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Yeah, depends what kind of resolution is required for analysis

Archimedes is also iffy above a certain porosity where the fluid will start to penetrate the sample. You will get the wrong measurement. You can smear some grease or coating on the outside to reduce interaction with the liquid.

The kinds you're describing are short-range systems that need to push off of something. So one could plausibly imagine a maglev train on the Moon, but not for deep space propulsion.

Magnets are involved in some types of ion engines (eg: hall effect thrusters), though those move more in common with conventional rockets.

Definitely completely porous is straightforward: Archimedes Principle will do that: weigh it, then immerse it in liquid and measure the volume change.

Where it's hard is semi-porous or mixed materials. Closed cell foam is an example of this - it's made of two very different materials, the plastic that makes up the foam, and the gas that makes up the voids. You can't non-destructively measure the density of the two parts together. The brutal approach is generally used - crush it to burst all the bubbles, then measure the density of the resulting lump.

This matters, because often porous materials contain trapped gas. So you measure the density as above and get quite different results from different samples. Think of immersing a sponge, then squeezing it - if bubbles of air come out you would have measured different densities before and after. How do you know that you have no closed cells containing gas? Answer: you crush it then measure it.

You can use helium pycnometry to determine real densities of solids, essentially you calibrate a pressure vessel to determine it’s volume, then fill the volume with a known mass of sample, and then recheck the volume of the vessel, giving density of your sample, you can do archimedes method as well, much less accurate and requires attention to solubility

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Take you dry sponge, put it on a scale, you have its mass. Take a graduated glass recipient with exactly 1L of water in it, put the sponge in it, read the new volume indicated, substract 1L to it, you have the volume of your sponge. Divide the mass by the volume and you have the volumic mass density of the sponge. Divide it again by the volumic mass of pure water (1000kg/m3 or 1kg/L) and you obtain its density.

Edit: corrected as I didn't get the correct english terms.

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It breaks some of the hardened salt (or sugar, or whatever crystalized substance) keeping the lid from turning. It doesn't work if the lid is stuck because of something viscous instead of hard; for those, a prolonged application of medium force works better than a very strong force of short duration.

The number of alleles that can be found in the gene pool of the population.

Us Finns for example descend from a very small number of survivors of some ancient famine and each of our genes can be one of only the few alleles that those survivors happened to carry and no other alleles. Our gene pool is one of the smallest as a direct consequence of that ancient bottlenecking event.

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You're talking about dentures and not implants, right? Generally the sensory feedback you get from chewing is from the teeth transmitting forces to the periodontal tissue, not actually feeling forces in your teeth through the enamel.

Dentures disupt this process not because they don't have nerves, but because they totally change the way forces are transmitted to the periodontal tissue. I believe that root-canaled teeth and bone-borne dental implants still allow force feedback like healthy teeth do.

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Thanks! It seems obvious in retrospect that protein folding is the mechanism here. Biochem was never my forte.

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>what is a cold receptor?

Aδ and C nerve fibers have a protein that responds to heat dissipation and triggers the “gentle cooling” effect. That protein is TRPM8 and on top of being activated by cold (due to part of the protein changing shape) it can also be activated by menthol and eucalyptol.

More on that here

I see it is a subjective feeling, but my nose is legitimately less stuffy after consuming these substances. How is that so?

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