1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Density and Volume

  1. Aug 8, 2010 #1
    1. The problem statement, all variables and given/known data
    A balloon is to be filled with helium(density = 0.178kg/m^3) while the air outside has density of 1.29km/m^3. The mass of the empty balloon is 80.0kg and it is to lift a load of 240kg. What minimum volume is necessary for the lift off? Assume that the balloon material and the load occupy negligible volumes.


    2. Relevant equations

    density = M/V

    3. The attempt at a solution

    I'm not quite sure how to start with this question. Can someone help me please?
     
  2. jcsd
  3. Aug 8, 2010 #2

    rl.bhat

    User Avatar
    Homework Helper

    The balloon will float when the weight of the displaced air is equal to the weight of the balloon with load.

    Volume of the balloon = mass of the balloon/density of helium.
     
  4. Aug 8, 2010 #3
    hmmm. not quite getting it.

    This is archemides' principle right? so wouldn't the equation be:

    density of balloon/ density of gas = volume of gas/ volume balloon??
     
  5. Aug 8, 2010 #4

    rl.bhat

    User Avatar
    Homework Helper

    Mass of the displaced air = Ma = (240 + 80 + VHe* ρHe) kg.

    volume of the air = Ma/ρa = ...... = Volume of the balloon.

    So Vb*ρa = (240 + 80 + VHe* ρHe) kg

    Ηere volume of the balloon is equal to the volume of He.
     
  6. Aug 8, 2010 #5
    k...just want to clarify. The balloon will float when the mass of the displaced air is equal to the weight of the balloon plus the load.

    therefore,
    Ma = (240 + 80)kg including helium??

    how come is volume of air = volume of balloon?

    sorry...a bit confused. I'm not great with this kind of physics.
     
  7. Aug 9, 2010 #6

    rl.bhat

    User Avatar
    Homework Helper

    (240 + 80)Kg is mass with out helium. Mass of the displaced air is equal to the (mass of helium + 240 + 80) kg. Balloon will float only when it is filled with the helium. Since the balloon material and the load occupy negligible volumes, volume of the helium = volume of the air.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook