. In fact, the pressure is very intense there. Please refrain from making comments about physics you have yet to learn properly.

- Warren

I don't know it you have fun proving 17year old kids wrong, but a mistake is a mistake. My vector addition was wrong, and I missed something. I hope you feel satisfied. Later.

 

[geometer]

Ok, an example; The Navy conducts saturation dives at depths over 2000 feet in the ocean. This is equal to about 1000 psi of pressure (as opposed to the 14.7 psi we are exposed to typically). Applied evenly btw, and the divers surely aren't crushed or even aware of any extra pressure they feel. Take the same person, lie them down on the Earth and put a 1000 lb concrete slab on them. What happens next is the person splits at the sides where the pressure is uneven.

Wow - is this true?! I spent 13 years in the Navy, and I don't remember any dives this deep!

 

[geometer]

Ok, an example; The Navy conducts saturation dives at depths over 2000 feet in the ocean. This is equal to about 1000 psi of pressure (as opposed to the 14.7 psi we are exposed to typically). Applied evenly btw, and the divers surely aren't crushed or even aware of any extra pressure they feel. Take the same person, lie them down on the Earth and put a 1000 lb concrete slab on them. What happens next is the person splits at the sides where the pressure is uneven.

After doing some quick reading on saturation diving, I find it is true! To do this, the divers raise the partial pressure of gases in their system to that of the depth they are operating at - in other words, they reduce the differential pressure between their insides and their outsides to zero.

 

[Cyrus]

I find this hard to belive. I think 1000psi inside the cabin of a sub would be terrific forces in all directions. I would think a persons ears would pop not to mention their body being crushed like a tin can. Its true the pressure in and out of the sub are about equal, but to a person inside the sub, isint he going to feel 1000psi pressure relative to inside the cabin? It would seem only insturments could servive that pressure, and big bulky ones at that.

 

[GOD__AM]

I find this hard to belive. I think 1000psi inside the cabin of a sub would be terrific forces in all directions. I would think a persons ears would pop not to mention their body being crushed like a tin can. Its true the pressure in and out of the sub are about equal, but to a person inside the sub, isint he going to feel 1000psi pressure relative to inside the cabin? It would seem only insturments could servive that pressure, and big bulky ones at that.

First off a submarine is not pressurized any different than on the surface, 14.7 psi. That is why they have to be so strong, to hold out the pressure.

Secondly in diving compressed gas is used and supplied through a regulator that supplies air in relative density to the water pressure around it(typical hi pressure scuba tanks hold about 3500psi or more). A diver then has to force air into his inner ear to "equalize" the pressure as he descends deeper. This is done for most by pinching the nose with the mouth closed and forcing the pressure into the ear canal. During ascent the equalizing occurs naturally as long as the ascent rate isn't too fast.

On a final note you say the people in the sub should feel the pressure (if it were pressurized). Well hypobaric chambers used in medacine and treatment and study of the effects of pressure on the body do just this. They actually pump compressed air into the chamber with the people in it. Other than some slight heating during the compression and the necessity to "equalize" the inner ear, there is no sensation of being compressed.

 

[geometer]

I find this hard to belive. I think 1000psi inside the cabin of a sub would be terrific forces in all directions. I would think a persons ears would pop not to mention their body being crushed like a tin can. Its true the pressure in and out of the sub are about equal, but to a person inside the sub, isint he going to feel 1000psi pressure relative to inside the cabin? It would seem only insturments could servive that pressure, and big bulky ones at that.

You don't feel pressure, you feel differential pressure. If the pressure of the gases inside your body is 1000 psi, and the pressure outside is 1000 psi, you will feel nothing. This is the trick in saturation diving - take the time to slowly and carefully equalize your "internal" pressure with the external pressure, and it would be just like you were sitting in your living room watching TV.

 

[Cyrus]

Hmm weard. Can you acutually go to 1000psi though? That seems wayyyyy wayyy too hight :-) I would think even 100psi on the body would be pusshing it. Thats nearly 1470 lbs per square inch on a human body. ouch.

 

[GOD__AM]

Hmm weard. Can you acutually go to 1000psi though? That seems wayyyyy wayyy too hight :-) I would think even 100psi on the body would be pusshing it. Thats nearly 1470 lbs per square inch on a human body. ouch.

Yes, as I said in a post you quoted, the navy has done dives at these high pressures. You seem to be missing the fact that the human body is mostly water and is uncompressable. Only the air spaces in our body will compress. Oh and your math is flawed. 100 psi on the body = 100 lbs per square inch not 1470.

Just consider this if you are having trouble with the idea. Animals live very deep in the ocean, possibly even in the deepest trenches there is life. Whales dive very deep, and they don't even have the benifit of breathing compressed gas to offset the pressure. So effectively the air spaces in their bodys are crushed proportional to the pressure of the water acting on them, yet they suffer no ill effects.

One more thing that may help you. If I take a plastic bag and fill it with water, then throw it off a boat into the deepest part of the ocean, as the bag decends into more and more pressure it still occupys the same amount of space or volume as it did on the surface. There is no compression.

 

[geometer]

Yes, as I said in a post you quoted, the navy has done dives at these high pressures. You seem to be missing the fact that the human body is mostly water and is uncompressable. Only the air spaces in our body will compress.

I think this is a little misleading. The water in the human body is incompressable, but the human body itself is eminently compressable. The water in our bodies is contained within cell walls which certainly can rupture and leak all this water out. As an example (rather gruesome I admit), think of what happens to an animal that has been repeatedly run over on the road; it gets very flat! As kids we used to refer these as "sail cats, or sail whatever animal." You could pry them up and use them like frisbees.

Theoretically, I suppose you could exist under any pressure, if there was no differential across your body. I am sure there are physiological and structural limits to this, but the trick behind saturation diving is to raise the partial pressures of the gases in the human body to the pressure of the depth you will be diving at - 0 delta P means 0 problems.

 

[Cyrus]

My mistake God_AM, I meant to say 100atm not 100psi. 100atm would be 1470 psi, my fault. I made a NASA units conversion mistake. ;-P

 

[GOD__AM]

The water in our bodies is contained within cell walls which certainly can rupture and leak all this water out.

So these cells rupture and spill out into what? For them to "rupture" and leak out there would have to be an area of much lower pressure some where near the cell. I thought we already agreed that we are talking about pressure applied evenly in all directions.

Theoretically, I suppose you could exist under any pressure, if there was no differential across your body. I am sure there are physiological and structural limits to this,

You can surely feel differentials in pressure across your body without being indured. I can poke you in the arm and create a differential, obviously the differential has to be quite large, or concentrated on a very small area.

but the trick behind saturation diving is to raise the partial pressures of the gases in the human body to the pressure of the depth you will be diving at - 0 delta P means 0 problems.

It's no trick...its a result of breathing gas in relative pressure to your enviornment. Saturation just implys that the pressure has had enough time to equalize all the gas compartments in the body. Typically saturation dives are conducted out of an underwater habitat that has a gas enviornment at the same pressure and mixture (helium/oxygen blends for real deep dives) that the divers will breathe when in the water. After a dive like this divers have to decompress for days as the disolved gas comes out of solution.

Further more I don't know what the limits are for how much pressure the body can take, but it is a great deal more than you seem willing to believe. Maybe some one can help and tell us where the "weak link" in the body is that will succumb to the pressure and render us leaking bags of cellular water. Maybe there is a point where the pressure turns us into ice?

 

[geometer]

So these cells rupture and spill out into what? For them to "rupture" and leak out there would have to be an area of much lower pressure some where near the cell. I thought we already agreed that we are talking about pressure applied evenly in all directions.

This happens all the time. Some snake venoms rupture muscle cells and circulatory system walls, and the fluid leaks out between the cells. A pressure applied evenly in all directions won't help you if the differential between your internal and external pressure is too large - you will get hurt.


You can surely feel differentials in pressure across your body without being indured. I can poke you in the arm and create a differential, obviously the differential has to be quite large, or concentrated on a very small area.

I agree.



It's no trick...its a result of breathing gas in relative pressure to your enviornment. Saturation just implys that the pressure has had enough time to equalize all the gas compartments in the body. Typically saturation dives are conducted out of an underwater habitat that has a gas enviornment at the same pressure and mixture (helium/oxygen blends for real deep dives) that the divers will breathe when in the water. After a dive like this divers have to decompress for days as the disolved gas comes out of solution.

Further more I don't know what the limits are for how much pressure the body can take, but it is a great deal more than you seem willing to believe. Maybe some one can help and tell us where the "weak link" in the body is that will succumb to the pressure and render us leaking bags of cellular water. Maybe there is a point where the pressure turns us into ice?

Actually, I believe that if the external to internal difference is small enough, the body could theoretically take any pressure. I suspect however there is some practical limit, but it's probably really high.

 

[geometer]

oops - sorry - I goofed up the quoting - some of my response is in the quoted section!

Ah well!

 

[conkermaniac]

I had read in a book that gravity decreases as you approach the center of the Earth because you feel only the Earth under you. What's all this about being pulled by gravity in all directions?

 

[Cyrus]

A consequence of Newtons law of gravitation conker

 

[HallsofIvy]

I had read in a book that gravity decreases as you approach the center of the Earth because you feel only the Earth under you. What's all this about being pulled by gravity in all directions?

The gravity form the Earth below you as well as the Earth on the "other side" of the center pulls you down but the Earth above you pulls you up. It can be shown that the pull of the Earth above you exactly (assuming a uniform earth) cancels the pull of the Earth on the other side of the center so the only pull is that of the Earth still "below" you.

 

[Cyrus]

I have another good question, (at least I find it puzzling). I agree that the pull of the Earth on either side of you will be equal and opposite so you won't go anywhere. But to a rigid body, it WILL feel too forces trying to pull it appart internally. But at the same time, the body will also feel the force pushing inwards due to the pressure of all the material above it. Do you think these two forces would be exactly equal in magnitue and opposite in direction, and so you really would feel weightless, despite all the hydrostatic pressures? You wouldent be crushed due to the weight maybe.

 

[Doc Al]

I agree that the pull of the Earth on either side of you will be equal and opposite so you won't go anywhere.

The gravitational force on a body at the Earth's center will be zero.

But to a rigid body, it WILL feel too forces trying to pull it appart internally.

What forces are you talking about?

But at the same time, the body will also feel the force pushing inwards due to the pressure of all the material above it.

As this discussed throughout this thread.

Do you think these two forces would be exactly equal in magnitue and opposite in direction, and so you really would feel weightless, despite all the hydrostatic pressures? You wouldent be crushed due to the weight maybe.

I don't know what you mean by two forces. A body will feel the pressure of the Earth pushing inward, and the body's structure will provide the reaction force pushing back on the earth.

 

[Cyrus]

Well, there will be two forces due to gravity won't there. You will have the gravitational force of all the mass above you pulling you upwards, and you will also have a gravitational force of all the mass below you pulling you downwards. The sum of these two forces add to zero. But for a body at the center, it will not be accelerated in any direction since the net force is zero, but that does not mean that it won't feel a tug on each side where the force is present. I am referring to someones post where two people tug on your arms. The net force is zero, but both of your arms sure do feel a force in either direction. By analogy, the force tugging on your arms could be the force of gravity above and below you. You don't go anywhere, because the net force is zero, but won't you feel it on both sides.

 

[Doc Al]

Well, there will be two forces due to gravity won't there.

No, there will be NO force due to gravity.

You will have the gravitational force of all the mass above you pulling you upwards, and you will also have a gravitational force of all the mass below you pulling you downwards. The sum of these two forces add to zero. But for a body at the center, it will not be accelerated in any direction since the net force is zero, but that does not mean that it won't feel a tug on each side where the force is present.

It's not just that the net gravitational force on a body will be zero, it's that the net force on every tiny piece of that body is zero. There is no "tug", there is no gravitational force. Another way to say it: there is no gravitational field at the Earth's center, so no gravitational force.

I am referring to someones post where two people tug on your arms. The net force is zero, but both of your arms sure do feel a force in either direction. By analogy, the force tugging on your arms could be the force of gravity above and below you. You don't go anywhere, because the net force is zero, but won't you feel it on both sides.

A bad analogy. There is no gravitational pull tugging you at all.

What post are you referring to?

 

[Cyrus]

someone earlier said something about that in tab 1 of this post. You should see it there.

 

[Doc Al]

Perhaps you're thinking of Hurkyl's analogy in post #10? If so, he's saying something quite different.

 

[GOD__AM]

I imagine the specific gravity of liquid nickel is very high (8.902?). If you were to make it to the center of the earth, you would likely be forced towards the upper layers of the liquid mantle.

Edit: Solid inner core? ...my bad sorry.

 

[Cyrus]

hey doc al, maybe I am just interperting something incorrectly then. I thought that Newtons law states that F = \frac{GmM} {R^2}. Let's do a simple case. Let's say I am at the center of the earth, and there is just a column of Earth above my head and below my feet that all has the same uniform density. The weight that is trying to push down on my head will equal the weigh trying to squash me from me feet, I think we both agree on that point. And that force would just equal the sum of the mass at its distance, times the gravity at that distance, added together over the distance. That would be the force pushing me down from my head, likewise there would be that force equal in magniude present trying to push me from my feet as well.

Now for the gravity. I understand that the NET gravitational force I will feel will be any mass that is below me and is unacounted for above me. What i mean is that if i don't stand directly in the middle of that column of earth, there will be more Earth one one side of me ( either above or below). As a result, there will not be a proper balance of attraction between myself and all the particles of earth, so there will be a net force in the direction where there is the larger amount of earth, and I will feel like I want to fall in that direction.

If I were in the center, there would be equal mass and distance both above and below me, so I would not feel any net gravitational force. You said it does not have to do with net gravitational force, but I do not see how you can avoid it. I thought it is TOTALY dependent on having that net force cancel out. For every particle trying to pull me towards it above my head, there is the same amount of force trying to pull me towards the column of Earth below my feet, and for the reason, the NET force on me is zero, and I go nowhere. But that does not necesairly mean that i would not feel myself trying to be pulled both up and down does it?


Hmm, now after all this typing I kind of see what you mean .haahhaaha. I will post it anyways. (SIGHHHHHHHHHHH) . If every particle on my body is feeling as thought it is being pulled both up and down, then as a whole, each particle in my body relative to each other, don't want to go anywhere.

I think what I was thinking about would be a gradient across my body. Because I take up space, I could not really be a point masss at the center, therefore, Techincally speaking, I should feel some small component of force from my head down to my toes. The thing was that I was considering the entire space I occupied as the center. In reality the center is a point, so a rigid body like me can't be the center, by definition, some parts of my body will be off center, by 3 feet or so on each side, I am a tall guy :-). So the parts of my body that are off center should feel some force due to the unbalance of force above and below it, but were talking about maybe 3 feet difference at most, I would think this to be a very very very very small force difference, and in affect, so small I would still think that my entire body is weightless. Thanks Doc Al you were right. I did not think about the situation hard enough.


As a side note, Shouldent that single particle feel a force on each side trying to pull it appart like the anlogy I made earlier? Would this force be strong enough to rip electrons off of that point mass, and be a reason why the center of the Earth would have a liquid plasma? Or am I still looking at the situation incorrectly.

Edit: I just made the same exact mistake to this situation. Each electron relative to the atom will not experience a great force tugging at it, thus this cannot be the reason for the plasma. The pressure has to be the only result of the plasma. great pressure = great temperature = energy to rip electrons from their orbits and turn stuff into a plasma state.

 

[LURCH]

Perhaps the problem here is that all points within your body cannot be in the exact center of thwe Earth; some will be above it , and some below. Is that the heart of your question?

 

[Tide]

I think Cyrus is trying to describe tidal forces! :-)

They will be extremely weak however as the tidal force varies as the gradient of the gravitational force.

 

[Cyrus]

what is a tidal force? Its 12:55 am, I am studying general chemistry. I just went to 7-11 and bought a sobe energy drink and now I am wired and good to go for another two hours. Tommorow morning is going to suck! ;-P Anyhoo, I would think that if i were off center from the center of the Earth by, say one atom, then the only force due to gravity would be the equivelant of the force between me and that atom and our distance squared, since all the other mass' gravitaional affects can be ignored, since they have a counterpart on the other side of me. So it really would be a weak force even if i am off center by just a tiny amount no? I made this into a hot thread, 600 views yes!

 

[Tide]

If gravity is pulling more strongly on one part of an object than on another part of the object then the object is effectively being pulled apart due to the differential. That is the tidal force.

 

[Cyrus]

I see. Makes sense to me. :-)

 

[Doc Al]

Perhaps the problem here is that all points within your body cannot be in the exact center of thwe Earth; some will be above it , and some below. Is that the heart of your question?

Even so, imagine a small sphere centered at the Earth's center. Assuming the usual symmetry, within that sphere there will be NO gravitational field due to the Earth's mass outside the sphere. So the only gravity pulling on a body within that (imaginary) sphere will be due to the trivial mass within the sphere itself.