Air's Weight

[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?

[abskebabs]

You could place some kind of container on a mass balance and evacuate the air using a vacuum cleaner or something like it to create a vacuum. if there are no leaks in your container; then when you zero your mass balance it should read zero[you'll have to figure out how to do this, perhaps you could use a remote controlled signal]. If you now allow air into your container you should immediately get a reading. The fact you have been able to measure its mass shows the force of gravity is acting on it and it therefore has a weight[really your only measuring its weight].

 

Hope this helped you.

[entwined]

take a small fan and turn it on.

 

If you can feel the air moving, if has weight.

[herpguy]

Simply remind everyone that there is no air in space, it is all pulled in by the earth's gravity. If something has no weight, gravity cannot effect it. So air must have weight.

[Sisyphus]

One easy way to show that air has weight is to fill a helium balloon. The balloon floats upwards because the helium inside it weighs less than the air outside it.

[Kedas]

Take a glas with water and a straw in it.

Drink from it via the straw.

If the weight of air didn't press the water in your mouth it would not be possible to drink from it.

[abskebabs]

One easy way to show that air has weight is to fill a helium balloon. The balloon floats upwards because the helium inside it weighs less than the air outside it.

I was gonna say that but it doesn't really show that air has weight, it just shows that helium is a lighter gas with faster moving molecules.

 

Also it shows that that the Helium molecules are moving too fast to be accelerated[initially decellerated if you consider their motion away from the centre of the Earth] back towards the Earth before they exit its atmosphere.

[Sisyphus]

A lighter gas than air, meaning air necessarily has weight. The speed of its molecules is its temperature. Filling a balloon with ordinary hot air would work also. The point is there's a difference in weight.

[EvoN1020v]

Don't you guys think mathemactially? How did scientists were able to determine the value for the gravity force?

 

I'm a mathematican myself, and this idea just appeared in my head from nowhere. Consider this:

 

Stand somewhere high above the ground, and hold an apple in your hand. Get a webcam and position it so it can capture the motion of the apple. Whenever you drop the apple, start the capturing video. The recorded video should tell you the time it hit the ground.

 

2 important values: height of initial dropping: ([math]V_i[/math]) and time the apple hit the ground ([math]T_2[/math]). You should know that ([math]T_1[/math]) should be zero too.

 

With those values, you might can determine the quadratic equation for the slope on a speed vs. distance graph. Then, the y-intercept value should be at [math]9.81m/s^2[/math].

 

I know you might not understand what I'm saying, because you're in grade 7. But this is a great experiment that you can determine the gravity's value. You can ask your high school friends to help you.

 

Hope this helps.

 

 

 

Note: I thought of the weighting scale at first, but that won't work, because those weighting scales are made to push gravity force which equals to 0.

[insane_alien]

take a container(about a litre or 2 should be ok) capable of withstanding sustained pressure, zero a scale with the unpressurized container, then pump lots and lots of air in. if you attach a pressure guage and do some maths then you should be able to calculate the weight of air with a bit of accuracy.

[swansont]

One easy way to show that air has weight is to fill a helium balloon. The balloon floats upwards because the helium inside it weighs less than the air outside it.

 

Or fill it with normal air at higher pressure (i.e. make the balloon stretch), and compare that to an empty balloon.

[entwined]

Or fill it with normal air at higher pressure (i.e. make the balloon stretch), and compare that to an empty balloon.

 

I am told that a scuba diving tank is measurably heavier (on a common household scale) when filled than when empty.

[insane_alien]

I am told that a scuba diving tank is measurably heavier (on a common household scale) when filled than when empty.

 

Yeah thats true. Its because the air is stored at very high pressure (300atm?)

so if its a 10litre tank when filled it has the equivalent of 3000 litres of air in it.

[John Cuthber]

When I did this (many years ago) as a high school experiment we got coke bottles fitted with bungs, and short lengths of rubber tubing. We weighed the bottles (etc) full of air then drew the air out with a vacuum pump (a vacuum cleaner doesn't produce a nearly good enough vacuum), tied the rubber tubing to seal out the air then re-weighed the bottles. I remember being quite suprised at how much the air weighed -it's about 1.2 g / litre. For most classrooms the air in the room probably weighs more than you do.

(BTW, Evon1020v I may or may not think mathematically, but I can't for the life of me see what measuring "g" has to do with the question.)

[swansont]

I remember being quite suprised at how much the air weighed -it's about 1.2 g / litre.

 

One mole of an ideal gas takes up 22.4 L, so 1.2 g/L would be about right for air. The exact value will depend on the humidity, pressure and temperature.

[Bluenoise]

Or fill it with normal air at higher pressure (i.e. make the balloon stretch), and compare that to an empty balloon.

 

Easiest method yet! this is the one to do.

[YT2095]

how about the old Classic as seen in some bars/pubs, take a shot glass fill it to the top place a crisp $20 or £20 (or even a beer mat) over the top of it.

with your finger over cover, invert the glass, then carefully remove your finger, the liquid stays in the glass seemingly defying gravity, how? the weight and subsequent Pressure of the air holds the mat/20 in place

[swansont]

how about the old Classic as seen in some bars/pubs' date=' take a shot glass fill it to the top place a crisp $20 or £20 (or even a beer mat) over the top of it.

with your finger over cover, invert the glass, then carefully remove your finger, the liquid stays in the glass seemingly defying gravity, how? the weight and subsequent Pressure of the air holds the mat/20 in place [/quote']

 

You have surface tension helping you out, but the main effect is that for the liquid to leave it must be replaced by air, and no air can get there. Same effect as in a barometer bulb, where you draw a vacuum on the top.

[Sisyphus]

It's not that the liquid has to be replaced by air as a general principle, its that the air pressure outside is greater than the weight of the liquid, and thus the liquid pushing down is more than compensated for by the air pushing up. (The same thing happens when you suck water into a straw, then cover the top, and the water doesn't fall out when you take it out of your drink.) This air pressure is caused by the weight of the air above you. A column of air from sea level up to the top of the atmosphere weighs about as much as 30+ feet of water, hence, you could do the same trick with up to 30 feet of water, but no more. After that, part of the liquid would fall out, leaving vacuum behind it until the weight of the liquid once again equals the weight of air. This is how a barometer works, and why air pressure is often measured in height of mercury - the amount of mercury equal to the air's weight.

[s pepperchin]

It's not that the liquid has to be replaced by air as a general principle, its that the air pressure outside is greater than the weight of the liquid, and thus the liquid pushing down is more than compensated for by the air pushing up. (The same thing happens when you suck water into a straw, then cover the top, and the water doesn't fall out when you take it out of your drink.) This air pressure is caused by the weight of the air above you. A column of air from sea level up to the top of the atmosphere weighs about as much as 30+ feet of water, hence, you could do the same trick with up to 30 feet of water, but no more. After that, part of the liquid would fall out, leaving vacuum behind it until the weight of the liquid once again equals the weight of air. This is how a barometer works, and why air pressure is often measured in height of mercury - the amount of mercury equal to the air's weight.

 

 

The principle behind why the liquid stays in the glass is because of pressure. The explanation of why pressure holds it there is because any amount of water leaving the glass would creat a vacuum at the bottom of the glass. This vacuum would have lower pressure than the air outside and that is not possible. The reason the water will flow when you lift the glass is that the area over which the air pressure is applied becomes great enough that at some point an air bubble gets inside and rises to the bottom of the glass.

[Sisyphus]

Yes, exactly. But it's not because of some principle of "nature abhorring a vacuum." The vacuum has nothing to do with it. It's simply a matter of the weight of the water vs. the weight of air. If you do the straw thing, where the area is small enough that surface tension prevents a bubble, then you can keep exactly as much water in the straw as is equal in weight to the column of air above it, which is about 31 feet at sea level. More than that, and it will start to create a vacuum at the top, and the rest will fall out until equilibrium is reached. In other words, if you put a 40 foot straw under water, then cover the top and lift it out, there will be 31 feet of water in the bottom of the straw, and 9 feet of vacuum at the top.

[s pepperchin]

the issue is not the weight of the air above the water it is a question of the pressure. If you could change the gas trapped at the top of the straw to something else then the water would shift so that the pressure on the top and bottem was the same. For a rough estimate you could use the ideal gas law to estimate the change in pressure for changing the gas. And you would be able to figure out how far the column would move.

[Sisyphus]

Right, and the air pressure exerted in the open atmosphere is the result of the weight of the air above it. Hence,

 

you can keep exactly as much water in the straw as is equal in weight to the column of air above it

 

If you let air in at the top of the straw, you've introduced a whole new source of pressure besides the weight of air and the weight of water, so it's not really relevant.

[swansont]

It's not that the liquid has to be replaced by air as a general principle, its that the air pressure outside is greater than the weight of the liquid, and thus the liquid pushing down is more than compensated for by the air pushing up. (The same thing happens when you suck water into a straw, then cover the top, and the water doesn't fall out when you take it out of your drink.)

 

Pressure isn't the whole story (though it's correct and a different way of saying what I did). It doesn't fall out in the case of the straw because of the surface tension/capillary action not allowing the water to flow. Try it with a larger diameter tube and the water will most certainly drain out once it leaves the reservoir.

[KFC]

If you had a big jar and it was just big enough to hold the Eiffel Tower the air in the jar would be heavier then the Eiffel Tower.