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Homework Help: Volume buoyancy problem

  1. Feb 7, 2010 #1
    1. The problem statement, all variables and given/known data
    "friend with a mass of 79 kg ponders the idea of attaching a helium-filled balloon to himself to effectively reduce his weight by 21% when he climbs. He wonders what the approximate size of such a balloon would be"
    solution is expressed as cubic meters
    apology if this entry is miscategorized


    2. Relevant equationsvolume =mass/density



    3. The attempt at a solutionvolume= change in mass/ difference in density=16.59/(1.2-1.78)=16.27m^3
    this amount is inaccurate, as is 13.27 m^3
     
    Last edited: Feb 7, 2010
  2. jcsd
  3. Feb 7, 2010 #2

    mgb_phys

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    You have the right idea, you just need to be a bit more careful about working it out.

    You want a bouyancy of 21% * 79kg = 16.59kg
    bouyancy = volume * difference in density

    Density of air = 1.27 kg/m^3 helium = 0.18 kg/m^2
     
  4. Feb 7, 2010 #3
    presuming:
    16.59 kg/1.09 kg/m^3=15.22 m^3 (volume)
    neither 15 nor 15.22 m^3 is the correct solution- according to my online homework program
     
    Last edited: Feb 7, 2010
  5. Feb 7, 2010 #4

    mgb_phys

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    Unless it gives you a value for the altitude or temperature, then assuming STP
    Density 1.2754 - g/l (air) 0.1786 g/l (he) = 1.0968 g/l (or kg/m^3)
    You need to lift 79*0.21 kg / 1.0968 kg/m^3 = 15.1 m^3
     
    Last edited: Feb 7, 2010
  6. Feb 7, 2010 #5
    these are the incorrect answers submitted:
    13
    13.2
    13.27
    13.28
    16.95
    16.23
    16.26
    1.38
    16
    18.08
    18
    15.22
    15...14.9.....15.1
     
    Last edited: Feb 7, 2010
  7. Feb 7, 2010 #6
    exact question:
    "A mountain-climber friend with a mass of 79 kg ponders the idea of attaching a helium-filled balloon to himself to effectively reduce his weight by 21% when he climbs. He wonders what the approximate size of such a balloon would be. Hearing of your physics skills, he asks you. What answer can you come up with, showing your calculations?"
    -
    solution is in m^3

    -thank you for the explanation, i have no more available attempts...should find out the correct answer tomorrow, will update
     
    Last edited: Feb 7, 2010
  8. Feb 8, 2010 #7
    according to Paul G. Hewitt, via webassign.net;
    the solution is:
    volume = 79 kg*.21/1.2 kg/m^3= 13.825 m^3
    no consideration of helium density and air density is considered 1.2
     
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