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Homework Help: BALLOON PROBLEM. please please help

  1. Jan 2, 2010 #1
    BALLOON PROBLEM. please please help :)

    1. The problem statement, all variables and given/known data
    I need to find the volume of a helium balloon, and 9 paperclips can sit on it so it doesn't float away (.004kg). Density of He=.18; density of air=1.29; mass of balloon=.0116kg.

    2. Relevant equations
    Free body sum of equations:

    3. The attempt at a solution
    using the equation above, I first isolated Fb, then I got:

    I don't know if this is right. And i don't understand if you solve for Vhe or Vdisp because aren't they the same things.
    Last edited: Jan 2, 2010
  2. jcsd
  3. Jan 2, 2010 #2
    Re: BALLOON PROBLEM. please please help :)

    Wait. for some reason, our teacher also told us to convert Liters to Meters...and I have no idea why. Thanks for your time.
  4. Jan 2, 2010 #3


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    Re: BALLOON PROBLEM. please please help :)

    First of all, you have some values that are incorrect without units. What are the units of the density of helium? What about the density of air?
    You seem to have some signs (negative/positive) wrong in that equation. Can you tell which ones?
    This is incorrect because you don't have the proper units on these values (.18, .0116, .004). But if you add the proper units, that's a good first step. Also, what value (with units!) should you put in for g?
    Yes they are the same thing - good that you noticed that! Since they are the same, choose one variable to use for both quantities. It could be VHe or Vdisp or even just V.

    Since you say you don't know if that expression is right: is there some particular reason you think it should be wrong? (except for the unit errors which I mentioned earlier)
  5. Jan 2, 2010 #4
    Re: BALLOON PROBLEM. please please help :)

    I know the units; it would just take too long to write. And I know that g=9.8m/s2.
    I just am not sure if the equation is correct, because I get an answer of .044m3 and that doesn't seem correct. This is just a regular balloon, like a helium happy birthday one. Thank you for your help though; I appreciate it.
  6. Jan 2, 2010 #5


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    Re: BALLOON PROBLEM. please please help :)

    Too long? Geez, what are you in such a hurry to get to? And why didn't you shave off another second or so by skipping some digits too? One thing you absolutely need to learn if you're ever going to do a physics problem properly is that a value which doesn't have the correct units is wrong. The density of air is (about) 1.29 kg/m3; it is not 1.29, just as much as it is not 1.29 seconds. When you're doing work on your own, sure, you don't need to write the units as long as you can keep track of them properly, but when you share your calculations with others, proper units are essential.
    k, cool
    Why doesn't it seem correct? (You're right, it's not correct, but you should try to understand why)

    The equation you've set up is missing a factor of g in one term - I just noticed that (and you would too if you were being careful with units) - but otherwise, if you solve it properly, it does give the right answer. If you can't find your mistake, post the work you've done to solve the equation and someone else can help you identify where you went wrong.
  7. Jan 2, 2010 #6
    Re: BALLOON PROBLEM. please please help :)

    how cute.
  8. Jan 3, 2010 #7
    Re: BALLOON PROBLEM. please please help :)

    This equation is seems right. What do you mean with p(fluid) here?
    I would say that Fb = (mass of displaced air) * g = (volume of displaced air) * (density of displaced air) * g
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