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Name: C. J.
Status: educator
Grade: 9-12
Location: FL
Country: N/A
Date: 1/25/2006


Question:
How can I prove air has mass?


Replies:
I think the simplest (and most intuitively convincing) way to show this is simply to move your hand quickly through the air. Wave it in a wide, fast arc in front of you or stick your hand out the window of a moving car. You can feel your hand hitting something. That something must have inertia (mass), otherwise it would not offer resistance and you would not feel it. We have all felt air push against our bodies on a windy day. Without mass, there would be no "push".

Christopher Perkins


C. J.

Weigh a balloon and the inflate it. Weigh it again. That works. If you want to demonstrate it better make a balance with a stick suspended by a string in the middle. Tie an empty balloon on each side to prove they weigh the same. Inflate one balloon and rehang it. That side of the balance will be heavier.

Air is really quite heavy. It is just that it has always been there for you and me so we do not notice. Asking a human if air is heavy is like asking a fish if water is heavy.

Every square inch of surface on the earth has about 15 pounds of air sitting on it. (Air is piled about 100 miles high on each square inch.) Just for fun, calculate the number of square inches on the top of your head and multiply it by 15. Wow... you are holding all that up!!??

Larry Krengel


CJ,

The problem with weighing air in an empty, heavy container is that you would actually have to evacuate the air already in it or pump in more air (both of which require a pump assembly). The problem with weighing an inflatable container is that you will get buoyancy issues since the injected air will displace some volume and it would have to be weighed in ambient air.

The work around is to use a gas that is heavier than air (such as exhaled breath which is rich in CO2. This way the exhaled-breath filled balloon will not be buoyed as much (especially if you use a heavy rubber balloon) by the surrounding air. Another approach is to use a chemical reaction that produces some kind of gas (for example Mg in aqueous HCl) and note that the mass of the reactants is actually higher than the *remaining* products - meaning that the escaping gas must have had some mass. -- In both these cases you would have to extend the reasoning that CO2 and H2 have mass, and we can assume that air must have mass too.

Greg (Roberto Gregorius)


The fact that it stays near the Earth, instead of wandering off into space, seems TO ME pretty compelling proof. What force other than gravity could cause this, and what besides mass responds to this force?

Tim Mooney



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