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Thread: Confused about density

  1. #1 Confused about density 
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    Hi forum

    New here - so please make allowances!

    I was watching the Brian Cox video tonight, where he goes to the NASA Vacuum chamber to conduct an experiment (Video is on "You Tube", but I am not allowed to post the URL)

    It shows how the speed an object descends, when put in a vacuum chamber, is independent of the object mass and density (the feather and cannonball descend at the same speed and arrive at the bottom at the same time, as wind resistance is removed from the equation). This is where I am confused. Hot air balloons alter their ascent / descent rate by adjusting the density of the air in the balloon. How is that possible, if descent rates are independent of density, as proved in the video?

    Thanks for your patience


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  3. #2  
    Genius Duck Moderator Dywyddyr's Avatar
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    You're comparing apples and oranges - fall rate in a vacuum chamber against a balloon in an atmosphere.

    A balloon works because of buoyancy: by adjusting the density of the balloon itself (not just the air in the balloon1). No atmosphere - no buoyancy: no working balloon.

    1 As the hot air expands in the balloon it expands the envelope of the balloon - taking up more space for no increase in mass, thus making the entire thing less dense.


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  4. #3  
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    Oh of course. Balloons work because of relative densities, and there isn't anything less dense than a vacuum! Thanks
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