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I want to write about an object that won't be pierce-able even with the fastest of arrows (notice I didn't mention guns, only arrows) but that can somehow be crudely cut by an albeit very razor-sharp weapon (say something as sharp but as brittle as obsidian which is 3 times sharper than diamond and between 500-1000 times sharper than a razor or a surgeon's steel blade).

Is there anyway to make sense of this under real world physics or am I just going to have to handwave this as "something something magic" ?

*Also I would like to mention that (however misguided and uninformed it may be) I'm going off the assumption that making obsidian/diamond tipped arrows would be ineffective since they would be very likely to shatter on impact thus nullifying any cutting power they might have had, had they been used for simple stabbing or cutting for example.

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    $\begingroup$ Those are different types of hardness. Polycarbonate, for example, is very impact-resistant, but can be easily scratched and cut. $\endgroup$
    – Alexander
    Apr 8 '20 at 20:05
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    $\begingroup$ Kevlar will catch an arrow but can be cut with a sharp knife. Not sure if that completely meets your criteria of ‘fastest arrow’ though. $\endgroup$
    – Joe Bloggs
    Apr 8 '20 at 20:11
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    $\begingroup$ Also of note: what kind of arrow? A Broadhead and a bodkin May have different penetrative properties. $\endgroup$
    – Joe Bloggs
    Apr 8 '20 at 20:13
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    $\begingroup$ the difference in force between an arrow and a stab can be huge, a person can throw their entire mass behind a stab. $\endgroup$
    – John
    Apr 8 '20 at 20:14
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    $\begingroup$ @Anonymousworldbuilding this is again my point about different types of hardness. An arrow delivers sharp impact, a hand weapon like spear delivers slower impact, a "can opener" delivers constant pressure. Material can respond very differently in those scenarios. $\endgroup$
    – Alexander
    Apr 8 '20 at 20:53
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I believe a non-Newtonian fluid may help out. Some of these (e.g. "oobleck") get harder when subjected to strong, sudden force. Conceivably, a fast arrow hitting this substance will cause it to almost-instantly "solidify", while a slow-moving razor blade can pass through it.

(BTW, it sounds like you want something that behaves like a Dune shield...)

The complication is I'm not sure if there are non-Newtonian semi-fluids... since it seems you don't want it to ever behave entirely as a liquid. That said, maybe there is a way to combine such a liquid with a porous substrate (think 'sponge'). An impact might slightly damage the substrate, but you'd have to keep hitting it in the exact same spot in order to penetrate.

Whether or not this is 100% hard science, it may at least be sufficiently plausible for your purposes.


Edit: I see that you mention stabbing in some comments. If you want stabbing to work, you probably don't want a non-Newtonian fluid/sponge/whatever, since its ability to resist penetration is somewhat proportional to the force of the attempt. (Not sure if it's actually proportional, or if the resistance "plateaus" once you hit some threshold.) Just like penetrating a Dune shield, the way to damage this hypothetical armor is going to be gently. Slow slicing will work, vicious stabbing probably won't.

If you really just want something that resists arrows, but not being stabbed, boiled leather might actually be closer, or possibly some sort of paper armor or even something like Kevlar. Or, if you when you said you want to be able to "cut" it, you care about stabbing it with pointy things rather than slicing it with something scalpel-like, try chain mail.

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    $\begingroup$ Thank you ! Perfect description of the kind of the general feel I was looking for, I think an object with non-Newtonian semi-fluid-like properties would the perfect explanation with the added advantage of making it sound somewhat plausible. $\endgroup$ Apr 8 '20 at 20:32
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    $\begingroup$ (Clarification for confused people; the previous comment is in reply to a comment that I deleted because I subsequently folded its contents into my answer as the "Edit:".) @Anonymousworldbuilding, you mentioned "someone putting their entire mass in the service of cutting/stabbing could be successful in causing it harm". A non-Newtonian material will also resist an initial jab. I think if the attacker can continue to "lean into it", though, they can get through, like how you sink in oobleck if you stop moving. $\endgroup$
    – Matthew
    Apr 8 '20 at 21:00
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    $\begingroup$ "Dune shield" Please, call it Holtzman shield; the author went out of his way to drop that name like 20k times every page ;D $\endgroup$ Apr 9 '20 at 6:15
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    $\begingroup$ I retract my previous statement. I looked it up and the non newtonian fluids used in modern body armor are viscus colloids used to soak the layers of internal fabric (kevlar). Because the molecules repel each other to a degree, they maintain a homogenous saturation instead of all being pulled down by gravity. $\endgroup$
    – Nosajimiki
    Apr 9 '20 at 19:14
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    $\begingroup$ Yes, it's been around for over a decade now. The newer stuff is actually so good you can hide a level IIIa strike and slash resistant vest pretty well under normal clothing. I can't find exact product specs, but from photos I've seen they appear to be less than a cm thick. $\endgroup$
    – Nosajimiki
    Apr 9 '20 at 20:41
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Option A: Textile Armor

Gambison was a historical style of armor made from layers of linen sewn together. A good gambison can stop nearly all historical arrows fired even at relatively close range as well as duller bladed weapons. Most arrows just bounce off, but when they do penetrate (like when you fire an English war bow or crossbow at close range), it typically askews the path of the arrow as it penetrates each successive layer until it is turned to travel between the layers of cloth instead of through it.

That said, melee weapons are less prone to loose their alignment on a hit because they are braced in your hand so spears, war picks, thrusting swords, and particularly sharp curved swords are all fairly good at penetrating it.

Option B: A Hide and Wicker Laminate Shield

A shield does not need nearly as much stopping power as armor to save you from an arrow. Whereas armor needs to completely stop a weapon's penetration to keep you safe, shields are held away from the body; so, even when an arrow head goes through a shield, the friction with the shaft can still stop it before it harms the user.

The Persian Empire took advantage of this fact when they designed their shields. Mobility and archery were major aspects of their military doctrine; so, instead of making heavy shields that would slow them down but block almost anything like most other ancient civilizations, they made these very light weight tower shields that could screen their entire body, but just from arrows. The shields could be easily hacked through with a sharp sword or over penetrated with a spear thrust making them almost useless in melee (as the Greeks proved on more than one occasion), but against arrows, they were a very effective countermeasure for their weight.

This same principle can also be applied to situations where you just need a "thing" to block an arrow. So, a thin wall or door could also apply as long as you are not standing directly against it.

Option C: Plate Armor

If you slightly shift your thinking to just being about overcoming the armor in melee, but not with any arrow, then plate armor may be another solution. Tests of historical recreations of plate-mail against historical recreations of all sorts of weapons pretty consistently show that cutting or piercing it was practically impossible. Killing a plate armored warrior with a bow generally required a lucky shot to enter through an eye socket or exposed joint. By the 14th century, plate armor had become so well made that it was virtually impossible to get around with an arrow.

That said, a knight could still be killed through his armor by blunt force trauma. Front-heavy weapons like maces, axes, and war-hammers could kill a knight without actually penetrating the armor at all. This was even more true in the ancient era before the wide-scale use of arming jackets to cushion the blow. Daggers were also a popular option because their shorter blades gave one enough tip control to aim it into the very narrow eye slits which an arrow or sword would almost never be lucky enough to get into.

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    $\begingroup$ Basically old school Kevlar. $\endgroup$
    – Joe Bloggs
    Apr 8 '20 at 22:40
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    $\begingroup$ The reason I didn't mention plate is because plate (assuming we're talking about metal, anyway, which is what people usually think of when you say "plate armor") is because it doesn't satisfy the OP's criteria of being able to cut it. However, if the OP wants to be able to stab it with a sword (I'm not entirely clear on this point), then yeah, looking at historical armor is the way to go... even if the "cool factor" is significantly less than my answer 😉. $\endgroup$
    – Matthew
    Apr 9 '20 at 18:37
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    $\begingroup$ @Matthew I agree, I added plate armor more as an additional suggestion than an exact answer. While not specific to the OP as asked, most people who ask this sort of question are really more interested in how to make a scenario that forces a melee than the specifics of how to do it. Good 14-15th century plate armor is nearly 100% effective against arrow after arrow whereas textile armors (including those made with non newtonian fluids) suffer more from repeated or lucky shots. $\endgroup$
    – Nosajimiki
    Apr 9 '20 at 19:15
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The problem would be EASIER with guns. If you shoot a 50 caliber bullet into a tank of water, the projectile either disintegrates or skews wildly. If you take a musket rifle with a mini ball, the projectile penetrates deeper and truer. They did this experiment on mythbusters. I can't quite picture the scenario, but I suspect your answer will involve a denser but amorphous substance LIKE water dispersing the force. A slow, sawing tool might be the best thing to cut your object.

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  • $\begingroup$ Thank you very much for bringing this to my attention, it never would have dawned on me. The way you describe it, I think an object with a fluid substance like properties would be the perfect explanation. $\endgroup$ Apr 8 '20 at 20:29
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The article for bulletproof vests in Wikipedia is in dire need of citations, but any site more aimed at explaining or selling such vests will confirm this:

Vests designed for bullets offer less protection against blows from sharp implements, such as knives, arrows or ice picks, or from bullets manufactured with hardened materials, e.g., those containing a steel core instead of lead. This is because the impact force of these objects stays concentrated in a relatively small area, allowing them a better likelihood of puncturing the fiber layers of most bullet-resistant fabrics used in soft armor. By contrast, stab vests provide better protection against sharp implements, but are generally less effective against bullets.

The reason stab vests are not great against bullets is because they handle impact differently. The bullet may not pierce you, but you still take the full impact at the contact point. Bulletproof vests on the other hand redistribute impact.

There is still the matter of arrows, which aren't properly stopped by either. You may use some handwavium for those.

Edit: thanks to Nosajimiki for this comment:

Just to clarify, this is true of textile vests like kevlar. Modern armor that is reinforced with ceramic plates will block a blade just as well as a bullet, the the plates ablate on impact so they don't take repeated blows to the same place very well; otherwise, they block it all.

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    $\begingroup$ Just to clarify, this is true of textile vests like kevlar. Modern armor that is reinforced with ceramic plates will block a blade just as well as a bullet, the the plates ablate on impact so they don't take repeated blows to the same place very well; otherwise, they block it all. $\endgroup$
    – Nosajimiki
    Apr 10 '20 at 14:10
  • $\begingroup$ @Nosajimiki-ReinstateMonica thank you, I added it to the post :) $\endgroup$ Apr 10 '20 at 14:16
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A fine chain mail curtain.

This is an imperfect solution for a bunch of reasons, but meets the literal criteria and might suggest other solutions.

  • Something that can move freely like a curtain would just be pushed backwards if hit at high speed, but if you grab it with one hand to put it under tension, you could cut it at close range. When free, the curtain can conduct kinetic energy away, like a non-Newtonian fluid does.
  • Something slanted at the right angle (e.g. a curtain pushed back by an impact) could deflect something like an arrow, but be vulnerable to a well-aimed well-angled stab.
  • Grain. Meats, fabrics, woods, and many other materials have a kind of grain that is very hard to cut across, but easy to cut along. Similarly, a chain mail curtain might be very hard to pierce, but a downwards cut that catches on one ring could create tension against the ceiling mount, and pull it straight and make it easier to cut.

Another questionable possibility: A giant block of very dense gelatin. (Or another substance)

  • The substance should be a barrier that is thick and dense enough that one arrow could not pierce all the way through, as it loses velocity with distance traveled.
  • A sword could get one through it, though, because you can land multiple sword strikes in the same location to progressively cut a hole through in a way you can't with arrows, both because you can't aim well enough and because arrows fill holes behind them with themselves.
  • If the substance is gelatinous enough, though, it could collapse to fill in holes pierced by an arrow, and make piercing it that way even harder. Using a sword would get harder too, but still be doable with care and planning.
  • Frankly, gelatin is too weak and wouldn't be a very good solution. Meat would work fine, though--a 10 foot cube of whale meat sitting in front of a doorway could not be pierced by arrows, but you could get through the doorway by hacking your way through the meat with a sword.
  • Would take a while though.

  • Also, kind of a disturbing image.

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  • $\begingroup$ Not sure that a giant block of gelatin is practical as armor 😉. $\endgroup$
    – Matthew
    Apr 9 '20 at 21:32
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    $\begingroup$ The question doesn't mention armor, just "an object". $\endgroup$ Apr 9 '20 at 21:44
  • $\begingroup$ I don't really agree with any aspect of this answer in terms of actual armor making, but I totally appreciate the whole new meaning to the term "meat shield". $\endgroup$
    – Nosajimiki
    Apr 9 '20 at 22:11
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    $\begingroup$ Huh, you're right... I had assumed the OP wanted armor. In that case, a giant blob of gelatin would actually work, and is almost plausible with stone age tech (a big enough vessel in which to set it is the only problem; gelatin is historically made from animals). Of course, in that sense, an animal carcass would work equally well 😃. Really, anything that will slow an arrow to a halt will suffice. $\endgroup$
    – Matthew
    Apr 10 '20 at 14:14

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