I'm using those packing polythene bags you suck air out of, & they seem to be a lot heavier once totally flat.
2007-01-22 00:50:56 · 15 answers · asked by rokeith62 1 in Science & Mathematics ➔ Physics
Depends if you are talking of gross weight or net weight.
Any immeresed body that displaces something else (usually a liquid or a gas) is subject to a buoyancy force equal to the weight of the displaced matter.
Air does have weight, so that when you remove air from inside a volume, the weight diminishes. If the volume stays the same, the buoyancy remains the same. Therefore, the overall effect is a greater relative buoyancy (force per unit of mass).
However, in your case, the volume is reduced. I suspect that the weight of air that is removed from the bag at a rate very similar to the rate at which buoyancy diminishes so that the net weight (gravity - buoyancy) remains the same. However, because the volume is a lot less, the density must therefore appear greater.
2007-01-22 00:58:40 · answer #1 · answered by Raymond 7 · 1⤊ 0⤋
Let the mass of the bag be m, the mass being determined under vacuum.
Now the bag is filled with air of mass m and closed. The pressure of air is the same as the atmospheric pressure.
When we weigh the bag in ordinary room, the weighing machine will show the weight corresponding to the mass of the bag alone and not the weight of the bag and air inside.
As the air outside and inside of the bag is of same pressure, the upward buoyant force is equal to the downward weight of air, the bags weight mg alone will be shown by the machine.
When the air is removed, the upward buoyant force will also be reduced correspondingly.
Hence again the mass and hence the weight will be the same.
Only when the pressure of air is higher than the air pressure out side, there will be downward force due to air inside.
Hence the buoyant force has no effect in this case.
Instead of air if the bag is filled with hydrogen, even though the pressure is the same the mass of displaced air gives an upward force which is greater than equal volume of hydrogen. The weight in this case will be less.
Therefore the weight of the polythene bags cannot increase. As many has stated, it is only a feeling of heaviness.
2007-01-22 12:12:28 · answer #2 · answered by Pearlsawme 7 · 0⤊ 0⤋
Raymond is the only one who has understood. The weight (ie the reading obtained if you hang it from a spring balance) is EXACTLY the same.
You have reduced the mass by removing some air... but you have also reduced the volume. The reduction in volume will cause a reduction in upthrust (the small upwards force we all feel which is caused by the fact that we are displacing air).
Assuming that you have removed pretty much all of the air, and assuming that the fabric itself is incompressible (once the air has been removed) then the weight of air you have removed is exactly balanced by the fact that you are now not displacing that volume of air.
It's the same as the famous Swiss cheese problem!
Hope this is clear!
2007-01-22 11:10:02 · answer #3 · answered by Perspykashus 3 · 0⤊ 0⤋
They may seem a lot heavier but they're only slightly heavier. Air has weight... approx 14 psi at sea level. So sucking air out makes them lighter. They seem heavier because it's a smaller package with nearly the same weight.
2007-01-22 09:18:48 · answer #4 · answered by MadMaxx 5 · 0⤊ 1⤋
No, they are not heavier. They probably SEEM heavier because they're denser. Once you suck the air out, the total volume decreases; the weight is about the same (less the weight of the air, which is negligible). Weight/volume = density. It's like the old pound-of-feathers vs. pound-of-bricks riddle; they weigh the same, but the bricks seem heavier because they're denser.
2007-01-22 08:58:38 · answer #5 · answered by Grizzly B 3 · 0⤊ 1⤋
Nothing gets heavier when you 'take out' the air.... actually, now there is no air inside, the package will be a tiny bit lighter !
The weight is concentrated in a smaller package - hence the 'feeling' that it's heavier ...
2007-01-22 08:59:27 · answer #6 · answered by Steve B 7 · 0⤊ 1⤋
No, air has weight. They would weigh less, but not appreciably. A cubic foot of air at standard temperature and pressure weighs a bit over 1 oz.
2007-01-22 08:53:47 · answer #7 · answered by gebobs 6 · 0⤊ 1⤋
That's an interesting observation. It should weigh slightly lighter but once the ait has been evacuated, the package feels hard, making it feel heavier. A soft, fluffy one makes you think it is lighter.
2007-01-22 09:00:39 · answer #8 · answered by Anonymous · 0⤊ 1⤋
no, unless you weigh something in a closed chamber and such the air out of the chamber, than that something will weigh more per volume by the specific weight of air, about 30 grams per m^3
2007-01-22 08:54:56 · answer #9 · answered by Anonymous · 0⤊ 1⤋
it will actually weigh fractionally less, it may appear heavier because it will be heavy for it's size which changes your perception of weight
2007-01-22 09:04:33 · answer #10 · answered by blue_cabbage 2 · 0⤊ 1⤋