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Air density, vacuum and gravity

  1. Oct 8, 2015 #1
    Hello to everyone.
    My first question on this forum (it was quite tricky thinking of a heading and prefix for this thread!):
    Imagine for a moment that there is a hot air balloon which has a strong shell forming the balloons shape rather than one made from material and it weighs the same as a normal hot air balloon, What would happen if air was gradually drained from the balloon, would it rise?
  2. jcsd
  3. Oct 8, 2015 #2


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    It would rise as long as the balloon was lighter than the same volume of air, regardless of what's inside.
  4. Oct 8, 2015 #3
    Thanks, that's cleared that one up.

    Another question: If an object was placed on a set of scales in a vacuum chamber and the air was removed, does it weigh much less without the weight of any air pushing down on it?
    Last edited: Oct 8, 2015
  5. Oct 8, 2015 #4
    The air removed from the vacuum chamber I suppose you mean, and not from the object.

    You might be able to answer that question yourself with a little help.

    When dropped from a height in air, which will reach the ground first:
    a blown up balloon, or not-blown up balloon. If you put them on a scale, which weighs more?

    If you then put them in a vacuum chamber, and drop them,:
    which one hits the ground first. On a scale in the vacuum chamber, which one weighs more?

    ( You can cheat by looking up something called bouyancy )
  6. Oct 8, 2015 #5


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    I believe so.
  7. Oct 8, 2015 #6


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  8. Oct 8, 2015 #7
    Not so.
    Do you weigh more on a scale when submerged in water than air.
    Would you weigh more on a scale when submerged in air than a vacuum.
  9. Oct 9, 2015 #8
    Yes, I mean the air being removed from the vacuum chamber and not the object.

    Funnily enough I recently watched a feather and hammer dropped side by side in a vacuum chamber and they both fell at the same speed and hit the ground together. I haven't seen two objects weighed in a vacuum chamber yet, would the blown up balloon would weigh more because of the extra weight of the air inside it?
  10. Oct 9, 2015 #9
    That should tell you something.

    In a vacuum, they both fall at the same rate.That's true.
    And on the scale in the vacuum, the blown up balloon would weigh more due to the air inside.
    So you are 2 for 2 - 100% so far.

    ( You didn't answer the first part when dropped in the air and weighed in the air, but since that's an everyday experience, so I'll give it to you. 4 for 4 )
    ( And no, we aren't going to worry about air friction. We will assume the friction is small and negligible )

    So the question is :
    Why does the blown up balloon weigh more than the non-blown up in a vacuum.
    But weighs less than the non-blown up balloon , when weighed in air.

    Have you looked up buoyancy yet?
  11. Oct 9, 2015 #10
    Does that mean every object has a certain amount of buoyancy whilst it is submerged in air, depending on the amount of air it displaces, and in a vacuum it would weigh more without any buoyancy?
  12. Oct 9, 2015 #11


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    If there is a pressure gradient in the air, for example due to gravity.
  13. Oct 9, 2015 #12
    Thanks, yes I've looked up buoyancy and displacement, it's forming a picture in my head gradually!

    Hmm, in a vacuum, there isn't any buoyancy at all, there is nothing to "squeeze" the balloon upward through layers of less dense atmosphere. There is more density to the balloon with the air inside it, so it would weigh more on the scales in the vacuum?

    Would the blown up balloon weigh less when weighed in air because it is displacing more air? I'm not sure if can see that yet because I think the inflated balloon would contain more air pressure than the outside atmosphere, which means it wouldn't be displacing enough air to make it buoyant?
    Last edited: Oct 9, 2015
  14. Oct 9, 2015 #13
    Somewhat more air pressure, but most party balloons are quite buoyant you will have to admit.
  15. Oct 9, 2015 #14


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    The content is irrelevant for the force of buoyancy.
  16. Oct 9, 2015 #15
    Yes. So the material of the balloon when inflated\spread out, is displacing more air than the non inflated one, making it more buoyant?

    Am I right in thinking that as an object is stretched out, the closer the physical matter of the object becomes to the density of air, until it eventually matches the weight of air and achieves neutral buoyancy. Then if it is stretched out further it becomes lighter than surrounding air and is squeezed upward until it reaches air density that matches it?
  17. Oct 9, 2015 #16


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    The volume of the object as a whole matters for the force of buoyancy.
  18. Oct 9, 2015 #17
  19. Oct 9, 2015 #18
    Right, think I've got it now! As a balloon is inflated and the skin stretches out, the actual skin itself displaces more and more of the surrounding air until eventually the air it has displaced weighs more than the balloons skin.

    If the balloon skin were stretched out far enough and it displaced enough air, it would begin to be squeezed upwards towards thinner layers of air by the weight of the air around it, until it matched the weight of the air it was displacing and achieved neutral buoyancy?
  20. Oct 9, 2015 #19


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    No. The balloon as a whole displaces more and more air.

    No. Balloons float when filled with gases of lower density than the surrounding air.
  21. Oct 9, 2015 #20
    Thank you.

    Supposing a sheet of rubber were stretched out until it is volume displaced more air than the rubber itself weighed, would it float?
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