Cutting power by blade thickness.

[Traab]

An odd little random question that popped into my head. I know that there are math formulas for determining how much force is being applied through say, a sword swing and that one of the factors is the width of the blades cutting edge as that determines how much force is being applied to a smaller surface area. A mallet being swung at 20 mph will impart a lower total force to its impact point than an axe at the same speed. What I was curious about is, can anyone do the math on this hypothetical? Lets imagine you have a sword, its cutting edge is one molecule and it is infinitely hard so it will STAY at that thickness without dulling no matter what you swing it into. What could an average human cut through with that blade that they couldnt cut through with a regular sword? Basically, what sort of numbers are we looking at if both swords are identical, both swings are identical, the only variable is one has a mono-molecular edge.

[Grey_Wolf_c]

A mallet being swung at 20 mph will impart a lower total force to its impact point than an axe at the same speed.

No, it will impart the exact same, but over a larger surface.


Lets imagine you have a sword, its cutting edge is one molecule and it is infinitely hard so it will STAY at that thickness without dulling no matter what you swing it into. What could an average human cut through with that blade that they couldnt cut through with a regular sword?

I'm not sure this would be a good example. The issue is that it's no longer about impact force: monomolecular blades tend to simply cut the atomic bonds of the item they are encountering, rather that engage in the usual transfer of energy - i.e. instead of Newton, we are looking at quantum physics to explain what is going on.

Grey Wolf

[Traab]

No, it will impart the exact same, but over a larger surface.



I'm not sure this would be a good example. The issue is that it's no longer about impact force: monomolecular blades tend to simply cut the atomic bonds of the item they are encountering, rather that engage in the usual transfer of energy - i.e. instead of Newton, we are looking at quantum physics to explain what is going on.

Grey Wolf

Yeah sorry I know I wrote that part badly, but you get my drift about the difference between hitting something with a flat surface as opposed to an edge and how that translates to ever decreasing edge widths. If monomolecular cant work because it goes past regular physics then how about dialing it back to the thinnest edge you CAN do the math for? What if the blade is 1/8th of an inch? 1/32? 1/128th? Is it a linear increase? And, at its best, what could a person cut through with that ultimately thinnest blade you can do the math for that he couldnt with a normal blade edge?

[Eurus]

You have a few different things going on here, and I'm afraid that the answer is probably going to be "whatever you want it to be, because you're in the realm of fiction".

For one thing, cutting is a kind of complicated process (https://physics.stackexchange.com/questions/134119/how-does-a-knife-cut-things-at-the-atomic-level). The way that an obsidian knife cuts a piece of fruit is different from the way a steel axe cuts a block of wood, or the way a water jet cuts a piece of metal.

For another thing, the biggest limitation of "really sharp blades" in real life is how fragile they are. You can't cut a metal bar with a glass knife, it'll just break apart. You can cut a metal bar with a really big metal blade, if it's heavy and tough enough, even if it's not very sharp. On the most basic level, pressure equals force divided by area, so making the cutting edge half the width while maintaining the same force means that you're applying twice as much pressure to the cutting edge and the object you're trying to cut. That's more useful for abstract math or very rough estimations, but think about it this way: talking about something "infinitely hard" means that it can probably cut anything, whether it's sharp or not. You just need to swing or drag it hard enough!

[Traab]

You have a few different things going on here, and I'm afraid that the answer is probably going to be "whatever you want it to be, because you're in the realm of fiction".

For one thing, cutting is a kind of complicated process (https://physics.stackexchange.com/questions/134119/how-does-a-knife-cut-things-at-the-atomic-level). The way that an obsidian knife cuts a piece of fruit is different from the way a steel axe cuts a block of wood, or the way a water jet cuts a piece of metal.

For another thing, the biggest limitation of "really sharp blades" in real life is how fragile they are. You can't cut a metal bar with a glass knife, it'll just break apart. You can cut a metal bar with a really big metal blade, if it's heavy and tough enough, even if it's not very sharp. On the most basic level, pressure equals force divided by area, so making the cutting edge half the width while maintaining the same force means that you're applying twice as much pressure to the cutting edge and the object you're trying to cut. That's more useful for abstract math or very rough estimations, but think about it this way: talking about something "infinitely hard" means that it can probably cut anything, whether it's sharp or not. You just need to swing or drag it hard enough!

The infinitly hard part is just to confirm that the edge wont be damaged by the swing so no one pulls the "The edge would be destroyed instantly if you swung it at something" argument. The point was basically to go with, you have the same sword, at the same size, weight, etc being swung at the same speed, and the only variable changing is the cutting edge itself. If the edge takes no damage from the swing, what are you cutting through by the time you reach the thinnest cutting surface you can do the math for? I dont mean through infinite swings either. I just mean /swing "Hey, this sword just cut through bamboo" /swing "Hey this sharper edge just cut through wood." /swing "Hey this insanely thin blade just sliced through steel!" /swing "OMG I JUST CAUSED FISSION WITH A SWORD SWING THROUGH THE AIR!" (Im joking with that last one)

[Keltest]

The infinitly hard part is just to confirm that the edge wont be damaged by the swing so no one pulls the "The edge would be destroyed instantly if you swung it at something" argument. The point was basically to go with, you have the same sword, at the same size, weight, etc being swung at the same speed, and the only variable changing is the cutting edge itself. If the edge takes no damage from the swing, what are you cutting through by the time you reach the thinnest cutting surface you can do the math for? I dont mean through infinite swings either. I just mean /swing "Hey, this sword just cut through bamboo" /swing "Hey this sharper edge just cut through wood." /swing "Hey this insanely thin blade just sliced through steel!" /swing "OMG I JUST CAUSED FISSION WITH A SWORD SWING THROUGH THE AIR!" (Im joking with that last one)

The problem with this question is that the answer you seem to be looking for doesn't exist. You cant just turn off one aspect of physics, like blade durability, and treat everything else the same. If the blade wont break, then it can cut through anything given sufficient force behind it, because that's the primary practical limiter on that sort of stuff. If it can break, then it doesn't matter how sharp it is because, as you noted, it will dull and degrade so quickly that it wont actually do any meaningful cutting at that sharpness.

[Erloas]

I know the question you're trying to ask, but it really does come down to "too many variables" to say anything useful.

If you hammer and sword weigh the same and are swung with the same force then pressure will be determined by the surface area. So if your hammer has a 3 square inch surface area and you hit with 100 pounds of force you're going to have 33.3 PSI at the contact point. If your sword has a surface area of 0.5 square inches that same force is 200 PSI at the contact point. So yes, in theory as you approach 0 for the surface area your PSI approaches infinite. So if we're going to simply ignore everything else about the process then your answer is essentially infinite. But even if we wanted to keep it reasonable and just go a minimum of say 0.1 microns (a quick search showed razors getting to around 0.25) and get your very high PSI number, everything else has it's own numbers too.

Hardness should be the property you're looking for. I know they're related but the math to get a reasonable conversion from force to cut versus hardness is not one I know, and is probably not a simple calculation. With that you could look up the various hardnesses of materials, look at your force from your perfectly sharp sword, and get a pretty good idea.

[LordEntrails]

It also depends upon what material you are cutting. Because different materials fail (i.e. get cut) in different ways.

Something like a ceramic is going to fail through cracking for even very sharp blades while gelatin is going to simply cleave into two parts with very little damage even with a soft and very dull blade. Some materials also behave differently depending upon how quickly the cutting edge contacts and passes through the material. And some cutting edges behave differently as well (i.e. a ceramic blade is great for slow cuts, not for fast or shock prone cuts).

But, assuming you have a strong enough (not necessarily hardness) mono-molecular blade (and you are ignoring material behaviors), it would cut through most things that have a weaker molecular bond than the blade material has.

[Traab]

I know the question you're trying to ask, but it really does come down to "too many variables" to say anything useful.

If you hammer and sword weigh the same and are swung with the same force then pressure will be determined by the surface area. So if your hammer has a 3 square inch surface area and you hit with 100 pounds of force you're going to have 33.3 PSI at the contact point. If your sword has a surface area of 0.5 square inches that same force is 200 PSI at the contact point. So yes, in theory as you approach 0 for the surface area your PSI approaches infinite. So if we're going to simply ignore everything else about the process then your answer is essentially infinite. But even if we wanted to keep it reasonable and just go a minimum of say 0.1 microns (a quick search showed razors getting to around 0.25) and get your very high PSI number, everything else has it's own numbers too.

Hardness should be the property you're looking for. I know they're related but the math to get a reasonable conversion from force to cut versus hardness is not one I know, and is probably not a simple calculation. With that you could look up the various hardnesses of materials, look at your force from your perfectly sharp sword, and get a pretty good idea.

Thank you, this was basically what I was looking for. The reason this popped up was someone was using what amounts to a light construct as a weapon (think green lantern) and was cutting through all sorts of things. It made me wonder if since its basically hard light and can be shaped however the person wants, if the reason it cuts so well is it has a super thin edge and because it isnt an actual material like steel it wont lose said edge when used, therefore is perfectly capable of cleaving through whatever. So I was curious to find out what sort of psi an absurdly thin cutting edge could be putting out with even a normal human swing and thus what sort of materials it could break through that a normal sword wouldnt do anything against. Or maybe penetrate would be the better choice of words as someone else pointed out, differing materials would react different ways to a blade hitting it.

[halfeye]

There is another aspect, the angle behind the edge. If the edge is backed by a 170 degree wide angle, it isn't going through anything (well nothing much stiffer than butter), even if the actual edge is a perfect one molecule thickness.

[wumpus]

I'd ignore the deep physical analysis if you are remotely concerned about historical blades, the edges weren't sufficiently uniform for such to matter (some of the edge could be sharpened much better than other bits).

Consider the falchion. Basically the same weight as any other sword (2-3kg?), but flattened out more for a smaller edge. A terrific cutter, but presumably only useful against less well armored foes.

So "cutting power" in this case meant "cutting through meat, not armor". And thinness was better. But when you had to worry about your sword hitting more maile, you needed a thicker blade that could take the abuse (and probably had to aim at seams in the armor or similar, and hope the armor could take the abuse through the battle when you missed).

these views were mostly taken from this video: https://www.youtube.com/watch?v=lKJyust0nS4&t=769s
Matt Easton's "Sword Blade Technology Informing Design"

[Keltest]

I'd ignore the deep physical analysis if you are remotely concerned about historical blades, the edges weren't sufficiently uniform for such to matter (some of the edge could be sharpened much better than other bits).

Consider the falchion. Basically the same weight as any other sword (2-3kg?), but flattened out more for a smaller edge. A terrific cutter, but presumably only useful against less well armored foes.

So "cutting power" in this case meant "cutting through meat, not armor". And thinness was better. But when you had to worry about your sword hitting more maile, you needed a thicker blade that could take the abuse (and probably had to aim at seams in the armor or similar, and hope the armor could take the abuse through the battle when you missed).

these views were mostly taken from this video: https://www.youtube.com/watch?v=lKJyust0nS4&t=769s
Matt Easton's "Sword Blade Technology Informing Design"

My understanding is the falchion was actually made for fighting armored foes. It was basically designed to be an axe that could parry along the entire length of the weapon, so not all of it had to be of the same sharpness.

[Tyndmyr]

The infinitly hard part is just to confirm that the edge wont be damaged by the swing so no one pulls the "The edge would be destroyed instantly if you swung it at something" argument. The point was basically to go with, you have the same sword, at the same size, weight, etc being swung at the same speed, and the only variable changing is the cutting edge itself. If the edge takes no damage from the swing, what are you cutting through by the time you reach the thinnest cutting surface you can do the math for? I dont mean through infinite swings either. I just mean /swing "Hey, this sword just cut through bamboo" /swing "Hey this sharper edge just cut through wood." /swing "Hey this insanely thin blade just sliced through steel!" /swing "OMG I JUST CAUSED FISSION WITH A SWORD SWING THROUGH THE AIR!" (Im joking with that last one)

With infinite hardness and a lot of speed/force, everything yields. Seriously, this isn't merely a pedantic answer, metals behave as liquids with high speed impacts.

If you want to know how sharp an angle should be on your knife, though, 13-16 degree angles are pretty sharp. Many start out as high as 20 degrees. In practical examples, tradeoffs exist somewhere in or around this range. A good edge is important, but angle is going to affect cutting power in practice.

[Frozen_Feet]

If the edge is practically indestructible, then what's going to eventually stop its path through the material being cut is friction between the flat of the blade and the material. So the properties of the edge quickly become of secondary concern. For example, comparing a sharp steel sword to a dull steel sword, the cutting efficiency will not improve as dramatically as the initial impact pressure would make you think.

Worse, if you're talking about a "hard light" construct, you might be better off looking at cutting lasers.

[John Campbell]

Thank you, this was basically what I was looking for. The reason this popped up was someone was using what amounts to a light construct as a weapon (think green lantern) and was cutting through all sorts of things. It made me wonder if since its basically hard light and can be shaped however the person wants, if the reason it cuts so well is it has a super thin edge and because it isnt an actual material like steel it wont lose said edge when used, therefore is perfectly capable of cleaving through whatever. So I was curious to find out what sort of psi an absurdly thin cutting edge could be putting out with even a normal human swing and thus what sort of materials it could break through that a normal sword wouldnt do anything against. Or maybe penetrate would be the better choice of words as someone else pointed out, differing materials would react different ways to a blade hitting it.

What color is the light?

By odd coincidence, I spent the last month writing 50,000 words (which I don't think you could possibly have been reading) about a superheroine whose powers include the ability to form arbitrary objects out of "solid light", and who has used those as cutting blades in various circumstances. The thing is, her resolution when forming solid light constructs is the wavelength of the light. For visible light, that's not really all that small... about 0.4–0.7 µm, which is thicker than the edge of an ordinary razor blade.

My character isn't limited to the visible band, so if she actually needed a really sharp blade, she could make one out of gamma rays with an edge measured in picometers, but she tends not to make things that shed ionizing radiation when there's anyone who gets fussy about being irradiated around. (And, I mean, the way radiation works in comic books, do you really want to be swinging a sword made of gamma rays around? That's just asking for supervillain origins.)

Her constructs are extremely hard and fairly strong, but very brittle. They're completely unyielding up to a certain level of force (which I've been deliberately vague about), but if pushed beyond that, they don't deform or even break... the whole construct bursts into a flash of ordinary light and disappears.

[Traab]

What color is the light?

By odd coincidence, I spent the last month writing 50,000 words (which I don't think you could possibly have been reading) about a superheroine whose powers include the ability to form arbitrary objects out of "solid light", and who has used those as cutting blades in various circumstances. The thing is, her resolution when forming solid light constructs is the wavelength of the light. For visible light, that's not really all that small... about 0.4–0.7 µm, which is thicker than the edge of an ordinary razor blade.

My character isn't limited to the visible band, so if she actually needed a really sharp blade, she could make one out of gamma rays with an edge measured in picometers, but she tends not to make things that shed ionizing radiation when there's anyone who gets fussy about being irradiated around. (And, I mean, the way radiation works in comic books, do you really want to be swinging a sword made of gamma rays around? That's just asking for supervillain origins.)

Her constructs are extremely hard and fairly strong, but very brittle. They're completely unyielding up to a certain level of force (which I've been deliberately vague about), but if pushed beyond that, they don't deform or even break... the whole construct bursts into a flash of ordinary light and disappears.

White light. Its from the web comic Star Power. There was this scene where she stabbed someone with her light blade and the blood remained on it, showing it was basically a solid object and not a lightsaber or something, which made me wonder how much damage you could do with a blade that is stupidly sharp and wont dull or break with roughly human strength then work from there. I was hoping there was some sort of formula like force plus mass times the surface area struck equals psi or whatever (Im throwing random words out there, I know thats likely not even close to a real formula) where you could keep altering the surface area struck for a thinner and thinner blade and tell what the total force would be. Instead I just got "Metal doesnt work that way" which wasnt even the question. So I have mostly moved on.

[Keltest]

White light. Its from the web comic Star Power. There was this scene where she stabbed someone with her light blade and the blood remained on it, showing it was basically a solid object and not a lightsaber or something, which made me wonder how much damage you could do with a blade that is stupidly sharp and wont dull or break with roughly human strength then work from there. I was hoping there was some sort of formula like force plus mass times the surface area struck equals psi or whatever (Im throwing random words out there, I know thats likely not even close to a real formula) where you could keep altering the surface area struck for a thinner and thinner blade and tell what the total force would be. Instead I just got "Metal doesnt work that way" which wasnt even the question. So I have mostly moved on.

If you like, we can just say "no, there is no equation where you can just change one variable and just get bigger numbers as a result" but that doesn't really explain anything.

As far as it goes, the hardness is another variable. Changing it changes the results of the equation.

[Frozen_Feet]

@Traab: the issue is that if you want to know total penetration of a cut, you need more than just formula for pressure of initial impact. You also need angle of the blade, friction coefficient between flat of the blade and the material being cut, etc.

So if I took the time, I probably could find you values for comparing a specific steel sword in various states of sharpness, but these values would not tell you much about a speculative "hard light" construct, as we don't know how "hard light" compares to steel in other ways than edge sharpness.

You can find a 152-page PDF called "dynamics of handheld weapons" by searching for "sword strike impact energy" (etc.) which will give you a lot of the relevant formulas, but you still won't get an intuitive feel for how much adjusting single variables matters, because physics of cutting are hideously complex. You might actually get a better answer with empirical testing: find a knife, hit something with it, sharpen it, hit something it with again.

[Traab]

@Traab: the issue is that if you want to know total penetration of a cut, you need more than just formula for pressure of initial impact. You also need angle of the blade, friction coefficient between flat of the blade and the material being cut, etc.

So if I took the time, I probably could find you values for comparing a specific steel sword in various states of sharpness, but these values would not tell you much about a speculative "hard light" construct, as we don't know how "hard light" compares to steel in other ways than edge sharpness.

You can find a 152-page PDF called "dynamics of handheld weapons" by searching for "sword strike impact energy" (etc.) which will give you a lot of the relevant formulas, but you still won't get an intuitive feel for how much adjusting single variables matters, because physics of cutting are hideously complex. You might actually get a better answer with empirical testing: find a knife, hit something with it, sharpen it, hit something it with again.

Ok so there just flat out isnt a formula that basically goes "force of swing, mass of object, surface area of impact = force of impact" And from there you go "tensile strength of whatever material and force of impact equals item cut/broken or not" Thats what was bugging me, for some reason i thought there actually was one along those lines but if its really got that many factors included then never mind. I mean I suppose you could just make assumptions such as ideal angle for cutting, friction coefficient of smooth glass (Its just light, I doubt it has a very rough texture on the blade) But it sounds like you would be making assumptions of like a dozen random factors so its just not worth it.

[Rockphed]

Ok so there just flat out isnt a formula that basically goes "force of swing, mass of object, surface area of impact = force of impact" And from there you go "tensile strength of whatever material and force of impact equals item cut/broken or not" Thats what was bugging me, for some reason i thought there actually was one along those lines but if its really got that many factors included then never mind. I mean I suppose you could just make assumptions such as ideal angle for cutting, friction coefficient of smooth glass (Its just light, I doubt it has a very rough texture on the blade) But it sounds like you would be making assumptions of like a dozen random factors so its just not worth it.

As with most interesting questions, the formula you seek is in the form of a set of coupled partial differential equations. For a thin material I am fairly certain we could find equations you describe, but consider swinging a 1" diameter wooden rod: what can you cut with it? The answer depends on the thickness of what you are cutting, its shear strength, and how fast you swing. That is wood with a 0 degree angle on the blade. As you move to harder and stronger substances, you need a sharper wooden blade to cut them, though at some point the wood simply deforms on impact.

[Knaight]

I was hoping there was some sort of formula like force plus mass times the surface area struck equals psi or whatever (Im throwing random words out there, I know thats likely not even close to a real formula) where you could keep altering the surface area struck for a thinner and thinner blade and tell what the total force would be.

The total force of the object doesn't change with sharpness. It's just mass times acceleration. As for force of impact that actually tends to be higher for blunter objects. Consider the case of a complete stop - whatever the mass is has been accelerated fast enough to stop it. Something that cut deeper has moved further in that time, giving a lower required acceleration. A blunt object that doesn't cut and comes to a stop quickly will require much more acceleration, and thus much more force.

Now consider a cut that goes all the way through, and doesn't slow down in the process. That wouldn't involve only enough force to counteract added force during the cut. How much this is gets complicated fast, but it's far less than the blunt swing example.

Going back to surface area - if you're looking at both force and surface area, it's probably because you're looking at pressure, defined as force divided by area. That said, it doesn't sound like you're looking for force at all, but about the amount of damage that could be done with this hypothetical weapon. Force is fundamentally the wrong metric there.

So, cutting. What a cut is, fundamentally, is the separation of contiguous material into more parts via the application of a wedge driven between them. In real systems this tends to be dominated by efficiency, which is the entire point of a sharp edge - a blunt object can be forced through a substance given enough energy, and it will cut it in a very technical manner. It will also do a whole lot of other things, which also takes energy.

Hard light being a fundamentally magical concept, you can assume that that stops being the case, and that it instead cuts perfectly efficiently, doing nothing other than the cut. At that point calculations start getting viable - basically you need to break crystal lattice energy (or similar things for noncrystalline material) for all the bonds intercepted by the cut. Those energies are recorder per mole of the substance, which can be converted to per bond, and you can calculate how many bonds intersect an infinitely thin plane. Then you can calculate energies needed to move the macroscopic objects far enough to accomodate the width of the blade, which is generally some combination of friction forces applied over a distance and change in potential energy due to height, both of which will generally get dwarfed by the chemical energies involved. Of course, once you get to actual material blades that infinite efficiency assumption goes right out the window.

[Traab]

The total force of the object doesn't change with sharpness. It's just mass times acceleration. As for force of impact that actually tends to be higher for blunter objects. Consider the case of a complete stop - whatever the mass is has been accelerated fast enough to stop it. Something that cut deeper has moved further in that time, giving a lower required acceleration. A blunt object that doesn't cut and comes to a stop quickly will require much more acceleration, and thus much more force.

Now consider a cut that goes all the way through, and doesn't slow down in the process. That wouldn't involve only enough force to counteract added force during the cut. How much this is gets complicated fast, but it's far less than the blunt swing example.

Going back to surface area - if you're looking at both force and surface area, it's probably because you're looking at pressure, defined as force divided by area. That said, it doesn't sound like you're looking for force at all, but about the amount of damage that could be done with this hypothetical weapon. Force is fundamentally the wrong metric there.

So, cutting. What a cut is, fundamentally, is the separation of contiguous material into more parts via the application of a wedge driven between them. In real systems this tends to be dominated by efficiency, which is the entire point of a sharp edge - a blunt object can be forced through a substance given enough energy, and it will cut it in a very technical manner. It will also do a whole lot of other things, which also takes energy.

Hard light being a fundamentally magical concept, you can assume that that stops being the case, and that it instead cuts perfectly efficiently, doing nothing other than the cut. At that point calculations start getting viable - basically you need to break crystal lattice energy (or similar things for noncrystalline material) for all the bonds intercepted by the cut. Those energies are recorder per mole of the substance, which can be converted to per bond, and you can calculate how many bonds intersect an infinitely thin plane. Then you can calculate energies needed to move the macroscopic objects far enough to accomodate the width of the blade, which is generally some combination of friction forces applied over a distance and change in potential energy due to height, both of which will generally get dwarfed by the chemical energies involved. Of course, once you get to actual material blades that infinite efficiency assumption goes right out the window.

:smallbiggrin: I think my eyes crossed three times working my way through all that. lol Yes, I suppose the right term I was looking for would be pressure, which, if im understanding the terminology properly is what the tensile strength of various materials would require to break/cut/smash? Now the part about efficiency you kind of lost me, as im not sure what the term means in this context. Do you mean something like dealing with a loss of energy via friction? And once you got to crystal lattices and such I just had to shake my head, thats way past my 20 year old high school science classes. I can understand the part about energy required due to friction and the like abstractly but thats about it.

[Knaight]

:smallbiggrin: I think my eyes crossed three times working my way through all that. lol Yes, I suppose the right term I was looking for would be pressure, which, if im understanding the terminology properly is what the tensile strength of various materials would require to break/cut/smash?
Tensile strengths are generally measured in pressure (usually megapascals). They're also really not relevant to cutting. Shear strengths are significantly closer there, but still off.


Now the part about efficiency you kind of lost me, as im not sure what the term means in this context. Do you mean something like dealing with a loss of energy via friction?
It's mostly not friction - basically efficiency is the amount of energy that goes into whatever you want to do divided by the amount of energy you spent doing it. Here getting the amount of energy it actually takes in an ideal system is already a bit messy, but there's a bunch of ways to lose it. Starting with all the material in the way getting pushed anywhere other than directly out of the way.


And once you got to crystal lattices and such I just had to shake my head, thats way past my 20 year old high school science classes. I can understand the part about energy required due to friction and the like abstractly but thats about it.
Basically it takes energy to break chemical bonds, or at a larger scale to overcome intermolecular forces. Cutting things requires this, even given a magic perfect efficiency hard light blade.