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Comment by wtvanhest | Hacker News Reader

For those that have studied physics in depth:

The article says: "5,000 times less dense than water, and six times lighter than air"… so what does it mean by lighter than air exactly? I presume in means that if it were in a vacuum that it would have less mass than whatever its outside edges of volume would be?

That seems misleading since if you break the material down to the smallest piece, then compared air volume to that piece it wouldn't actually be lighter right?

Am I thinking about this correctly?

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We've been down this road before. I think the misleading part is that it doesn't displace air to any significant extent.

By this logic, a roll of chain link fence is three times lighter than water[1]. But it still sinks.

EOM

[1] Calculations follow…

50 feet of 6 foot tall 9 gauge chain link fence rolls into an 18" diameter cylinder 6 feet tall and weighs 216 pounds. http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDi...

Mr. A. Wolfram says that is a volume a 80 gallons. http://www.wolframalpha.com/input/?i=volume+of+a+cylinder+18...

Water is about 8 pounds to the gallon, so the water of equivalent volume would be 640 pounds.

Steel fencing is one third the density of water! Write the press release!

Sorry about the units, world, at least I didn't break into microfortnights.

Great analogy, helped me understand why I felt this whole six times lighter thing was strange.

What's the smallest piece? A carbon atom? The only reasonable way to compare densities is to take a sizeable chuck of the material and compare it to the same volume of a different material.

If you break it down to the smallest piece, you're talking about Carbon atoms with an atomic mass of roughly 12 versus (mostly) Nitrogen and Oxygen with approximate masses of 14 and 16. So it's lighter as atoms but not by six times.

Most of the Oxygen in our atmosphere is O2, so there's a bond and space between the atoms as well as space between the molecules and others. In the same way, perhaps the carbon is more like closed cell foam that has voids that exclude other atoms. These voids have no mass but contribute to the effective volume.

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