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Comment by Stratoscope | Hacker News Reader

> six times lighter than air

If it's lighter than air, doesn't it float away? Wouldn't this make it a bit hard to work with? Why doesn't the article say anything about this obvious consideration?

OTOH, the piece in the photo with the drop of water does not appear to be trying to float away - unless it's the water drop holding it down. So maybe it isn't lighter than air? Or is the piece glued down?

> Non-conductive objects, such as plastics, could be coated with aerographite to make them conductive — without gaining weight.

While the material may be very light, I'm certain its weight is nonzero (and non-negative). This doesn't even make sense.

Fascinating material, dumb article.

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A (dry) sponge is much lighter than water, but it might not float; when you put it in water, the holes fill up with water, and the question becomes whether the rest of the sponge is light enough to float.

It's the same thing here. When they say it's lighter than air, they're counting the whole structure's volume, with all its holes in it. When you put it in air, and the holes fill up with air, the question then becomes whether the rest of it is lighter than air on its own. (Probably not)

Coating plastic in this material would add mass, not weight. You can fill a balloon with helium, and in a vacuum it would be heavier because it has more "stuff" in it. But in our atmosphere, it is less dense than the air, so it floats because the air it displaces is pulled by gravity with greater force.

So the weight is non-zero, but it's actually negative in our atmosphere. It's mass is positive.

Or to simplify the same thing, "it behaves like a bubble in water".

Buoyancy. The weight is still positive (net force of gravity downward), but the air pressure is greater. In the case of the balloon, the added weight of the helium is overpowered by the increased air pressure due to the larger displacement volume, and the balloon follows the path of least resistance (lower air pressure, at higher altitude).

I think it's probably lighter than air if you weight it in a vacuum.

I think like a sponge in the photograph it has a fair amount of air in itself.

Just a guess though.

Air also has a fair amount of air in itself.

If 1 cube of a meterial has x% aerographite + (100-x)% air, while air has x% air + (100-x)% air, the material is still going to be lighter than air.

Specific to the water drop: that's a huge drop of water for that piece. Weigh that volume of air and compare it to a drop of water that size - there's no way it's even close. So even if it does float, the water would be more than sufficient to keep it down.

> If it's lighter than air, doesn't it float away?

You are confusing density with buoyancy.

Can you explain a bit more and help me with my confusion? Thanks!

Simply: Buoyancy is the result of a difference in weight when one less dense material displaces another.

What is the displacement of the aerographite samples? The article doesn't say, but the samples are small enough that I'd guess the forces of moving air (breathing near the sample) is more problematic than forces of buoyancy.

> While the material may be very light, I'm certain its weight is nonzero (and non-negative). This doesn't even make sense.

I don't know, if you coat a plastic with air, does it get heavier?

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