How much does a cubic meter of air weigh?

[masraum]

Originally Posted by Superman

Interesting. If you filled a balloon with one cubic meter of air and put it on a scale, then subtracted the weight of the balloon, you'd have your answer. In terms of "weight."

Only if it's a mylar balloon. Rubber balloons are elastic. Therefore they exert pressure on the air inside. To blow up a balloon, you have to have more air inside than the volume at the same conditions to actually expand the balloon which means that you have more air inside the balloon's space than you would have if the balloon wasn't there.

[Superman]

Originally Posted by Flieger

No, you would just have just the weight of the rubber assuming the air inside the balloon is the same density as outside.

If so, then one cubic yard of air has zero weight.

[Superman]

Okay, so if you filled a mylar baloon with air, at exactly one atmosphere of pressure, and if you set it on a scale and the scale read exactly the weight of the mylar baloon, then the air inside has no weight. It might have mass, but no weight.

[Flieger]

Originally Posted by masraum

Only if it's a mylar balloon. Rubber balloons are elastic. Therefore they exert pressure on the air inside. To blow up a balloon, you have to have more air inside than the volume at the same conditions to actually expand the balloon which means that you have more air inside the balloon's space than you would have if the balloon wasn't there.

Good catch! I hadn't thought of that. So the air inside can't be at the same density.

Originally Posted by Superman

Okay, so if you filled a mylar baloon with air, at exactly one atmosphere of pressure, and if you set it on a scale and the scale read exactly the weight of the mylar baloon, then the air inside has no weight. It might have mass, but no weight.

The scale would read the weight of the air and balloon minus the buoyant force on the whole thing. That means the air inside the mylar balloon would have zero net weight and the weight of the rubber would be reduced by a tiny amount equal to the weight of a volume of air (same density as inside and locally outside the balloon) equal to the volume of the mylar.

[slodave]

I'll just add this in here for the heck of it.

At sea level, the standard atmosphere consists of a barometric pressure of 29.92 in. Hg. (1013.2 millibars) and a temperature of 15*C (59*F). This means that, under these standard conditions, the weight of a column of air at sea level will weigh 14.7 lb/in².

[RWebb]

Dave, how much is that in Pascals?

[slodave]

Turbo Pascal?

[slodave]

I had to look things up.

101325 Pa

[svandamme]

Originally Posted by Superman

Interesting. If you filled a balloon with one cubic meter of air and put it on a scale, then subtracted the weight of the balloon, you'd have your answer. In terms of "weight."

At that point, I would begin to wonder whether the "mass" of the balloon would be different from its "weight."

Well no.. because the content of the balloon were already on the scale before you put the balloon with the air in it on the scale.. + you got all the weight of all the air above the balloon, still on the scale, on top of the balloon.


so in short, you can't weigh "air" with just a scale.

unless you first put the scale in a room that has been pulled vacuum..
and then you put your filled balloon in there.

Ideally you take a non elastic , box that is 1 square meter..
close that box at regular athmospheric pressure

Seal it
take it to a vacuum chamber
reset the scale to zero
put the box on the scale

weigh it

open the box
reset the vacuum in the room
weigh it again

subtract.

[scottbombedout]

Wouldn't the balloon explode?

[Flieger]

We did the vacuum thing in a measurement lab as a demonstration. Marshmallows get really big. We did not try balloons.

We had to measure the mass of diesel fuel we were going to combust in a bomb calorimeter and we had to correct for the bouyancy since we measured in ambient air and to get the mass more accurate. The scale also had a box around it to keep the air currents from causing noise in the measurement.

[Jim Bremner]

I'm sure that LA Smogair weights in more than say air from Idaho.

[Hugh R]

If you have a hummingbird in a box on a scale and the box weighs 10 grams and the bird weighs 10 grams, (total weight of 20 grams) and the hummingbird then takes off and hovers inside the box, what is the weight reading on the scale?

[Jim Bremner]

Originally Posted by Hugh R

If you have a hummingbird in a box on a scale and the box weighs 10 grams and the bird weighs 10 grams, (total weight of 20 grams) and the hummingbird then takes off and hovers inside the box, what is the weight reading on the scale?

10 grams. it took me a second to get the hummingbird into the box. I had to readjust my tare since my box weighted in heavier.

[Hugh R]

Uh, no. Waiting for other responses.

[scottbombedout]

The birds wings will create downward thrust equal to its own weight?

[Rick V]

Equal or greater than?

[scottbombedout]

Dunno Rick, I am still waiting to see if my original calculation should include the 43% oxygen present in the wood

[Rick V]

Nerd

[RWebb]

Originally Posted by Hugh R

If you have a hummingbird in a box on a scale and the box weighs 10 grams and the bird weighs 10 grams, (total weight of 20 grams) and the hummingbird then takes off and hovers inside the box, what is the weight reading on the scale?

Initially, it will be the wt. of the box plus the wt. of the increased pressure downward caused by the wing beats

Later on, it will be the wt. of the box + wt. of the hummingbird

Much later on, it will be the wt. of the box + the non-water wt. of the hummingbird