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Myth: Deadly Throwing Knives

The hero is cornered by the bad guys! Thinking quickly, she pulls out a brace of throwing knives. She flips the knives at two of her attackers. They go down, one clutching the knife embedded in his chest, the other lying still with a knife in his eye.

Stop doing this. Writers, moviemakers, everyone — just stop. Unless your hero is also wielding a magical get-out-of-physics free card, thrown knives don’t work that way.

One of the most important aspects of a weapon is its stopping power. That is, its ability to stop your attacker in his tracks and render him incapable of hurting you any more. Some weapons, such as pepper spray, do this by inflicting severe pain and blindness. Others, like a sword, inflict enough physical damage to make further attacks impossible. Thrown knives are usually shown to be the latter sort of weapon. In reality, thrown knives have negligible stopping power because they lack three crucial elements: Mass, velocity, and accuracy.

It takes quite a lot of force to push a knife through skin and muscle and bone. The force with which a weapon hits its target is determined by its mass and its velocity. Knives are relatively light — an alleged throwing knife that I borrowed from a friend weighs only 50 grams (~1.8 oz). The heaviest knife in my collection still only weighs 310 grams (~11 oz). Throwing spears are far heavier. This modern example weighs in at 1134 grams (2 lbs 8 oz). A person who uses a knife in hand-to-hand combat benefits from the ability to put their body weight behind each thrust. A thrown knife, on the other hand, has only its own weight to work with. When you see a thrown knife in a movie that has buried itself up to its hilt in the bad guy’s chest, what you are seeing a cliché with no basis in reality.

Light projectiles, such as bullets, have to rely on the other side of the equation: Velocity. The muzzle velocity of the popular 9 mm cartridge is usually around 400 m/s* (and this round is still not considered powerful enough for self-defense by many experts**). Compare this to a fastball, which travels at around 40 m/s — a speed that is still far above what can be achieved with a thrown knife. In fact, the fastest speed I could find for a thrown knife was only 16 m/s! The essential difference between the thrown knife and other ranged weapons is its lack of mechanical advantage. Bows use the energy stored in the curve of the limbs. Guns use chemical energy stored in gunpowder. Atlatls and slings are essentially big levers, multiplying the reach of the thrower’s arm and thus the speed with which the dart or stone is thrown. A throwing knife, in contrast, relies solely on the velocity that a human arm can give it.

Finally, there’s the question of accuracy. Even a light, slow projectile can be deadly if it hits its target in just the right way. Pointy projectiles — bullets, arrows, darts, and spears — travel pointy-end-first, making them aerodynamic. They often spin on their long axis for stability, like an American football. Knives, on the other hand, spin end-over-end. This creates comparatively large amounts of wind resistance. Thus, a thrown knife will lose what little velocity it has very quickly, making it next to useless at long range. The end-over-end spin also means that a knife spends very little time with its pointy end towards its target. Even a talented knife thrower is more likely to hit her target with the side or the butt of the knife rather than its point when the target is moving, as in a melee. Combine that with the knife’s limited range, and your hero would probably be better off walking up to the bad guy and stabbing him in the face.

My favorite depiction of how thrown knives could be used in hand-to-hand combat is in Steven Brust’s Taltos series. Our hero, Vlad, frequently gets himself into scrapes where he’s outnumbered by people who are bigger than he is. One of his tricks for winning these fights is to throw a knife. The flying piece of pointy steel makes his opponent flinch, giving him an opening. He does not expect the knife to hit the person point-first, and he certainly doesn’t count on the knife to kill anyone. Killing someone with a thrown knife is not impossible. It just isn’t something that a character can rely on in a life-or-death situation.***

Even with years of training, throwing a knife is still slightly less effective for self-defense than throwing a large rock (rocks are cheaper, heavier, and sometimes more aerodynamic). Please, the next time you’re arming your hero or her sidekicks for combat? Leave the throwing knives at home. They’re silly, they’re clichéd, and physics doesn’t work like that.

* This number is a rough approximation. Actual speeds will vary by load and barrel length. However, it’s still way faster than a knife.
** I will not indulge in the fast & light bullet vs. heavy & slow bullet debate here — .45 ACP still goes way faster than a knife.
*** Hunters who use throwing knives are usually after small game, like rabbits and squirrels. They also use very heavy knives.

Comments (13)

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  1. Joanne says:

    I am now so sad that I have never questioned this! I just watched Prince of Persia this morning and a lot of people die from thrown knives… but the hero does also appear to have a “get-out-of-physics-free” card. I will now be watching for this in stories (but I will probably still be ok with it if the writing is otherwise good!). (the writing in the movie was not good… not good at all.)

  2. Dirk Bruere says:

    And while we are on the subject…
    Time bombs that have flashing lights, a nice countdown, and colour coded wires – terrorists (or just sensible bombers) do not do things so conveniently. Add in one more cliché here – the above bomb connected to large slabs of plastic explosives, with the hero sweating about which wire to cut. He never thinks of just cutting away the explosive from around the detonator, or even removing the detonator from the mass.

    Then we have the hero escaping in a car, with the baddies shooting at it from less than 50m distance using (say) AK47s. In reality, he and everyone in that car are Swiss Cheese in about 3 seconds.

  3. Adam says:

    Good points, but sour grapes.
    If we were all perfectly concerned about the laws of physics, would we be fans of sci fi and fantasy? Don’t think so. (Read: Warp drives, Dyson Spheres, etc., etc.)
    We can make the distinction. Reality is great, but you dismiss “fun factor” for cold reality. If we were that concerned about it, we’d all be reading non-fiction, now wouldn’t we…?

  4. What you said goes double for shuriken. Watching a villain drop dead of a throwing star embedded a half inch in a chest or forehead is definitely a “naaah” moment.

  5. cs says:

    Sorry to disagree, Sarah but I do. I feel that there is room enough in literature, cinema etc for different mechanics.
    What you suggest is

  6. cs says:

    *please delete me last unfinished comment. My keyboard has crossed wires and now seems to send things without my intention.

    Sorry to disagree, Sarah but I do. I feel that there is room enough in literature, cinema etc for different mechanics.

    What you suggest is just one style, one is which the laws governing velocity in the story resemble the ones in our own world. But, as I often do, we turn to stories for escape.

    Sometimes I want to see Dragons who breath fire, spaceships the break light-speed, and (yes) ninjas with deadly throwing stars.

    Maybe say replace, “Stop doing this” and more “be clear what kind of story your writing”. Because I think there is room enough on my shelves for both types of story.

    -Cs

  7. julian says:

    You should check out this youtube channel because real serious knife throwing is no-spin knife throwing, where the knife tip is ALWAYS pointed at the target.

    http://www.youtube.com/user/HouzanSuzuki

    I’m sure this guy’s videos don’t show his worst throws, but when you are no-spinning the knives they are always pointed at the target, and probably don’t have the wind resistance of spinning it either.

  8. Janet says:

    Sara’s physics are spot on. (In round-numbers terms, anyway.) and the fact that a guy can stick a steel dart into celotex (soft wood-fiber insulating board) at the range of a few yards still does not make it a deadly weapon.

    Have you ever wondered why surgeons prefer to close large wounds with surgical staples instead of traditional stitches? Simple! It’s amazingly hard to jam a needle big enough to drag a thread through skin, so Staples? Yes, Surgical glue? Yes, traditional stitching? Not so much, unless it’s a place where self-dissolving sutures are really needed.

  9. Black Mage says:

    “Stopping power” as you imply is represented by kinetic transfer is generally a poor indicator of projectile efficacy. Bullets are lethal because of their mode of injury, specifically: deep tissue damage, CNS damage and rapid exsanguination.

  10. The main advantage of throwing knives in a fight is as suppressing fire. Unless you get lucky and poke someone’s eye out or nab their hand (always a good possibility), the best advantage of using a throwing knife (or star) against someone holding a gun is to get them to duck. Knives have a small advantage over other things like salt shakers and rocks in that they can draw blood and inflict acute pain, adding to the psych-out factor.

    But you’re right, they’re very rarely deadly weapons.

    Great post, very enjoyable!
    -Dan

  11. mike says:

    I was curious, so I did a short study. Throwing knives can’t be compared to guns, but all special forces learn them. Those circus knives are for show, but the heavy military and hunting knives are dangerous. Mark knives as “possibly deadly.”

  12. Chris says:

    I can’t believe I’m actually commenting on this, buuuut… I’m a knife thrower.

    The knives I throw could bring down a large deer – easily. It just depends on where you hit them. The size and weight of the knife (and in the movies if it has poison on it or something) is important.

    The knives I throw resemble pointy chunks of high carbon steel (Cold Steel Sure Flight Throwers) that weigh about half a pound each. I throw them in a style like a baseball at speeds up to 70mph with a high degree of accuracy up to about 25ft – consistently with point-on-target. I’ve thrown them through a half inch of decent/new plywood. Flesh is a lot weaker. If the knife catches an artery, which it probably would since the cross-section is huge… that’d be the end. And more than likely the knife would go straight through them.

    The problem with the little knives you see in movies is they’d have to be extremely accurate and heavy… or of tipped with some kind of highly concentrated neurotoxin.

    That’s my 2 cents. :-)

  13. Ez Day says:

    re: Adam:
    Who said anything about being “perfectly concerned about the laws of physics”?
    This post is about a common cause of “expectations as to what is allowed to happen” being broken.
    As much as some people refuse to acknowledge it, experience tells me that most or all people do have a threshold for suspension of disbelief – they simply take their personal standards for granted on a subconscious level and fail to recognize how arbitrary they are.
    Going by the “just enjoy it, don’t worry about x” philosophy, shouldn’t we all be perfectly enjoying all media entirely regardless of content? “instant-death throwing knives” happens to be a common breaker of these standards for some people.