Does air in an open bag add weight?

by fcacciola
Tags: weight
 P: 5 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?
 P: 907 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.
 P: 5 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?