Howdy folks,
Ok, I’ll just say right off the bat that this is probably a dumb question. So, /disclaimer.
I work with a knife currently, so cutting things has been on my mind. (In a good, legal way :smack:). My point is, that, when I cut a vegetable or some fish, I’m severing the actual, physical, molecular bonds at the point of the cut. I assume that’s true?
Well, with air it’s a different story. When I wave my knife around a little in the air, there’s no proof of cutting anything. Am I cutting the air but new air replaces it so fast that it’s impossible to notice? Or perhaps the molecules dont divide but it’s more like a pushing motion?
So, I’ve cut wind before, but I am not sure if I have ever cut air.
I’m not sure there’s a distinction between cutting and pushing aside molecules when it comes to air. It depends on your definition of cutting. However you want to define it, the knife is moving between molecules which are themselves moving. Once the barrier of the knife is removed, the molecules move to fill that space. Same with liquids. A knife is pushing aside molecules in more solid substances as well, but they don’t move as easily to rejoin, so we define that as cutting.
If you fill the air with smoke particles or some other colored material you can actually see this effect. The smoke will curl around the edge of the knife and regroup but a definite line through the smoke can be observed as the knife moves.
Or around you yourself, were you to walk through smoke.
Even solids can act in this way. Tie small weights to each end of a string, say, 1 foot long. Allow the weights to hang, and the middle to rest on an ice cube. After a momment, the string will have passed completely through the ice, yet the ice will still be intact above the cut.
There was a piece in an old issue of MAD. I think it was supposed to be a magazine from the (then) future . it had an ad for a “block” of New York City Air – “Harvested Fresh Daily”. But i don’t think we’re that bad yet.
Of course, you could freeze the air until it solidified, then cut it with a knife. No real trick there, except that you shouldn’t expect to be able to cut the portioon that’s helium. (See any good text on cryogenics. Or read Fritz Leiber’s “A Bucket of Air”)
>when I cut a vegetable or some fish, I’m severing the actual, physical, molecular bonds at the point of the cut. I assume that’s true?
Of course it’s true. Ignore the skeptics. If you mix up some epoxy and let it harden, every atom in the blob is chemically bonded to the same big complicated molecule, so in the sense of molecules it’s one giant molecule. The same is true of any single crystal. You can certainly cut these things. Now, in cutting fish, not every atom on the surface of the cut was bonded to an atom on the other side of the cut, but certainly some can be, more or less of them depending on the substance. It is not especially different from the point of the knife whether there are bonds or not. You can also argue about what is cutting and what is stressing until they pull apart, because the knife is probably dull on the scale of a bond (they’re usually something like a half a nanometer).
Gasses aren’t bonded together, except within one molecule. Each molecule goes flying around a distance like 1000x its own size between collisions with other molecules. You don’t cut gasses because they’re not really together.
You do cut liquids. Liquids have tensile strengths of usually a few hundred PSI. However, liquids have almost no notch strength, no resistance to tearing. So cutting them is trivial. Molecules in liquids have much weaker attraction to one another than they have within themselves, so cutting liquids generally doesn’t sever bonds. You could arrange it, though, if you shear a long-chain liquid like polydimethylsiloxane (eg Silly Putty) at low temperature and then cut it rapidly across the direction of shear.
I suspect that depends entirly on the solid. On the molecular scale the blade of a knife is so flat I can’t imagine it does anything but push small molecules aside. Polymers on the other-hand are at the other extreme (you mentioned epoxy) and most certainly you are breaking covalent bonds apart.
I think what he means is that the pressure from the string causes the ice under it to temporarily melt. There may not be much weight on it, but the string is tiny so the pressure is pretty high. That’s how ice skates work.
Anyone else remember the Camelot 3000 comic? At the end, Arthur swings Excalibur and we see the point splitting an atom. The next frame is a nuclear explosion.
You can’t cut air. The act of cutting is the breaking by physical force of chemical bonds in a solid. These chemical bonds don’t exist in a gas or liquid, so they can’t be broken.
I don’t know what your definition of a chemical bond is but as far as I know there is no kind of bond in the solid phase that doesn’t also exist in the gas and liquid phase. And once again, I think the size of the molecules in question determine if you are literaly breaking chemical bonds or not when you cut a solid. On the molecular scale, the surface of a very sharp knife is as flat as a piece of paper. (OK flat may not be a good description, but it does not come to a sharp point.)
