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Homework Help: Helium Balloon Question

  1. Sep 22, 2007 #1
    Helium Balloon Question!!!

    Hey I have a buoyancy question here, anyone know how to solve it? A 15 gram rubber balloon is filled with 18 L of Helium (density 0.179kg/m^3) and tied with a string to a child's wrist. Find the tension in the cord. The answer is 0.050N but what is the solution?
    Im not sure if yuo need it but the denisty of air is 1.29kg/m^3. Thanks.

    2. Relevant equations
    Equations i used include F buoyancy = F tension + m helium * gravity + m balloon * gravity

    and F buoyancy = density * gravity * volume displaced

    3. The attempt at a solution

    I tried to solve but got completely different answers, what is the volume displaced? is that the right thing to use?
  2. jcsd
  3. Sep 22, 2007 #2


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    Looks right to me. Can you show your calculations?
  4. Sep 22, 2007 #3
    F tension = (18000 m^3 * .179) (9.8) + (15/1000kg * 9.8) + Force of Bouyancy

    But im not sure how to get the force of buoyancy, can anyone help?
  5. Sep 22, 2007 #4


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    I think the buoyancy force is the volume of the balloon, times the density of air * g.
  6. Sep 22, 2007 #5


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    The bouyancy is the difference between the weight of 18 L of air and the weight of the balloon.

    (You are TOLD that the volume is 18L.)
  7. Sep 22, 2007 #6
    ok thanks for the help, i will try this
  8. Sep 22, 2007 #7
    Still doesnt help. My equations must be wrong. Lets see: the downwards forces are the tension, weight of helium, and weight of balloon, and the upwards force is force of buoyancy. the question is asking for tension force. the answer is apparently 0.050 N. did anyone get that answer, cause i sure cant.thanks..
  9. Sep 22, 2007 #8


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    It should be: F tension = -(0.018 m^3 * .179) (9.8) - (15/1000kg * 9.8) + Force of Bouyancy.

    fbuoyancy = 1.29*0.018*9.8

    Working out the above, gives me 0.05N.
    Last edited: Sep 22, 2007
  10. Sep 22, 2007 #9


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    18L = 0.018m^3, not 18000m^3.
  11. Sep 22, 2007 #10
    lol ur rite
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