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Balloon in elevator

  1. Nov 19, 2004 #1
    so you have a girl in the elevator with a balloon. the elevator begins to descend in free-fall, and the girl releases the balloon in surprise. my book says that the balloon will stay at the same level relative to the girl as before, which brings me to my question: do buoyancy forces not work when the whole system is in acceleration, then?
    could someone please explain this to me?
  2. jcsd
  3. Nov 19, 2004 #2


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    Balloons float from a high pressure area to a lower pressure area.
    When the elevator goes down the air moves to the top making that the high pressure area.
    You get the same effect in a car. The balloon will move forward when the car accelerates
  4. Nov 19, 2004 #3


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    In a free fall, there are no buoyancy forces. Air pressure would equalize through out the elevator. Before the freefall, gravity's pull on the air resulted in a slight pressure variance, higher at the bottom, lower at the top. The net pressure differential at the top and bottom of the elevator would equal the weight of the air inside the elevator (about 2 lbs per cubic yard).

    If the baloon were released at the moment of free fall, there would be some upwards movment of air as the pressure equalized, so the baloon would move upwards a bit.

    If the girl's reaction time to the free fall delayed the relase of the balloon until after the air pressure equalized, then the balloon would remain in free fall, just like the girl and the air inside the elevator.
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