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.