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PSI and buoyancy

  1. Apr 24, 2015 #1
    A question about PSI and buoyancy. How does the air pressure of a container effect whether the container will sink or float? For instance, if two containers of the same mass and area (and the mass of the container itself is negligible), one container is filled with 10psi of air and the other 100psi, then weights are attached to them, will it take 10 times as much weight to sink the first container as the second or 1/10 the weight?
     
  2. jcsd
  3. Apr 24, 2015 #2
    Have you heard of Archimedes principle?
     
  4. Apr 24, 2015 #3
    Yes I have this leads me to believe that the 100 psi container would sink quicker due to the increased density but it seems counter-intuitive, wouldn't a balloon with high pressure take more effort to sink than a balloon of equal area but with a much lower pressure?

    I have the feeling I'm missing something blindingly obvious here and I guess I'm fishing for my own "Eureka" moment.
     
  5. Apr 24, 2015 #4
    The volume of the fluid displaced by the object is what determines the buoyant force. Suppose you had two equal volume containers but one had feathers and the other had lead. The buoyant force on each container would be equal if they were fully submerged in a fluid, but the force of gravity is certainly different.

    I think the balloon is perhaps a potentially confusing example because the higher pressure will result in a greater volume and thus in a greater buoyant force. Imagine that the balloon is stuffed into the container and not allowed to expand. Would a greater pressure inside the balloon in this case make it float more readily according to Archimedes?
     
  6. Apr 24, 2015 #5
    I think maybe your confusion comes from the fact that usually a balloon with higher pressure will displace more water, and because of that it would rise faster. But one of your constraints was that the volume of the balloons are the same (which makes them rather un-balloon-like, and rather more like solid spheres), in which case really only the total weight of the object matters.
     
  7. Apr 24, 2015 #6
    A balloon was probably a bad example; I was attempting to highlight the fact that I was referring to how air pressure relates to buoyancy.

    Let me redefine: If two identical static, sealed containers are placed in a tank, with one pressurized to 10psi of air and the other to 100psi, the 100psi container would have ten times the buoyancy, since to reach ten times the pressure in containers with identical area would require the volume of air to be 10 times higher. Is this correct?
     
  8. Apr 24, 2015 #7
    Nope, that is not correct.

    The real question is, do you fully understand Archimedes' principle? Because this right now seems like a battle between your intuition and Archimedes.
     
  9. Apr 24, 2015 #8
    My guess would be that Archimedes is correct...

    Would it be the opposite; the 100psi container has less buoyancy due to the greater density?
     
  10. Apr 24, 2015 #9
    Yup. All other things equal, the container with more pressure contains more molecules, and thus it is heavier.
     
  11. Apr 24, 2015 #10
    That's what I was thinking at first but for some reason it just didn't seem right. For some reason I was thinking that a highly pressureized object would be harder to sink than a less-pressurized one.

    Anyway thanks for the insight. I'm going back to the garage; motors make my brain hurt less.
     
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