Wikipedia says that its density is 0.9 kg/m^3
At sea level and at 15 °C, air has a density of approximately 1.225 kg/m^3
Am I missing something here or is this just Boeing PR fluff?
The metal lattice itself weights 0.9 kg/m3; they're not counting the air between the metal bits, since there's no closed pockets in it.
It's the same reason that when weighing, say, a gold ring, you don't also have to count the air encircled by the band.
What he's trying to say is that the value they have given is the density of the overall object, not the material it's made of.
For example, if a 1L plastic bottle weighs 25g, then if we seal the bottle, it's density of the object is 25g/dm^3 or 0.000025 kg/m^3.
So yeah, PR fluff.
What's important here is the amount of weight/force it can take per kg, which is probably quite high due to the structure.
>if you filled it with vacuum and enclosed it in a shell of negligible mass so the air couldn't get in.
why do you need to do that? they claim it's less dense than air, you shouldn't need to fill it with a vacuum or enclose it in anything. a life preserver is less dense than water and has a giant hole in the center, it floats because it's less dense than water (weight of the water it displaces is greater than its own submerged weight).
For comparison, an average car has a total volume of about 6 m^3, and weights around 2,000 kg.
So the car as a whole, has a density of 333 kg/m^3. You should notice that's ~1/3 of the density of water, but cars don't tend to float on water.
tl;dr the density of an object doesnt mean shit if most of the object is empty space, you need the density of the constituting materials
Because the thing that matters for bouyancy is the *displacement* of fluid.
The lattice takes up space, sure, but because it's an open lattice it displaces very little air at all.
Let's say I have, I don't know, a 100 kg block of copper. If I drop it into a pool, it will sink.
And now let's say I take the exact same block of copper, cast it into a zillion tiny copper wires, and solder them into a toothpick-thin lattice that fills an entire 1-m3 cube.
This object is now 1 m x 1 m x 1 m and still weighs 100 kg, giving it a density of 100 kg/m3. This is 10x less than water. It will still sink.
> the thing that matters for bouyancy is the *displacement* of fluid
correct. the metallic microlattice is less dense than air, then the volume of air it displaces weighs more than the microlattice proper, then it should float. but it doesn't :)
No numbnuts, the microlattice is less dense than air, if you drew a little box around the microlattice and used those dimensions to calculate the volume. If you melted it down and made a very small box out of it that box would be denser than air.