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Homework Help: Buoyancy Balloon

  1. May 12, 2010 #1
    Hi i have this question:

    A balloon is filled with helium gas which weighs approximately one-seventh of what air weighs under identical conditions. The balloon has a diameter of 10 m and carries two people, each weighing 70 kg. Assume the density of air is 1.16 kg/m3 and neglect the weight of the ropes and cage. Determine:

    (a) The acceleration of the balloon when it is first released

    This is how ive been doing it

    F= 1.16/7 * 9.81 * 523.6 = 852.7 - Buoyancy equation

    Then i got the weight by doing

    W= 140 * 9.81 = 1373.4

    First of all im not sure if these are right and if they were ive been taking away the buoyancy force from the weight to get 520.7

    Then to get accleration i divided it by the mass 140

    to get 3.72

    But the answer giving said its 16.465

    Can anyone please tell me where im going wrong thanks

    And weirdly theres a secound question thats says find the maximum load that it can carry and the answer is 520.61 which is very close to my answer

  2. jcsd
  3. May 13, 2010 #2
    Surely someone can help me?
  4. May 13, 2010 #3

    Doc Al

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    Staff: Mentor

    What does Archimedes' principle tell you that the buoyant force equals?

    The buoyant force is an upward force; the weights of the people and the balloon are downward forces.
  5. May 13, 2010 #4
    yeah, i thought i should take away the weight of the balloon and the people from the buoyancy force and that would leave me the force pushing it up and accelerating it. is this wrong.

    does the buoyancy force=weight
  6. May 13, 2010 #5

    Doc Al

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    Staff: Mentor

    No, that's not wrong.

    Buoyant force = weight of the displaced fluid (air, in this case)
  7. May 13, 2010 #6
    so should i calculate the weight of the air using -
    row(density of air) * 9.81 * volume of balloon

    then do the buoyancy force minus the weight of air and the weight of the people(m*g)

    but if this is correct im not sure why my buoyancy force is lower than the weight as the weight of the air that i calculate is 5958.3 N compared to my buoyancy force of 852.7

    thanks for you help so far,
  8. May 13, 2010 #7

    Doc Al

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    Staff: Mentor

    Yes. That will give you the buoyant force.

    That should be minus the weight of the balloon (filled with helium, not air) and the weight of the people. That will give you the net force on the balloon+people.

    Your calculation of the buoyant force is incorrect:
    You divided by 7. (That gives you the weight of the balloon, not the buoyant force.)
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