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Why do Balloons filled with Air not fall at 9.6m/s/s?

  1. Sep 8, 2009 #1
    I've been racking my brain to figure this one out, I really don't understand it.

    If you fill a balloon with air from your lungs, just manually blow it up, it should be carbon dioxide, no?

    Density of Carbon Dioxide at Sea Level: 1.977 g/L
    Density of Air at Sea Level: 1.2 g/L

    So it's heavier. I know I'm pulling these numbers out of seemingly nowhere, but I'm sure someone can verify they are accurate.

    So why on Earth would a balloon filled with a gas heavier than the surrounding gas in it's environment not drop at the normal speed. Aren't I right to think that bouyancy only takes effect when the gas inside the balloon is at least a bit lighter?

    Or is there some critical property I'm missing?

    Please enlighten me, thank you.
  2. jcsd
  3. Sep 8, 2009 #2
    Do you recall that Gallileo dropped a feather off the Tower of Pisa, as well as a 1 pound weight and a ten pound weight:
    http://www.jimloy.com/physics/galileo.htm [Broken]
    Astronauts tried Gallileo's experiment on the moon. See:
    Last edited by a moderator: May 4, 2017
  4. Sep 8, 2009 #3
    If you fill a balloon up with your breath it would be mostly nitrogen just like the air you breathed in.

    Also, with a density so close to that of the air it is prone to the slightest disturbances from the air.
  5. Sep 9, 2009 #4


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    1] As pointed out, your exhaled breath is almost exactly the same as the air. It has an extra 5% CO2 and is shy by the same amount of oxygen.

    2] You've got a volume of air, that weighs maybe a gram or two more than the surrounding air, but it has a giant surface area. It's terminal velocity is on the order of a few feet per second.
  6. Sep 9, 2009 #5
    Ahh, Terminal Velocity. Now that makes sense. :)

    Thanks for pointing that out.
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