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Archimedes principle - special case

  1. Dec 14, 2013 #1
    1. The problem statement, all variables and given/known data
    We have a chinese lantern (balloon) made of paper, cylinder shaped with the following sizes:
    base diameter - 45 cm, height - 70 cm. Mass of the balloon is 57 g (21 g from this is mass of "fuel" - the fuel is wax paper!). Fuel is then ignited at the centre of the base, which is open. Therefore the balloon will soar. The question is, how much additional weight can be hung to the balloon and the balloon still takes off. Outside temperature is 5 °C (Let us denote it as cold air).

    2. Relevant equations
    Just see the text.

    3. The attempt at a solution
    First I can compute volume of the balloon, which is of course V = π*r2*h = π*22.52*70 cm3 = 111330.189662 cm3≈0.11133m3.
    Now the gravitational force of air pressed up by the balloon gives the magnitude of buoyancy:
    Gcold_aircold_air*V*g = 1.2697*0.11133*g≈1.38723 N.
    Now we should determine the volume of the fuel, which is
    Vfuel= mfuelfuel=0.021/650 = 0.00003 m3.
    Now we will compute Ghot_airhot_air*(V-Vfuel)*g = 1.1277*0.11130*9.871373 ≈ 1.23175 N. Now we can use Archimedes principle:
    Gcold_air = Ghot_air + m*g + M*g, and we want to solve this equation for M, which is our burden, that can be carried:
    M = (Gcold_air-Ghot_air-m*g)/g, which gives us M≈-0.04116 kg, which is rubbish...

    There are just too many determinants, that were omitted:
    1) temperature of hot air in the ballon, I chose 40 °C, but how can I know? What if it is 60 °C, then the density would be higher, but how can I know?
    2) It would be best if all fuel would just burn out, then the temperature would be the highest and the mass the lowest, so should I wait till the fuel is burn out and only after that I bind the burden?
    3) I omitted the air pressed up by the wax paper and the burden.
    4) ... inf)
    There are just too many...
    I'd be so much grateful for your help!
     
    Last edited: Dec 14, 2013
  2. jcsd
  3. Dec 14, 2013 #2

    CWatters

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    Do you know the energy content of the fuel and the specific heat capacity of air? That may allow you to calculate the temperature of the air in the balloon after the fuel is burnt. Some assumptions may have to be made!
     
  4. Dec 14, 2013 #3
    This is all I know, we should probably make some of our own assumptions, yeah...
     
  5. Dec 14, 2013 #4

    haruspex

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    Since you only need it to take off eventually, yes, you can restrict attention to the time at which the fuel is almost gone. I suspect that assuming all the heat stays in the balloon will give a significant overestimate of the temperature. You could calculate the volume of gas from the combustion products, assume that mixes uniformly with the original air, find the resulting expansion, and hence find how much heat is lost by spillage out of the envelope. (There'll also be radiative losses, but they're probably a lot less.)
    Of course, the gases in the balloon will no longer be just hot air - there'll be a lot of CO2, affecting the density.
     
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