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  1. A Gravity Question

    #95012012-01-29 20:08:16 *Fieyr said:

    I need the help of a physics major...

    A friend and I were having a discussion about the nature of gravity and we were unable to agree on how exactly gravity works.

    The scenario is this: Let's say you were able to go to the center of the earth.... encased in a hollow sphere with the same atmospheric pressure as on the surface of the Earth... so you could breathe easily.

    My friend looks at gravity as a force that would be pushing in from all sides. In other words, the person in the sphere would be feeling pressure from all sides and therefore it would be somewhat difficult to move.

    My theory is that gravity is greater depending on where the mass is relative to you. For example... when you sit in chair.... you have the entire mass of the Earth pulling you down. However, if you were at the center of the Earth, you would only have half the mass of the Earth, in any direction, affecting you. And beyond that... instead of it pushing inwards... you would actually be pulled outward in all directions by 1/2 the amount of gravity you would experience on the surface of the Earth.

    What do you all think?

  2. #95092012-01-29 20:37:16VivoDePyre said:

    While many people theorize about what happens in the center of the earth, one can at least assume that you are being pulled, not pushed. In theory, if you were in the exact center of the earth, a large majority of the forces would balance. Mass isn't equally distributed around earth, so one side may pull more than another. However, gravity is (by most reasonable explanations) a force in which a small mass is pulled to a greater mass. Rather than thinking of some force pushing you from above, it's a force pulling you down from below. When you reach the center, the mass is all around you, 360. At that point, you would be pulled in every direction at once.

  3. #95172012-01-29 20:58:35halcy said:


    I'm no physics major, but:

    In the center of the earth, the mass around you will pull you equally in both directons, which means they cancel out - you'll effectively be weightless.

    This assumes that the earth is spherically symmetric, which it roughly is, so that's probably fine.

    Fractional distances, I'm not sure how the gravity would be - the earth has different layers with different densities, so it's probably not a linear decrease from 9 point something m/s acceleration to 0 (which would be the case if it was uniform, I am pretty sure), but something that looks relatively wacky. It'll be decreasing as you get closer to the center though, that I am pretty sure of.

  4. #95272012-01-29 22:07:34Noodle said:

    That gravity would pull you, rather that push seems more legit to me. The question I have is; would that force rip you apart or would it leave you weightless?

  5. #95342012-01-29 23:08:05Fieyr said:

    I agree with what has been said so far. Thanks for the input.

    @Noodle - Well considering you're at the center of the planet... it should be half the gravity that you would experience standing on the surface of Earth.

    So assuming a person weighs 140 pounds... it would be 70 pounds of pull in all directions. Imagine if you were hanging from your finger tips from a ledge... it wouldn't' be comfortable, but it wouldn't kill you.


    Not all parts of your body are stationary... more specifically... blood. I imagine that your blood would be pulled to the outer extremities of your body. I'm not sure if that gravitational force would be able to overcome the pressure of your circulatory system though... I doubt it since we are able to both stand normally as well as upside down without dying. However, if blood is actually pulled away from your internal organs, then it could kill you.

  6. #95432012-01-30 00:10:06 *TokoyamiSenshi said:

    You can't imagine what a force really is from a newtonian perspective(well you can but it's not as close to the real world as this). Instead of imagining forces acting upon you like some poles trying to pull or push you around, a better description is imaginig all mass and energy as little balls on a "potential blanket". Naturally in the analogy, the balls will always want to roll downwards. It's what people like to call fields. Fields are nothing but potential blankets... 3D blankets. But imagining it all on a 2D plane is just as effective.

    In that model, every 'force' is just a consequence of a field, just a natural tendency of energy to roll down the potential blanket. So if the field is a hill, it will make the surrounding energy roll away and vice versa.

    Note though that this is a trap. This analogy fails to integrate electromagnetic fields and gravity(how can a ball be on two different blankets at the same time?!), and the description gets weird if you consider positive and negative charge(they roll in opposite directions on the same blanket).

    Anyway, applying this approach here, the earth creates a slope in the potential blanket, forcing the surrounding energy to tend to roll in. If you could just let yourself roll down to the bottom of the potential pit(analogous to going to the center of the earth), well you would have nowhere else to roll to, and the effect of the field would be minimal.

    A further explanation as to why you wouldn't be ripped apart is amazingly simple. In order to rip something apart, you need to put an object in a situation where one part of it has a much greater tendency to move than the other one(that's intuitive, right? since you want them to go away from each other), but the potential field created by a large object such as the earth changes so slowly that you'd have to be ridiculously big to have sufficient difference in potential to even feel it.

    Actually, there is a situation where gravity rips things apart. If you have never heard of Roche limits, and you're into this kinda stuff, I suggest you read up on it a little. Basically, planets are large enough to feel each other's potential difference so they can pull each other apart.

    A lot of text again ^^

  7. #95522012-01-30 01:09:42Fieyr said:

    For those who aren't familiar with the blanket metaphor that @TokoyamiSenshi is using. You can start the video around 3:40 and get an idea of what he means. The 'blanket' is the grid that the planets/sun are rolling around on.

    Someone needs to come up with a better metaphor for this though. A blanket isn't really a good way to describe 3D space...

    I've met too many people that see black-holes as a 2D hole in space, when in reality it's more like space is being pinched from all directions. I blame the blanket metaphor for this confusion.

  8. #95842012-01-30 06:39:44Nomi said:

    I think that gravity is something that man made up. They told us gravity is what keeps us onto the earth. So you listen to them and you start to believe that if you roll over your bed you'd fall off it on to the floor ... BUT ... what if gravity was never "discovered"? Would you still fall? Would we have been able to fly if no one told us that something much stronger than ourselves was pulling us to earth ... :P

  9. #96082012-01-30 09:49:34 *TokoyamiSenshi said:

    @Nomi: Who knows, maybe that's how it works. But since we all heard of it already and "believe" it, no point wondering about it now. After all, we can't just forget it.

    There is some validity in that though, as all physical models(which we believe) are just assumptions based on incomplete images of the universe, but you see, just because it made sense many years ago that everything revolves around earth doesn't mean it really did. Also, even if our perception of gravity is wrong, it doesn't mean that real gravity still won't smack your head to the floor if you let it.

  10. #96482012-01-30 14:29:51Fieyr said:

    @TokoyamiSenshi @Gargron

    Well if we agree that the definition of sound is the act of an ear translating the vibrations into a sensation, then if no one is around (therefore no ears) then no it doesn't make a sound. It does make a vibration though.

    @Nomi - Just like we were simply balls of energy before someone discovered atoms? I miss those days... floating around and junk...

  11. #96552012-01-30 15:46:27TokoyamiSenshi said:

    @Fieyr: But we agree that sound is a longitudinal mechanical wave, whether or not human ears can catch it.

    Following that logic, we could agree that the earth is the center of the universe(it is the center of observable universe, by definition), but that would be like sticking a stake into Math's ass and spinning it round and round. Good definitions tend to follow the rule of least entropy. If it has exceptions, it sucks.

    This is in no way "the right way" to think and define things, but it is by all means the easiest way. If you know vector spaces from mathematics, complicating is like using more than n vectors to define an n-dimensional space. Sure, you can do it, but it's redundant.

  12. #96592012-01-30 16:04:31Fieyr said:

    @TokoyamiSenshi - Haha true enough. That's why I enjoy arguing definitions and theories because every once in a while, you may discover that your definition/theory, while seemingly simple, beautiful, and entropy free is incomplete. If the great minds of that past didn't do this... I imagine we would still be at the center of the universe.

  13. #98682012-01-31 17:21:03Fieyr said:

    @Flywalker37 - Well I mean, I think it's pretty important to try to understand gravity. Otherwise how are we ever going to figure out how create anti gravity vehicles so we can just float everywhere?

    Sure, sounds crazy now... but we'll never know if all the scientists are like... "fuck it... that could never happen"