I have in mind this magic system that allows people/animals to change density. Now, from What I understand the denser a thing is the more resistant it is to being damaged or broken.

My question revolves arownd how dense could you make a human/animal body before it breaks under its own weight or before you couldn't stand up. Would it make any difference if you were able to augment the densities in different amounts?: the muscles 2 times as dense and the skin 3 for exaple. Would a density change in the muscular fibers affect your strenght? Could you stop bullets, avoid being cut/piercet by weapons/claws or could you at least resist more the punches of someone else?

Edit: To clarify. Mass increases while volume remains the same. How much denser could a normal human body get before handwaving becomes necessary? And what aspects would require to be handwaved?

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    $\begingroup$ If bones are denser, then they're also stronger. But it's unclear how magic changes "density". Density is the ratio of mass to volume. Is it packing more osteocytes into the same volume? Do they have the same number of molecules in their bone, but the mass is cranked up somehow? If the latter, that wouldn't help you at all, your bones are as weak as ever, but carry a higher load because the planet still pulls on you proportional to your mass. As it stands now, this is nearly a nonsense question. $\endgroup$ – John O Jan 30 at 18:01
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    $\begingroup$ This question can't be answered as written because it's entirely dependent on where you're drawing the line between realistic physics and handwaving. remember that most of a human body is water and generally speaking water CAN'T be increased in density without turning it into something that's not water, which would prevent the chemical interactions your human being depends on from occuring, and killing him outright. For example. $\endgroup$ – Morris The Cat Jan 30 at 18:22
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    $\begingroup$ Edits should not invalidate existing answers. $\endgroup$ – BLT-Bub Jan 30 at 21:36
  • $\begingroup$ Density is defined in phyics as $\rho=\frac m V$. Muscle density and Bone density have physical limits, Human muscles can't become denser without making them stronger, and trained human muscle is just as dense as any other mammal muscle. Human bones are not the most dense, cattle bones are at times factor 2 to 3 to times more dense than human bones. That's it. $\endgroup$ – Trish Jan 30 at 21:59

My question revolves around how dense could you make a human/animal body before it breaks under its own weight or before you couldn't stand up.

That's not what you have to worry about.

If your mass remained the same, but the density was altered, than you would shrink. I am assuming this to be the case, as you say that the magic system allows for an adjustment of density, but not mass so I'm assuming that the law of conservation of mass is unaltered in this case. This is where I invoke the square-cube law, but the thing is that we're assuming the mass remained the same, so the only change here is relative surface areas. (We're going to assume that you adjust the density ratios on the fly to keep the proportions intact in the smaller form.) This is bad, because the surface areas shrink, limiting the rate of exchange for important things, like gas exchange in the lungs.

That is to say, when density increases, you'll have trouble breathing. Doubling the density of a human being would lower the surface area by 1/4, given that you're essentially cutting the size in half. And that isn't just the lungs that's affected, it's anything that needs exchanges across a membrane all across the body, effectively cutting everything to a quarter. Which is more or less deadly. Honestly, if you want to be science-based and biological about this, increasing density without improving mass is a death sentence.

Increasing density while increasing mass isn't that much better, by the way, because then your body needs to upkeep that mass as well and you run into the same problem. Humans operate at the current density because it works well for us, adjusting the density by a major factor would be very, very problematic. Unless you just want to use magic to ignore all these problems.

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  • $\begingroup$ I assumed mass was not conserved in the OP's post; I assumed volume was. $\endgroup$ – Zxyrra Jan 30 at 18:41
  • $\begingroup$ @Zxyrra Last paragraph addresses that - increasing mass means that you have more body to support, and the current human anatomy isn't designed for that. $\endgroup$ – Halfthawed Jan 30 at 18:48

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