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Simple Buoyancy Problem

  1. Sep 22, 2009 #1
    1. The problem statement, all variables and given/known data
    A 0.900 g balloon is filled with helium gas until it becomes a 25.0 cm-diameter sphere.
    What maximum mass can be tied to the balloon (with a massless string) without the balloon sinking to the floor?


    2. Relevant equations

    Fb=VSphere*Pair*Gravity
    Fg=Vsphere*Phelium*Gravity

    3. The attempt at a solution

    I tried to set them equal to each other (so that the net force is 0) and found that...

    Vsphere(Phelium-Pair)*9.8-0.0009kg*9.8(mass of ballon)=Mass attachable/9.8

    I got 7.5g but that's wrong. Anyone know where I messed up my process?
     
    Last edited: Sep 22, 2009
  2. jcsd
  3. Sep 22, 2009 #2
    Still looking for some help. I've run the calculation several times and keep get 7.45g...
     
  4. Sep 22, 2009 #3
    I still get 7.45g...weird.

    My balloon circumference is 25 cm. The balloon weighs 0.9g. Pair=1.2, PHelium=0.1785

    Therefor my radius is 0.125m and my balloon mass is 0.0009kg.

    4/3pi(0.125)^3*1.2=Mass+0.0009kg+4/3pi(0.125)^3*0.1785

    Mass=0.007457kg=7.5g...

    Anyone see where I'm messing up.
     
  5. Sep 23, 2009 #4
    I was right...I just had different constants than the book...

    Dumb mistake.
     
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