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Homework Help: Easy density question--no answer given

  1. Nov 24, 2015 #1
    < Mentor Note -- Poster has been reminded that they need to use the HH Template and show their work >

    A distant alien world has an acceleration due to gravity of g=100 m/s^2. Beings on this planet float in an atmosphere with a density of 5 kg/m^3, and they have a mass of 1 kg. What is their density?
    Last edited by a moderator: Nov 24, 2015
  2. jcsd
  3. Nov 24, 2015 #2
    I would take "floating" to mean neutral buoyancy - like a helium balloon after a few weeks. It just drifts with the breeze without trying to rise or fall.
    What has to happen to a helium balloon to get it to that state? As the helium leaks out of the balloon, the balloon gets smaller and its overall density increases and so it becomes less and less buoyant. What density does that helium balloon need to be neutrally buoyant?
  4. Nov 24, 2015 #3
    The density would need to be less than 5 kg/m^3 correct?
  5. Nov 24, 2015 #4
    Yup! Actually, it needs to be exactly that.
  6. Nov 24, 2015 #5
    Exactly 5 kg/m^3? Not less? I thought I would still need a calculation to find out the actual density of these beings despite knowing that it would be less than the density given. I'm not overlooking a formula that relates mass and acceleration, am I?
  7. Nov 24, 2015 #6
    I suppose less is okay. It would rise until its density matched the atmosphere.
    That extra information isn't very useful. Even if you wanted to determine how far up they would float if they were less dense, you would need the atmospheric pressure - which is not given.
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