Does Gas defy Gravity?

Discussion in 'Physics & Math' started by Zardozi, May 31, 2007.

  1. Zardozi Isvara.... . 1S Evil_Lau Registered Senior Member

    Messages:
    443
    How is it posible that based opon lightness it can defy gravity? Doesnt gravity pull down everything?

    ie- helium is lighter than air but how does it escape the force of gravity?


    also,
    water does not act as a wall in a bong to air?
     
  2. TruthSeeker The Truth is Out There Valued Senior Member

    Messages:
    15,142
    Gravity is a weak force.
     
  3. mikenostic Stop pretending you're smart! Registered Senior Member

    Messages:
    4,624
    Yes, gravity (if it's strong enough...big enough planet) does pull down gases. If it didn't, do you think Earth would have an atmosphere? Do you think Jupiter/Saturn/Neptune and Youranus would be gas giants if not? No, they would just be gas in space.
     
  4. Oli Heute der Enteteich... Registered Senior Member

    Messages:
    11,890
    Why does a boat not get pulled to the bottom of the sea?
    Think on that and apply to gases.
     
  5. Read-Only Valued Senior Member

    Messages:
    10,300
    You answered your first question yourself - it's lighter than air and therefore it floats upwards in it - just like an air bubble in water. It's all a matter of density and specific gravity. (You might want to look up those two terms and learn some VERY basic stuff.)

    And the answer to the last question is that you apply a partial vacuum to the bong so air pressure pushes the smoke through the water. More very basic stuff.
     
  6. spidergoat Give me heat, and then I'll add the wood. Valued Senior Member

    Messages:
    47,117
    Gravity still works on it, but it works more on denser gasses, thus bringing the denser gasses closer to the center of the planet, thus displacing the lighter gasses, forcing them up.
     
  7. darksidZz Ahimsa to you! Valued Senior Member

    Messages:
    4,850
    Yes... what of the gyroscopes?
     
  8. Read-Only Valued Senior Member

    Messages:
    10,300
    What about them? Get one spinning and drop it - it falls.

    I do realize you're talking about it's ability to resist having it's orientation changed - like balancing it on a string. But that's nothing but a matter of inertia and nothing to do with gravity. Remove the string and it will fall as quickly as anything else.
     
  9. darksidZz Ahimsa to you! Valued Senior Member

    Messages:
    4,850
    :(~~~~~~~~~~
     
  10. James R Just this guy, you know? Staff Member

    Messages:
    27,059
    It doesn't. It's just that there is an extra force on it - buoyancy.

    Helium floats upwards in air for exactly the same reason that a block of wood floats upwards in water. The wood doesn't defy gravity; the water buoys it up.

    The upward force is a result of the fluid density increasing with depth. Just as water pressure increases with depth, so air pressure increases lower down in the atmosphere. The pressure differences create net buoyant forces on objects.
     
  11. Read-Only Valued Senior Member

    Messages:
    10,300
    You were doing just fine until you made this misleading statement: "The upward force is a result of the fluid density increasing with depth. :D

    While both that statement and this one: "The pressure differences create net buoyant forces on objects. are both technically correct, they lend a slight misrepresentation in answering the direct question that was asked.

    The fact is that no increase in fluid density is required to make helium bouyant - the other gasses present already have a higher specific gravity than helium does. Meaning they are already more dense and heavier. ;)
     
  12. James R Just this guy, you know? Staff Member

    Messages:
    27,059
    Sorry - The word "density" was incorrect. My statement ought to have said: "The upward force is a result of fluid pressure increasing with depth."

    Obviously, buoyancy still occurs even in fluids of (approximately) constant density.
     
  13. Read-Only Valued Senior Member

    Messages:
    10,300
    But my main point was that even if the other gasses were only a single atom/molecule thick on the body of a perfect sphere, the helium would still float on top of them because it has a lower specific gravity. No "increase" in depth is even needed.

    And it's not the fact that pressure increases with depth that keeps wood floating, it's the fact that right at the surface of the water, the wood is less massive than an equal volume of water (specific gravity once again). :)
     
  14. Fraggle Rocker Staff Member

    Messages:
    22,777
    Let's answer this question first. You can push your face into a sink full of water and blow bubbles through it, right? A bong works the same way. When you suck on the inhaler, the air pressure on top of the bowl blows air and smoke bubbles through the water. It's exactly the same thing.

    Water is not a solid, so there's no way it could possible act as a "wall" under ordinary circumstances. Its molecules happily move aside to let the air molecules pass through. That's how liquids and gases behave, totally unlike solids.
    Most of the other answers are correct but Mike gets the prize in the following post, because he simply does a better job of explaining it, without using complicated scientific terminology, and without defining "buoyancy" as a force, which is a bit of a problem.
    Good job Mike. I hope you are a teacher. Everyone else's explanations are a bit too complicated for a layman to understand. You point out the obvious: Helium obviously does not escape the force of gravity because it is pulled toward the earth.

    The fact that helium is on top of the stack of gases is a separate and interesting question, but it doesn't contradict the fact that the helium is nonetheless firmly captured by the earth's gravity and does not float off freely into space. (Well actually if there's any free hydrogen it would be topmost because it's even lighter than helium--but all those rocket ship exhausts have probably burned up what little free hydrogen might have been there. :))
    Uh... "buoyancy" is a bogus force like "suction." I would avoid using those terms to explain science to laymen because it can actually make the explanation more convoluted. And some day they'll find out there really is no such force as buoyancy and it will be like Santa Claus: Why didn't you just tell us the truth so the universe would not seem so complicated? :) There is no force pushing the helium up. There is a force pulling it down, and that force is gravity. The problem is that the force of gravity pulls more strongly on all the other gases because they are denser: oxygen, nitrogen, carbon dioxide, etc. So they push their way past the helium and leave it sitting on top of them. But it is still being firmly pulled by gravity just like smoke, water, humans, rocks, and all other matter.
    I have had more success getting people to understand gravity by simply explaining that water is denser than wood so it pushes its way past the wood and sinks beneath it. This way you're only explaining one force rather than two.

    There are only four elementary forces in the entire universe: electromagnetism, gravity, the strong nuclear force and the weak nuclear force. I believe that postulating buoyancy as a force, when it is merely a local relative negative gravity, does not prepare people for their science classes. I vote for giving Mike the Science Fair Award. :) And I hope to the goddess that he is a teacher.
     
  15. James R Just this guy, you know? Staff Member

    Messages:
    27,059
    I disagree. I'm not convinced that a single molecular layer would result in the helium bubbling to the top. What happened in that case would depend in a very specific way on the surface chemistry.

    Unless we're talking about the same thing in different ways, I think you are wrong. What keeps wood floating is that there is a larger pressure on its bottom surface than on the top surface, producing an upward force. That force exists when the block is totally immersed.

    Your "specific gravity" explanation is not wrong, but it is a secondary explanation. The relative densities of fluid and object only become relevant because of the differential pressure in the fluid.

    Also, there are no buoyant forces in the absence of gravity, so buoyancy is not solely a result of differing densities, as you seem to be implying.
     
  16. Fraggle Rocker Staff Member

    Messages:
    22,777
    Moderator: This should probably be in Math & Physics.
     
  17. James R Just this guy, you know? Staff Member

    Messages:
    27,059
    Fraggle:

    I have no problem at all talking about "bouyant forces". Immerse an object in water and the net force on it consists of two separate effects: (1) gravity pulling it down + (2) pressure forces (buoyant forces) pushing it up.

    In terms of fundamental forces, we're dealing with two forces here - gravity and electromagnetism - not just gravity. Buoyancy cannot be explained in terms of gravity alone, if you want to get "fundamental" about it.

    This does not explain anything.

    Take a rock immersed in water. It "weighs" less than it does in air. Thus, we conclude that there is, in addition to its weight, a separate upward force on it, which we call the "buoyant force". This force exists even when the rock is held stationary in the water, so it cannot have anything to do with denser or less dense water pushing past it.

    Hopefully, you can appreciate why that is wrong, and you'll explain it better in future.
     
  18. Facial Valued Senior Member

    Messages:
    2,212
    Certainly fluids try to displace every volume under gravity. This is because there is always more hydrostatic pressure as depth increases, resulting in uneven forces, leaving an upward resultant. The only vertical force left to consider is weight - if it is heavier, it sinks. But the buoyant force is still there. Even something as dense as a rock will weight heavier when inside a vacuum because of the atmospheric buoyant force.
     
  19. Pete It's not rocket surgery Moderator

    Messages:
    10,165
    A misunderstanding of buoyancy lies behind a number of supposed perpetual machines.

    For example:
    [​IMG]
    The material of the ring floats is less dense than the blue fluid and more dense than the red fluid, so it will spin on its own... or will it?

    Another:
    [​IMG]
    The ball floats in the fluid on the right and falls through the air on the left. The fluid stays up on the right side because of the gates G1 and G2, which are timed to quickly open and close just long enough for the ball to pass . Assuming perfect (or even "good enough") engineering, the ball will cycle endlessly... or will it?

    Here's a similar one:
    [​IMG]
    This time, there are a series of buoyant balls connected to a chain, passing through a cleverly sealed container of fluid.
    Will it continually turn?
     
  20. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

    Messages:
    8,966
    James---there is a cruicial paper written by Archimedes 2300 years ago which you could refer people to, in which all the details are explained.
     

Share This Page