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    Pixie in the Playground
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    Sep 2016

    Default gravity and antigravity calculation question, the floating house

    I have a question about the calculation of the results of gravity in a hypothetical situation.

    The formula for the gravitational pull is: FG= G*m1*M2/r2.

    If an anti-gravity generator would exist ( of relatively small size) and I would anchor it into the ground. My question would then be. Would it be possible to place a then floating house (anchored so it wouldn't drift of place) above it, where you could live relatively normal ( sufficient gravity and right side up). Or would the super-positioning of both gravity wells not provide a point where the pushing of the antigravity well changes back into a attraction (positive force vector).

    I have some problems getting the formulas right.

    Would the equation allow for something above it to remain in place of would the antigravity well actually push things above it further away indefinitely.

    I would think that a constant anti gravity well in between the 2 masses ( earth and house) would actually cause the house to be pushed away.

    Is that correct?

    (earth)-----(anti grav well)-----(house)

  2. - Top - End - #2
    Titan in the Playground
     
    BlackDragon

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    Default Re: gravity and antigravity calculation question, the floating house

    The problem here is that anti-gravity does not exist and I'm pretty sure *cannot* exist according to the laws of physics as we know them, so it's pretty much up to you how this works. I would say that if the anti-gravity is affecting everything above it then you have a potential problem with the atmosphere draining off into space because there's nothing holding it down where your house is!

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    Troll in the Playground
    Join Date
    Jan 2007

    Default Re: gravity and antigravity calculation question, the floating house

    Assuming that this antigravity works like negative mass, it can be done

    Earth is big and the gravity field around surface is nearly constant (at least for anything the size of a house). A field generated by a small antigravity device would fade with distance just like one generated by mass, so barring any horizontal movement for simplicity an object would indeed float at a set height above the antigravity device (if it goes higher, Earth's pull takes over, if it goes lower, device's push gets stronger). Moreover, above the center of mass of the floating house things would still feel gravity in the regular direction even if diminished (this can be fixed to a degree with good geometrical design of the whole construction).

    There is a different problem though: such a construction would be inherently unstable (as far as I can tell) with respect to rotation, since you have a force acting toward the inside and attached to the edge of the house. A bit like trying to balance a pencil on a finger.
    In a war it doesn't matter who's right, only who's left.

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    Pixie in the Playground
     
    Kobold

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    Aug 2019

    Default Re: gravity and antigravity calculation question, the floating house

    Quote Originally Posted by Radar View Post
    There is a different problem though: such a construction would be inherently unstable (as far as I can tell) with respect to rotation, since you have a force acting toward the inside and attached to the edge of the house. A bit like trying to balance a pencil on a finger.
    The antigrav field could be easily engineered into shapes. If it has a bathtub shape, you dont even need to anchor the house. With respect to rotation, clever asymmetries in the mass distribution of the house and the field could give a rotational and local stability. Its like floating a ship (=house) on water (=antigrav field). Only that you probably float in your basement or walk upside down there.

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    Troll in the Playground
    Join Date
    Jan 2007

    Default Re: gravity and antigravity calculation question, the floating house

    Quote Originally Posted by Rydiro View Post
    The antigrav field could be easily engineered into shapes. If it has a bathtub shape, you dont even need to anchor the house. With respect to rotation, clever asymmetries in the mass distribution of the house and the field could give a rotational and local stability. Its like floating a ship (=house) on water (=antigrav field). Only that you probably float in your basement or walk upside down there.
    If antigravity works like antimass (that was my assumption), then no, you cannot shape the field like that. To do a bathtub-like field, it would have to fade with distance quicker then regular gravity. As for the ship analogy, it does not work. What stabilises a ship is that gravity works on mass, but buoyancy works on volume - if you shift the center of mass below the center of volume, you have stability. With antigravity you do not have that, since both forces work on mass.
    In a war it doesn't matter who's right, only who's left.

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    Pixie in the Playground
     
    Kobold

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    Default Re: gravity and antigravity calculation question, the floating house

    Quote Originally Posted by Radar View Post
    If antigravity works like antimass (that was my assumption), then no, you cannot shape the field like that. To do a bathtub-like field, it would have to fade with distance quicker then regular gravity. As for the ship analogy, it does not work. What stabilises a ship is that gravity works on mass, but buoyancy works on volume - if you shift the center of mass below the center of volume, you have stability. With antigravity you do not have that, since both forces work on mass.
    Regarding the shape, you'd need several generators distributed under the foundation with varying strength. They'd add up and create basically any regular shape.

    You are right that it is trickier with antigravity working just on the mass. But even then you only need a potential field with a local minimum with respect to the mass distribution of the house. Which is doable. My example would be a tripod resting in three indents of a potential field. Its impossible to move or rotate the tripod without leaving the optimal state.

  7. - Top - End - #7
    Troll in the Playground
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    Jan 2007

    Default Re: gravity and antigravity calculation question, the floating house

    Quote Originally Posted by Rydiro View Post
    Regarding the shape, you'd need several generators distributed under the foundation with varying strength. They'd add up and create basically any regular shape.

    You are right that it is trickier with antigravity working just on the mass. But even then you only need a potential field with a local minimum with respect to the mass distribution of the house. Which is doable. My example would be a tripod resting in three indents of a potential field. Its impossible to move or rotate the tripod without leaving the optimal state.
    Local minimum of gravity field means that in the middle there is a positive mass. If it were possible to immitate positive mass with negative one, then it would also be possible to immitate negative mass with positive one. The easiest way to see the limitation is to consider Gauss law: if there is a local minimum, then the gravity field has locally a non-zero divergence. By Gauss law it means that there needs to be a mass of appropriate sign and magnitude in the middle. So there are strict limiation on the shape of a field.

    You can always consider dynamic stabilisation, so that the antigravity generators are adjusting in real time to keep the structure in place. Or have static antigravity generators and some auxiliary thrusters and gyroscope arrays instead.
    In a war it doesn't matter who's right, only who's left.

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    Pixie in the Playground
     
    Kobold

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    Aug 2019

    Default Re: gravity and antigravity calculation question, the floating house

    Quote Originally Posted by Radar View Post
    Local minimum of gravity field means that in the middle there is a positive mass. If it were possible to immitate positive mass with negative one, then it would also be possible to immitate negative mass with positive one. The easiest way to see the limitation is to consider Gauss law: if there is a local minimum, then the gravity field has locally a non-zero divergence. By Gauss law it means that there needs to be a mass of appropriate sign and magnitude in the middle.
    I think thats only true for strict local minima and point charges. And i said minimum potential field wrt house. You test the sum of gravitational forces on the whole house, not a single point.
    Generally with antigravity you should get a surface, where the gravity and antigravity cancel out wrt point charges, thats a minimum in the potential field right there.
    I really get what you say, thats instable for points.
    The trick is to shape the surface such that a house with its size gets stuck, so to say.
    I think I cant explain it better without painting a picture.

    Edit: In real life this whole arrangement would be horribly unsafe anyway. Any storm would send the house flying and tumbling, possibly turning your house upside down.
    Last edited by Rydiro; 2019-09-21 at 10:31 AM.

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    Troll in the Playground
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    Jan 2007

    Default Re: gravity and antigravity calculation question, the floating house

    Quote Originally Posted by Rydiro View Post
    I think thats only true for strict local minima and point charges. And i said minimum potential field wrt house. You test the sum of gravitational forces on the whole house, not a single point.
    It actually makes little difference, since for stability you need to have the anchor points in actual minima.

    Quote Originally Posted by Rydiro View Post
    Generally with antigravity you should get a surface, where the gravity and antigravity cancel out wrt point charges, thats a minimum in the potential field right there.
    Only in the up-down axis, so this is a stationary point but not exactly a minimum. In the horizontal plane at best you would be able to have no forces, or at worse would have forces pushing the house away.

    I just realised though that I overestimated the problem of random tumbling - this would happen only if the house has a more vertical distribution of mass. If it is wider rather then higher, it will not flip on its own the same way as rafts or barges ramain stable. Rotation in the vertical axis can actually also be fixed to be honest.

    The remaining problem is that it could slide away from the antigravity cushion and here the nature is not as forgiving, but it is nothing active control could not handle.

    Now if the antigravity worked like the Meissner effect, then we could also count on flux pinning, which gives excellent stability. This would be neat.
    In a war it doesn't matter who's right, only who's left.

  10. - Top - End - #10
    Titan in the Playground
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    Default Re: gravity and antigravity calculation question, the floating house

    If gravity had both positive and negative masses, then it would behave like electromagnetism. It is not possible to create an electrostatic or magnetostatic arrangement that results in anything being supported stably. It is, however, possible to create a system that dynamically adjusts field strengths to hold something in an unstable position, and it is even possible to construct dynamic arrangements that will do so automatically.

    But any sort of gravity manipulation that works at all analogously to how we manipulate electromagnetism is going to be phenomenally impractical. We manipulate electromagnetism, fundamentally, by moving charges around. The analogous property for gravity would be to move masses around. But that requires planet-like masses, if you're doing it with matter of ordinary density.
    Time travels in divers paces with divers persons.
    As You Like It, III:ii:328

    Chronos's Unalliterative Skillmonkey Guide
    Current Homebrew: 5th edition psionics

  11. - Top - End - #11
    Pixie in the Playground
     
    Kobold

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    Aug 2019

    Default Re: gravity and antigravity calculation question, the floating house

    Quote Originally Posted by Chronos View Post
    It is not possible to create an electrostatic or magnetostatic arrangement that results in anything being supported stably.
    Do you have a reference to the corresponding theorem?

  12. - Top - End - #12
    Titan in the Playground
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    The Land of Cleves
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    Default Re: gravity and antigravity calculation question, the floating house

    Time travels in divers paces with divers persons.
    As You Like It, III:ii:328

    Chronos's Unalliterative Skillmonkey Guide
    Current Homebrew: 5th edition psionics

  13. - Top - End - #13
    Pixie in the Playground
     
    Kobold

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    Default Re: gravity and antigravity calculation question, the floating house

    Quote Originally Posted by Chronos View Post
    Okay, you convinced me. Even if i dont fully understand the extension to whole bodies. So the house cannot be fixed just by antigrav generators. You'd always have some wobbling or rotation invoved.
    Last edited by Rydiro; 2019-09-23 at 02:44 AM.

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