Throwing Knives

Getting down to the physics of throwing knives: E = 1/2 M V^2. The velocity-squared term in the equation says that the kinetic energy of your knife goes up much faster with velocity than with mass. It also has no term related to the length of the knife. Energy is what you need to break the mechanical bonds holding your target together.

The reason you don't go for a one ounce knife is that you can only throw so fast and when you approach your limiting hand velocity your best bet is to make your projectile heavier. Throwing knives, of course, are toys more than tools. The real throwing weapons were javelins. They have a low ratio of weight per length, but an optimum overall weight. I would pick a knife that weighed somewhat less than a javelin as a first choice.

The optimum depends a lot on the weight of your arm and the speed of your muscles. I did my most intense knife throwing practise as a 145 pound teen. The ideal weight for a knife scales a lot with your body weight since the energy of your throw is divided between energy you put into the mass of your arm and the mass of the knife. To find your personal optimum, consider what you personally can throw farther, a softball, a baseball, or a golf ball?

Of course if you had an arm-extending gadget the optimum would to be to go with a lighter projectile going faster. If you have a sling (ala David & Goliath) you can throw a golf ball sized projectile over a hundred yards. If you have a spear throwing stick your best javelin starts to scale down towards arrow size.

The large weight to length ratio recommended for throwing knives supports long distance and imperfect rotation control. A heavy knife that has under-rotated when it hits the target tears a triangular hole like an old style beer can opener. (PS There are basic physical reasons why a knife does not like to rotate edge-on like a saw blade and tends to hit can-opener-style at distance).

So in the end your most reliable throwing knife is going to be pretty heavy relative to your hand weight, but if you're light and fast you can get more performance out of a light knife than you would expect.

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"Defense against knife attack:
Option 1. If you have a gun shoot him."
 
Though I'm not taking a side on the issue, the same see-saw argument exists in the firearms world. Heavy bullet and lower velocity or light bullet and higher velocity. Kinetic energy gives one faction grist for its mill while momentum is the darling of the other.


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Regards,
Arizona Desert Rat


 
But muscle speed is a much narrower constraint than the chemistry/technology used in firearms. Except perhaps for very rare individuals, most humans can not throw fast enough to make a light knife penetrate, and indeed there comes a point where lighter does not equate to higher initial velocity, unlike firearms because the limit of the arm is more severe. In most cases, and for most people, heavier will be better where hunting with thrown objects are concerned. One of the reasons tomahawks were popular weapons...
 
Jeff, 1/2 m v^2 only accounts for linear kinetic energy. There is also a 1/2 I w^2 term which represents the rotational kinetic energy. The I is the moment of inertia and length is a factor in its value.

-Cliff
 
Cliff--
Of course that goes without saying, but I thought I had enough trouble explaining KE. The key concept is that it is energy that breaks bonds, not for example momentum. The other critical factor is geometry that optimises which bonds you break to achieve your destructive purpose.

With regard to light knives not penetrating clothing, it is only partly true. If you have a razor sharp hollow ground edge thrown like a baseball pitcher it will go through cloth and disappear in your target. The key is that you have to have a real good fastball. My more serious throwing knives are more the weight of a short bayonet.

[This message has been edited by Jeff Clark (edited 30 June 1999).]
 
I've heard about the 175lbs. boar too. It was written in the front of some knife throwing book, but is it officialy documented anywhere? I hunt boar, I know they're real tough and have very thick, I've seen 3/4" and heard upt to 2", hard "shields" aroung their neck and shoulders that make a formidable obstacle to anything try to get to the vitals. It is rumored that they have on occasion stopped bullets. Given these conditions I have a hard time believing three large throwing knives, well at least if all were thrown that is, could stop a boar. I could by two thrown to disable, one that was stabbed with to kill. I do pretty much the same thing but use an arrow instead of a knife.
 
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