Does air in an open bag add weight?

  1. This is a dumb question but I can't find a definite answer.

    If I have, say, a nylon groserry bag and I weigh it *when is open*, then I close it, then weigh again.
    It's quite clear that, depending on how I close it, different amounts of air will be locked in the bag so it will weigh differently, but, when it is open, can I account for *any* of the air that is "inside" the bag?

    In a conversation I had, I argued that when the bag is open, none of the air "inside" it counts, at all, so when you close it, it always, neccesarily, weighs more.

    How is it?
     
  2. jcsd
  3. jbriggs444

    jbriggs444 1,569
    Science Advisor

    The air in the bag has weight. But it also has buoyancy. If you are weighing this bag in air then the net is zero -- the buoyancy matches the weight. The downforce on the scale will be unaffected by any air that is considered to be inside the closed bag or outside the open bag.

    If the air inside the bag were for some reason lighter than the air outside the bag then this could affect the scale reading -- think hot air baloon for instance.
     
  4. Ahhh, interesting, I was thinking that the weight of the air is actually based (IIUC) on its pressue but then there is pressure on the outside as well, so, I was sort of coming to the conclusion that the "sideness" of the air might not affect at all, "solving" the apparent dicotomy.

    So, in efect the air within and around the bug just doesn't count at all, right?
    And even if the bag is closed at different positions, with different amouts of air, it would still all scale exactly the same (assuming like you said uniform conditions of pressure and temperture), correct?
     
  5. Nugatory

    Staff: Mentor

    As long as it's all the same kind of air, yes. Take your sealed bag full of air into a vacuum chamber and put it on a scale, and it will have a certain weight; poke a hole in the bag and the weight the scale reads will go down as the air escapes. Fill the bag with carbon dioxide ("heavier than usual air") and it will weigh more than if you fill it with hydrogen ("lighter than usual air").
     
  6. At the risk of complicating things... a bag full of air has greater mass. So if you were to spin round like a Top the one full of air would pull on your arms slightly more because there is a greater rotating mass. (Obviously you couldn't actually do that experiment easily because the bag full of air is larger and has higher drag). Stop spinning and they both appear to pull on your arms equally for the reason others have given.
     
  7. Interesting...
    Now speaking of mass vs weigh, if the bag is open, can you consider that there is any amount of air mass "within" the bag.. if yes, how would you measure it?
    My initial intuiton was that, not being any clear boundary between air inside vs outside, then none of the air would have to be considered as being on the inside unless the bag is closed (which is why I argued that being closed it weighs more since it "contains" air.. now I see that I was actually thinking in terms of mass, not weigh).

    If I replace air by water then clearly there is water "inside" regardless of how open the bag is, or how the water would end up spilling off the bag... unless, the open bag is fully summerged into a water tank... so, how is it?

    IOW, what exacty draws the disctinction between being inside or not of an open container? A glass with half water also contains half of air, or just half of water??
     
  8. HallsofIvy

    HallsofIvy 40,382
    Staff Emeritus
    Science Advisor

    First, when you set up a scale, you have to "zero" it which means that the weight of the air sitting on it is discounted. If you put a bag on the scale, to weigh it, the air in the bag, whether the bag is closed or open, just displaces the air that was already there and so does NOT add anything to the weight of the bag.
     
  9. Regardless if the bag is opened or not the air inside has weight but as already stated psi inside and outside the bag are equal but the weight can be determined by zeroing a scale inside a vacuum then weighing a sealed bag of air in the vacuum. Zero the scale in a vacuum then bringing it into normal atmosphere will give the weight of air inside and outside the bag due atmospheric pressure on the scale. Also an open bag of air always weighs the same but a sealed bags weight changes depending on where you filled it versus where you weigh it. If you fill a bag at water level it will be heavier atop MT Everest and lighter in Death Valley due to the difference in atmospheric pressure.
     
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