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Thread: gravity/weight of empty ball

  1. #1 gravity/weight of empty ball 
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    question i've been haunted by: If i had a 1 pound worth of carbon fiber shaped into a ball(radius=6 inch) and extracted all the air, what would the ball weigh? and why. Does the negative space of a vacuum effect gravity field above or below ball? Any feedback is greatly appreciated.


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    but if i filled it with helium it would be lighter?


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    Genius Duck Moderator Dywyddyr's Avatar
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    Quote Originally Posted by supanova View Post
    but if i filled it with helium it would be lighter?
    It would weigh 1 kg plus the weight of the helium - ergo heavier (than one with vacuum inside).
    "[Dywyddyr] makes a grumpy bastard like me seem like a happy go lucky scamp" - PhDemon
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by supanova View Post
    but if i filled it with helium it would be lighter?
    It would weigh 1 kg plus the weight of the helium - ergo heavier (than one with vacuum inside).

    Bouyancy of helium at sea level would make it lighter, no?
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  6. #5  
    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by supanova View Post
    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by supanova View Post
    but if i filled it with helium it would be lighter?
    It would weigh 1 kg plus the weight of the helium - ergo heavier (than one with vacuum inside).

    Bouyancy of helium at sea level would make it lighter, no?
    If you are accounting for the effects of buoyancy, then the vacuum filled sphere would be the lightest in terms of effective weight.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  7. #6  
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    so say there is no carbon fiber and just say you have the vacuum...what would the buoyancy of the vacuum be? at sea level
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    Quote Originally Posted by supanova View Post
    so say there is no carbon fiber and just say you have the vacuum...what would the buoyancy of the vacuum be? at sea level
    For a 6 in. radius sphere, about 2/3 of an ounce.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  9. #8  
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    so how did you come up with that number?
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  10. #9  
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    It's the weight of a 6 in radius sphere of air at sea level.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    but this is not air. It's a vacuum that can magically hold a shape for arguments sake.
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    Quote Originally Posted by supanova View Post
    question i've been haunted by: If i had a 1 pound worth of carbon fiber shaped into a ball(radius=6 inch) and extracted all the air, what would the ball weigh? and why. Does the negative space of a vacuum effect gravity field above or below ball? Any feedback is greatly appreciated.
    Would you still be asking the same question if the air in the ball was replaced by molten lead?
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  13. #12  
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    Quote Originally Posted by supanova View Post
    but this is not air. It's a vacuum that can magically hold a shape for arguments sake.
    Which displaces that much air.

    Look at it this way. Going back to your original idea. You take 1 lb of material and form it into a hollow sphere, enclosing air within. It still weighs 1 lb (enclosing the air does not change its effective weight.) Now remove the air, it now has an effective weight 1 lb minus the weight of the air you removed. That's the net buoyancy effect. (Another way to look at it is that the weight of the air in the sphere when you first enclosed it exactly cancels out the effect of buoyancy.)

    Now magically remove the shell (1 lb) and you have a net weight of minus that of the air removed.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    I've dropped the carbon ball part and now asking if a 6 inch raduis ball shaped vacuum has any buoyancy at sea level.
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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by supanova View Post
    but this is not air. It's a vacuum that can magically hold a shape for arguments sake.
    Which displaces that much air.

    Look at it this way. Going back to your original idea. You take 1 lb of material and form it into a hollow sphere, enclosing air within. It still weighs 1 lb (enclosing the air does not change its effective weight.) Now remove the air, it now has an effective weight 1 lb minus the weight of the air you removed. That's the net buoyancy effect. (Another way to look at it is that the weight of the air in the sphere when you first enclosed it exactly cancels out the effect of buoyancy.)

    Now magically remove the shell (1 lb) and you have a net weight of minus that of the air removed.

    Thanks man. So a ball of helium would be buoyant and a ball of vacuum would be even more buoyant. But how much more buoyant?
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    Quote Originally Posted by supanova View Post
    I've dropped the carbon ball part and now asking if a 6 inch raduis ball shaped vacuum has any buoyancy at sea level.
    And I've answered that question.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by supanova View Post
    I've dropped the carbon ball part and now asking if a 6 inch raduis ball shaped vacuum has any buoyancy at sea level.
    And I've answered that question.
    meant to quote rob before posting that.
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by supanova View Post
    but if i filled it with helium it would be lighter?
    It would weigh 1 kg plus the weight of the helium - ergo heavier (than one with vacuum inside).
    HTH would you weigh it though, given the buoyant effect of the helium? joc
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    Quote Originally Posted by supanova View Post
    I've dropped the carbon ball part and now asking if a 6 inch raduis ball shaped vacuum has any buoyancy at sea level.
    A ball of vacuum has NO buoyancy at any elevation, IMO. Practically, it cannot exist without confining media, and theoretically, in the absence of such media, it simply disappears as atmospheric air rushes in to fill the "void". Interesting idea, though! jocular
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  20. #19  
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    Just say i did have an ounce of magic material that could hold a vacuum the size of 6 inch radius ball. It would have no buoyancy? -or- would it still weigh 1 ounce? I'm no scientist but i think if i got 1 ounce of substance that usually takes up a certain amount of space and stretch it out to take more space it makes it lighter. Like a propane tank is lighter the less compressed it gets.
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    Quote Originally Posted by supanova View Post
    Just say i did have an ounce of magic material that could hold a vacuum the size of 6 inch radius ball. It would have no buoyancy? -or- would it still weigh 1 ounce?
    See post #8.
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  22. #21  
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    I think it would be lighter because it wouldn't have as much weight as the air sitting on top of it.
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  23. #22  
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    It's not hard. The buoyant force is equal to the weight of fluid displaced. Archimedes knew this before 200 BC.
    Archimedes' principle - Wikipedia, the free encyclopedia
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    About 15 psi or so,at sea level.
    Last edited by Hill Billy Holmes; November 18th, 2013 at 11:36 AM.
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  25. #24  
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    nevermind i just don't understand. but does the empty space effect the magnetics from the earth on the molecules around the ball? above or below
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    Magnetic field? In carbon? Maybe if it were supercooled. Will it affect the gravitational attraction? Gravity is a mass mass relationship in curved space, inverse square, so it depends on the two masses. If they are unchanged, then there should be no changes in gravitational relationships.
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