Weight of Air experiment

[totocyn]

Hey, I'm a grade seven in school, and my topic for a science fair is air's weight. Does anybody know an experiment that can show that air has weight? (P.S. If anybody's gonna suggest the pop can one, please explain to me how it has anything to do with air's WEIGHT... I can understand that the atmosphere changes around the can or something, but i don't get how it shows that air has weight)

[pagey]

The fact that air is held around the planet by gravity proves it has weight - but I am unsure of how to prove this in an experiment. You could weigh a container of air and a container of vacuum but you will need very sensitive scales!

[jake-the-peg]

http://www.newton.dep.anl.gov/askasci/gen01/gen01355.htm

The reason a tin can is crushed when you pump air out of it is because of the weight of the air on top of it. When the can's full of air the air presure inside the can balances the weight of the air on top of the can. Pump it out and the weight of the air crushes it.

[Hamish]

Get a 2-3l flask, weight it. Evacuate it, weigh it again, the differance must be the mass of air removed. Perhaps not easy to do in the average school, but not technically difficult in the scheme of things.

[brachiopod]

Weigh a deflated bicycle tyre. Then inflate it, and weigh again.

What you are doing when you are inflating the tyre is "squashing more and more air in". Since air has mass (what you call 'weight'), the inflated tyre will have a greater mass.

[Gnisy]

I can't really remember how the experiment goes but you're supposed to blow up two balloons and tie them to both ends of a rod. Then hang the rod at the middle so both balloons are hanging freely. Do two sets of these with the first set comprising of two balloons of the same size ( and hence they will more or less balance ) and the other set with one bigger sized balloon and a smaller sized balloon.

The hypothesis is that since both balloons are of the same weight and hanging freely ( no external force is acting upon either balloons apart from gravity ) the only thing that is causing the second set of balloons to be lop-sided is that air ( inside the bigger balloon ) has weight!

[Space]

has anyone actually managed to measure a difference between an inflated baloon/flask/bicycle tyre and a deflated one ?

I suspect that the inflated ones will be pushed upwards by the buoyancy force that deflated objects dont have, and that will compensate for the added air weight ! so how can a difference be measured?

If the pressure of air inside the object is nearly the same as the outside pressure, I would be surprised that a difference can be measured...
I suppose the only way to measure a difference is to make sure there is a high pressure difference between the object and the outside ...
I would start with finding an object that can handle high pressure ( either positive pressure of vacuum )

[mibn2cweus]

If a balloon can sustain enough pressurization of the air inside popping one of them should effect the balance?

[mibn2cweus]

If the balloons weigh the same initially the one with more air in it (provided the air inside is sufficiently pressurized to be measurably differant that the air outside) should be heavier.

[mibn2cweus]

As Space has noted the pressure differential would be crucial to the success of the experiment.

[Gef]

Very easy and I do it every year with my year 12 students ( as part of another experiment). Weigh a 500ml round bottomed flask fitted with a stopper, tubing and clip. Evacuate the flask by connecting to a vacuum pump, close the clip and reweigh - it weighs less.

[tony1941]

I seem to remember learning that air at normal temperature & pressure weighs close to 1Kg per cubic metre. This equates to only 1milligram per litre so you will need a decent balance to see the effect in a small flask - 500ml would only be half a millligram.

[brachiopod]

Space, this 'bouyancy force' you mention certainly does count. We all experience an upthrust force due to atmospheric pressure, just as we would when immersed in another fluid - water.

"I suspect that the inflated ones will be pushed upwards by the buoyancy force that deflated objects dont have, and that will compensate for the added air weight ! so how can a difference be measured?"

Whaa-aa-at??? A tyre, say of fixed volume, (enough to keep it in a tyre shape), will squash if you sit on it. However, because of its volume, experiences an upthrust due to the fluid in which is immersed (ie. the atmosphere).

Now, inflate that tyre by putting a lot of extra air into it (which has mass) then it weighs more - it's volume, and therefore the upthrust, however, remain the same. Hence the increase in mass is due to the air added internally.

"If the pressure of air inside the object is nearly the same as the outside pressure, I would be surprised that a difference can be measured." - quite right, space, that's why I like to make sure that the pressure inside the tyres of my bike and on my car are considerably higher than 'the outside pressure'.

[maelogman]

At the risk of adding to a long list! If you take an ordinary balloon and put it on a top-pan balance before and after inflating it, it does weigh more when inflated. Balance needs to be fairly sensitive, but a litre of air weighs about 1.4 g.Sitting it on the balance pan eliminates a lot of the upthrust/buoyancy.Good luck

Maelogman

[Top_Gooner]

when i was at school, we used a leather football, deflated and then pumped up and weighed the difference.