New material 100 X lighter than styrofoam!:

Originally posted by WoodPush
You might be confusing weight and density, which aren't the same (the figures in the article and most of this thread were density).

But to answer your question:

Air doesn't contribute to weight measured by a scale, in general, because the scale is already calibrated with air pressure considered. So unless you change the density of the air (and with pressure. But the scale might have a different reading due to the change in air pressure.

Air pressure has nothing to do with what reading you get on a scale, since air is all around it pushing equally on every surface, for instance, a spring scale like you have in a grocery wouldn't give a crap if it was in total vacuum on the moon or on Venus, the weight would be the same with the exception of lighter gravity on the moon and such. For instance, if you used a balance scale, weights on one end and the stuff too be measured on the other side of the fulcrum, air only speeds up the rate at which it would settle down to a single reading but in a vacuum on Earth, it would do exactly the same except take longer to settle to a single reading because of no air drag.

Originally posted by sonhouse
Air pressure has nothing to do with what reading you get on a scale, since air is all around it pushing equally on every surface, for instance, a spring scale like you have in a grocery wouldn't give a crap if it was in total vacuum on the moon or on Venus, the weight would be the same with the exception of lighter gravity on the moon and such. For instance ...[text shortened]... ld do exactly the same except take longer to settle to a single reading because of no air drag.

Yeah, you're right. I recall that high precision scales have to be re-calibrated when air pressure changes, and I was trying to rationalize that. As you say, the air pressure really shouldn't make a difference. But I suspect that has more to do with the buoyancy of the scales mechanisms itself, and vaporization of moisture on top of the scale, than my previous explanation.

I was thinking perhaps parts of the scales mechanisms might just be in a vacuum, but given that they try to remove friction, that doesn't seem likely...

Originally posted by WoodPush
Yeah, you're right. I recall that high precision scales have to be re-calibrated when air pressure changes, and I was trying to rationalize that. As you say, the air pressure really shouldn't make a difference.

But my helium balloon example shows that it does make a difference - which has nothing to do with the mechanism used in the scale.
It has to do with the fact that every object in air experiences buoyancy from the air equal to the mass of the air it is displacing. (exactly the same way that buoyancy in water works).
So, if you have two 1kg masses, and one has a volume of 1cm3 and the other a volume of 2cm3, then the 2cm cubed mass will, in air, be measured to weigh less than the other by the mass of 1cm3 of air.

Originally posted by twhitehead
But my helium balloon example shows that it does make a difference - which has nothing to do with the mechanism used in the scale.
It has to do with the fact that every object in air experiences buoyancy from the air equal to the mass of the air it is displacing. (exactly the same way that buoyancy in water works).
So, if you have two 1kg masses, and on ...[text shortened]... cm cubed mass will, in air, be measured to weigh less than the other by the mass of 1cm3 of air.

Agreed, i thought we already covered that. Your examples are cases of using a scale to try to determine weight, and failing.

sonhouse and I were taking about affects of air pressure on the scale itself. Not on what the scale measures.

I thought we already agreed you were doing the wrong thing if you're trying to measure the weight of an object without considering its buoyancy, or doing the experiment in a vacuum.