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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This is actually an interesting question. Why does something sharp cut ANYTHING?
The main reason it doesn’t push cells to the side is that cells themselves are bound together to other cells by these things called “ligand” proteins. Ligand means “binding”.
Think of it kind of like tying the sides of boxes together from the inside with zip ties.
The other main reason is the cells aren’t rhe only thing there. Most of our tissues are made of things cells MAKE as well as cells, primarily proteins, and things like bone which are proteins plus minerals, things like skin which have living cells in the underlayment but layers of toughened dead cells on top, etc.
So the blade may or may not cut actual cells, but will cut tissues, made of proteins.
>the sharp edge just doesn’t push the cells to the side?
Well…they do. They just don’t go back together.
Because a sharp object does not cut by “dividing through” anything. The reason a sharp object cuts is because it applies force to a very small surface area. Because pressure is equal to force/area, a force applied to a small surface area results in a very high pressure. The high pressure punctures the surface, which is what makes the cut. A dull cutting instrument fails to cut because the cutting edge is rounded and therefore presents a larger surface area, resulting in a larger denominator and lower pressure.
To address your second question:
Atoms are very, very, very, very, *very* tiny. If you were to blow up the size of an atom to a beach ball, then all but the sharpest of blades would essentially be like an office building crashing down on it. And as the outer shell of your blade would be composed of electrons (charged particles), your blade would push aside any atoms it came across well before it could even possibly split the atom.
And even if that didn’t happen – the nucleus of an atom (the part you need to cut to split an atom and start a nuclear chain reaction) is an incredibly tiny morsel within the atom itself. If the nucleus was the size of a basketball, the atom it was contained within would be about 7 miles across.
So you would have to hit that with your blade, and even if it did, you probably wouldn’t break the nucleus. It’s held together by the strong nuclear force, which is… well, strong. It would be like firing a BB gun at a tank.
And even if you did all that, you still wouldn’t be in any danger. Splitting an atom does release a lot of energy relative to its size, but we’re talking atomic scale. For reference, your smoke detectors are “splitting” atoms constantly.
Splitting an atom is only dangerous under certain very specific conditions.
On a side note. Some surgical needles are taper point. They don’t actually cut anything. They just push through tissue
So this might not be super ELI5, but our tiny cells are not just bound together physically (like pushed together), but they’re bound together through chemical processes, like glue. Once it’s dried (the cells have formed and been created and specialized), they’re more “rigid”. Normally skin stretching is like sticky putty that pulls apart but snaps back together. That knife or cutting surface breaks that “rigid” bond and just like glue, after its set, you can’t just stick it back together and have it reform.
Same thing that happens when cutting as pulling apart. Once it reaches its limit of distance apart it’ll break. Cause the knife or whatevers edge is so fine, it wedges itself between those cells and pushes them apart much quicker. It’s much more efficient to do this than to pull for that reason, that sharper angle increases the tension.
That’s why your body needs to send clotting factors and fill the gaps, it’s starting that gluing process again. As your scab is forming, your body is sending tiny little packets of glue through your blood to bit by bit stick to the edges until it closes the hole. These then dry/set in place and are pushed outward by more cells that keep piling on. Your body is pretty much constantly making these new bits of glue at the bottom later of your skin and pushing the old ones outward.
The topmost layer of your skin is actually a lot of old dead skin that is not alive but serves as a protective layer before it flakes off.
I think it’s best to think of your scab as a gluing process but its coming from the inside instead of out. You still need to wait for it to dry.
When you get a cut you mostly do just push the cells aside. Some of them will be damaged, but the majority are just pushed out of the way.
As for atoms, they really really hate touching other atoms. There are very strong forces that makes sure atoms are mostly empty space. So to actually cut an atom it takes extremely strong forces to overcome this.
Which is why nuclear bombs are initiated by a shaped charge of conventional explosives- it takes a bomb to force the atoms close enough together to start smashing into each other.
A knife doesn’t split atoms. The force that holds nuclei together is very strong, and cannot be overcome by a knife wielding human.
Knives work on a larger scale, by either pushing molecules apart or by splitting them up. The knife concentrates enough force in a given area to disrupt the molecules the object is made from.
Back to your original question, a knife both cuts cells and pushes them apart. It breaks up the molecules they’re made from, but some will be aligned just right so that the knife is aligned on the boundary and pushes them apart instead of breaking them up.
> DISCLAIMER: this is not the “right” answer, but i think it is clear enough without being utterly wrong.
Why do sharp things cut *at all* ?
Whenever something is bound to something else, this binding has a strength.
It works for people holding hands, cells sticking together in a tissue, molecules with other molecules, atoms to atoms and even sub-atomical particles with other particles.
The nature of this bounding varies from case to case (muscular – electro magnetic – strong nuclear force) but the final result is that the two “particles” resist being separated.
An external or internal “agent” can sever this binding by opposing and overcoming the strength that defines it.
In our most basic description of these bindings, the strength that defines them is the **energy density** -> it means how much **energy** is contained in a specific **volume** of the material.
This means that if the binding has a strength of 1 energy density, if you apply more than 1 energy density, you will break that bond.
**Energy density** is (by definition) equal to **pressure** (how much **force** is applied to a specific **surface**).
Sharp blades offer a very small surface, allowing for high pressure even with small forces. This means that the **pressure** of a blade is enough to overcome the **energy density** of some of the bindings that define your body.
A blade, for example, can break the binding between two tissue molecules, but can’t break the binding between atoms inside a molecule, and even more so can’t break the binding between sub atomic particles.
It can all be described as matter of how energy dense is a binding, and how energy dense is your cutting agent.
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