Physics / gravity question

[Alex Franke]

Say you're in a room at the dead center of solid iron & rock sphere some 8,000 miles in diameter, which is floating in space. My guess is that you'd be floating and feeling pretty bloated, being pulled in every direction by about 1/2-g. But a good friend seems to think that you wouldn't be bloated because you're at the center of gravity.

Any informed opinions out there? (not that I expect this will happen anytime soon, but it will be geed to know how I'd feel if it ever does...)

[Russianwolf]

since the mass outside of where you are should have some effect (think about the effect the moon has). The effect wouldn't be able to move you since it is counteracted by the mass in the opposite sirection, but it may stretch you a bit. If that's what you mean by bloated, then I agree.

[Ken Massingale]

I have no idea, Alex. But I do think you better get to the shop and get your mind on something else!

Sometimes you guys make my head hurt. ;-)

[Ed62]

The last time I did that, I didn't feel bloated.

Ed

[cgallery]

I'm afraid I agree with your friend.

Gravitation pull inside a sphere should be zero. Everywhere inside the sphere. Assuming uniform wall thickness.

[jseklund]

Preface- I'm not great at physics, but I like to watch movies on the subject

The answer would depend on how thick the wall of iron is that you are inside and how close it is to you. Every piece of matter exerts gravity on every other piece of matter. The force of gravity is also inversely proportional to the distance between the two objects. So if it's a 4,000 mile radius from you to the edge of the wall vs. a 10 foot radius from you to the edge of a 4,000 mile thick wall will make a HUGE difference.

In theory, I guess, you will be pulled equally in all directions- so the amount of "bloatedness" as you put it, would depend greatly on the mass pulling in each direction and your proximity to it.

However, gravitational force is much weaker (to a ridiculous degree) than the other forces in nature that we know of - electromagnetism and the strong/weak nuclear forces. If there was enough matter in the wall, and you were close enough to it- then it could rip you apart, but at this point- the wall would probably collapse and crush you with it unless there was another force keeping the center empty (anti-gravity?). With an 4,000 mile distance between you and that wall though- there'd have to be a lot of matter to rip apart an electromagnetic bond.

So, my disjointed answer is- I would agree with you to a point, but in reality your friend is right (if for the wrong reasons maybe)- you would either feel nothing or close to nothing, or you would be crushed because the wall would collapse in on itself.

Glad I could clear that up for you

[Stytooner]

If there is nothing else in the iron sphere, then you too would be one with the wall. Evidently the iron was so hot that it absorbed it's core material into itself.
Now if you had a Star ship and get beamed to the central core, I imagine it would be like floating in space. You are outside of any gravitational pull by being on the inside.
Also, let me know which phone service you get reception on while in there.

[cwsmith]

I'm not sure, but when such things are in doubt, I think I'd play it safe and change your vacation plans to one of the more popular spots.


CWS

[Alex Franke]

I agree it wouldn't be the most delightful place to visit -- the pressure and heat would be absolutely terrible. But suppose you were teleported into a 10x10x10 room at the center of the planet -- and somehow that room protected you from the pressure and heat so that your surroundings did not implode in on you -- and you didn't have to worry about the cohesiveness of your subatomic particles.

It seems to me that the mass around you would be pulling you out in every direction -- that essentially you'd be floating in the middle of the room with your center of gravity aligned with the planet's center of gravity, being pulled in every direction at once.

Stytooner -- I wonder if that "can you hear me now" guy has been down there with his "network"... if so I can just ask him...

[Anna]

Say you're in a room at the dead center of solid iron & rock sphere some 8,000 miles in diameter, which is floating in space. My guess is that you'd be floating and feeling pretty bloated, being pulled in every direction by about 1/2-g. But a good friend seems to think that you wouldn't be bloated because you're at the center of gravity.

Any informed opinions out there? (not that I expect this will happen anytime soon, but it will be geed to know how I'd feel if it ever does...)

Sorry, can't help butting in:

Gravity is a force, force is a vector. If you're being pulled to the left by a force F and at the same time being pulled to the right with an equal value of force F, you get a net force of zero.

You can look at the situation inside the sphere as being pulled by many forces in all directions, and this is true whether you are in the center of the sphere or anywhere else inside the sphere. Gravitational force is proportional to the mass of the material exerting the force, inversely proportional to the square of the distance. Sum it up in all directions (and you can rigorously prove it with integration if you really want to) and you get zero net force, not .5g.

By the way, the term "center of gravity" probably means differently than what your friend intended, I think.

Added: Made a mistake. If you're off-center, then there will be a smaller gravitational force by a sphere with a radius from your position to the center of the sphere.

[crokett]

You wouldn't feel anything, assuming you could survive the pressure and heat. The gravitational pull would be exactly equal in all directions.

[gerti]

0.5g-0.5g=0g

[LCHIEN]

I believe, regardless of the phsyical size of the object, the classical physics way of handling two masses and their effects on each other is to treat each as a point mass located at the center of gravity of each object with a mass equal to the total mass of the object.

Therefore being located at the exact enter of an object of uniform density
you will be located at the center of gravity as well.

This all sort of depends on the objects being solid, in actuality ojects like the earth for example are very much like a liquid plastic in that the heavy stuff moves to the center and gravity tends to distribute everything else in a sphere.

back to the original problem statement, the mass of the sphere would be huge, somewhat like the earth in total mass, the attraction between two objects is inversely proportional to the distance between centers of mass
thus you would be pulled very strongly to the center point even if you moved an inch to either side of center.

[Russianwolf]

I believe, regardless of the phsyical size of the object, the classical physics way of handling two masses and their effects on each other is to treat each as a point mass located at the center of gravity of each object with a mass equal to the total mass of the object.

Therefore being located at the exact enter of an object of uniform density
you will be located at the center of gravity as well.

This all sort of depends on the objects being solid, in actuality ojects like the earth for example are very much like a liquid plastic in that the heavy stuff moves to the center and gravity tends to distribute everything else in a sphere.

back to the original problem statement, the mass of the sphere would be huge, somewhat like the earth in total mass, the attraction between two objects is inversely proportional to the distance between centers of mass
thus you would be pulled very strongly to the center point even if you moved an inch to either side of center.

assuming the mass is liquid, then gravity would pull you until you were at the center of mass. Once there, gravity would be equal on all sides so you would stop moving through the liquid. Or at the very least move in small amounts as gravity tries to maintain balance on a non-spherical object.

But the effects on a human could be something else. AS the moon rotates around the earth, it has an effect on the human body. When it is "directly" overhead it pulls on our bodies and we are "taller", while when it is on the opposit side of the planet the gravity is cumulative and we are at our "shortest". While we are talking about miniscule amounts between the two the effects are none the less real. Even though we don't really feel this effect.

So in our sphere, with mass being equal on all sides, we would be being pulled in all directions which could stretch our bodies since we are not a solid form. Just like the moon does. This could be termed "bloated"

[cgallery]

Here is the math:

http://www.grc.nasa.gov/WWW/K-12/Num...ng/grvtysp.htm