So, we’re all made out of cells, right? And these cells are really super tiny, so why is it when we accidently cut ourselves, like a papercut or from glass, the sharp edge just doesn’t push the cells to the side?
Further, do those edges push aside out individual atoms too, or somehow have we just been avoiding death via splitting atoms?
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Humans are mostly held together fairly tightly (we maintain a fairly rigid shape), even in the soft fleshy bits, so the flesh resists the force being imposed by the piercing object. Some of the cutting does pierce cells, break their resistant cover (membrane), but cells are somewhat squishy (deform rather than rip) so a lot of the work done by the piercing implement is in forcing apart cells rather than ripping them open.
You might get an idea of how the cells “want” to cling together with other cells when you look at a scrape and see partially-detached strips of skin or flesh (flaps of flesh that you can sort of put back into place).
As to the atom-level question, in a certain sense all cleaving is the forcible separation of neighboring atoms that have some sort of attractive force keeping the neighbors together in a group, so the separation is the imposition of a force that exceeds the force of atom to atom attachment, but that view fails to account for the many different ways that atoms glom together, and the very different levels of energy involved between the different atom-atom attachments. Sort of also ignores the different behavior of fluids (liquids, which move around fairly easily) versus solids, and the intermediate plastic-behavior shown by many mixtures or solutions.
As a general idea, it is usually easier to make a clean cut on a low-liquid content item than its “wetter” equivalent because low fluid content means most of the energy is taken at the point of contact rather than being diverted by movement of liquid away from the point of contact. This is why we dry wood before we convert it into lumber or chop it into firewood, for example. Or well-cooked meat is generally easier to cut cleanly than red or rare-cooked meat.
When you get cut, its likely that much of the effect is in fact due to pushing cells aside rather than cutting through them.
The mechanical integrity of our skin and bones and blood vessels is for the most part not based on cells (which are soft and flexible), rather all the stuff around and in between cells.
Keratin and collagen in our skin, collagen and laminin in our soft tissues, hydroxyapatite in our bones, all that stuff is *outside* of our cells.
When something cuts through your body, it might cut through some cells, but much of the effect comes from cutting the extracellular matrix that actually supports weight, keeps blood inside of vessels, etc.
Some cells, like nerve and muscle cells, are long and immobile and thus susceptible to being cut, but a lot of the cells in your skin and fat and glandular tissue will just get pushed aside.
The way I understood it, when you cut yourself with a knife, the blade of the knife actually does just essentially push your cells aside and separate them. The reason this cuts you instead of your skin just deforming like a bean bag and then going back into shape is because:
Our cells have a sort of bonding to eachother, which makes skin a somewhat rigid surface which would make it easier to cut – for example – if you stuck a knife into a jar of sugar, it wouldn’t cut the sugar granules because the sugar granules are loose so the knife just pushes past the granules, whereas if you pressed down on a sugar cube with a knife, it would break the sugar cube or cut it because the sugar granules are bonded together so it’s a rigid structure meaning the knife can’t just push past them because it’s not loose.
And following on from this, yeah, the knife just for the most part delegates your skin cells and pushes them aside. The thing is, once they’re pushed aside from eachother, the cells can’t just instantly reattach to eachother, so they stay separated, and this is what the cut is. Then when the cut heals it repairs any damaged cells and essentially bonds/glues the cells back together.
About the atom splitting part, I might be wayyy off on this, I haven’t studied chemistry since highschool but here goes. The bonds that hold the nucleus together are incredibly strong, so strong that you with a knife are not strong enough to break them. To build on this, nuclear bombs actually require a conventional explosive inside the nuclear bomb to initially split the first atom – basically the conventional explosion rams one piece of nuclear material that’s shaped to be pointy, into another piece of nuclear material with such force that it splits the atom. Your arm obviously doesn’t have the force of an explosive, so you’re not going to accidentally split an atom because of how strong a nucleus is held together. Idk if im just repeating myself lol so sorry if I am.
remember that on an atomic scale, the surface area of a knife blade is like the width of a football field (side note that’s not actually an accurate scale, but in ELI5 terms the surface area if a knife blade is still absolutely massive on an atomic scale), and a single atoms nucleus is absolutely tiny.
And even if you did miraculously split an atom with a knife, I may be wrong on this but I don’t think you’d even notice anything happen. Atoms store a tremendous amount of energy **relative to their size**. The energy stored in a single atom, if released, you literally wouldn’t even notice I think. The reason nuclear bombs are so powerful is because it creates a chain reaction. It splits one atom, which splits another atom, which splits another atom, and then within a very very short space of time, gazillions of atoms have been split, all releasing their energy in that short space of time.
I don’t know how many atoms there are in the nuclear material parts of a nuclear bomb, but for reference isn’t it meant to be true that there’s more atoms in a tin can than there is grains of sand in the world? I mean…it’s not like they have counted every grain of sand, but you get the idea, there are a lot of atoms in something as small as a tin can. Now think about how many atoms there are inside a nuclear warhead. Whatever the number is, it has a lot of digits to it. That’s why a nuclear blast is so big and powerful, and why a singular atom being split and releasing its energy would be unnoticeably small and pathetic lol
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