It's no secret that martial artists are basically as superhuman as you can get without a radioactive spider crawling down your shirt. Of all the incredible feats they can do, punching through solid objects like wood, concrete and even rocks is one of the most impressive.
But how do they do it? The answer, as it is with everything awesome, is science.
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I'm just gonna start by leaving this here:
My hand is in the greatest possible mortal pain (slight exaggeration, but whatever) when I accidentally slam it in a drawer. How could it possibly go through a plank of wood. Through solid rock?
Go fast, son
Collision mechanics, boy. Here's how the physics work.
Large objects moving at high speeds hit harder than smaller objects moving more slowly. And when you're a badass martial artist trying to break a board - you want to hit it as hard as possible. So obviously a big part of being successful is making sure you move your hand as quickly as possible. Simple? Simple. But what makes up a "hard" strike?
Jon Chananie explained there are two ways to answer this question - both of which are as accurate as the other - in his scientific examination, "The Physics of Karate Strikes".
The first looks at the collision in terms of force and momentum, the second looks at the collision in terms of energy.
Hard science time:
Force (F) is acceleration (a) times mass (m): F = m· a. Momentum (p) is mass times velocity (v): p = m· v. Since acceleration measures change in velocity over time (t) (put another way, acceleration is the derivative of velocity with respect to time), force is the derivative of momentum with respect to time.
Equivalently, force times time equals change in momentum, or impulse (∆p): ∆p=F· t. This is significant because momentum is a conserved quantity. It can be neither created nor destroyed, but is passed from one object (the hand) to another (the board).
The reason for this conservation is Newton's third law of motion, which states that if an object exerts a force on another object for a given time, the second object exerts a force equal in magnitude but opposite in direction (force being a vector quantity) on the first object for the same amount of time so the second object gains exactly the amount of momentum the first object loses. Momentum is thus transferred.
With ∆p a fixed quantity, F and t are necessarily inversely proportional. One can deliver a given amount of momentum by transferring a large force for a short time or by transferring small amounts of force continuously for a longer time.
The reason why you'd need to move your hand as quickly as possible, Chananie explains, is because a fast moving hand will decelerate quicker in response to the force the board exerts on it upon collision - Newton's third law at play once again. The reason you want it to be a fast transfer in momentum is because this means the amount of force that will be transferred to the target all at once will be large.
- Fast transfer of force
- The part of the object struck moves faster in relation to the areas around it
- Success AKA destroyed object with your bare hand because you are, indeed, a total badass
The answer can also be discovered by looking at energy transfer. "In the simplest possible terms," Chananie says, "if the board is infused with more energy than its structure can handle, it breaks."
Looking at this more closely, it's the energy transfer makes the board (or other object) dent - and if the area stuck dents enough, it breaks. The amount it will dent depends on the amount of energy transferred. This, in turn, depends on the velocity of your hand.
And that's the story of why your hand needs to be travelling fast for this to work.
But the part of your hand you hit with makes a difference, too.
Reduce the surface area you are striking with
Now, you can't just smash any part of your body into a concrete block and expect it to not destroy you. There's a science (and an art - a martial art - get it?) to the technique used in breaking boards, blocks, and goddamn solid rocks.
Force, momentum, and energy are all at play here. Minimising the amount of striking surface on the hand that hits the board in turn minimised the area of the target to which force and energy are transferred - which maximises the amount of force and energy transferred to the area.
Here's the maths, as explained by Channie:
Consider a martial artist capable of striking with 190 joules (J) of energy. A typical human hand is about 15cm long including the fingers and 10cm across, which means that a strike with the entire hand disperses those 190 J over 154 square cm, about 1.2 J per square cm.
If, however, the martial artist strikes with only the fleshy part of the palm, about 5cm across and 14cm long, the 190 J will be dispersed over only 19 square cm. That strike will deliver about 10 J per square inch, inflicting many times the amount of damage the whole hand could.
Basically, if you can have the same amount of energy, concentrated in a smaller area, it's going to be far more successful. This doesn't just work for hands, either - you'll see this employed in kicks, elbows, and other strikes as well. I still haven't worked out that head one, though.
It might be this tip:
Visualisation (not the hippy version)
Remember all that talk of speed? It means nothing if you slow down just before you hit the board. Don't get me wrong, slowing down is a normal thing to do when you're about to nsmash your fist into a concrete block. So how do we stop this from happening?
It turns out that maximum hand velocity is achieved when the arm is around 75 per cent extended. When you think about it, this makes total sense - the arm can't move forward further than your arm can reach. Velocity at full extension is zero per cent. Not ideal for smashing through stuff.
So, if you imagine the target is actually sitting at 25 per cent of your arm's length away, you hand will be at its maximum velocity as it effortlessly (lol) powers through solid rock.
You need to hit in the right place, too - which varies depending on the material, and its ability to respond the strain of the strike.
Use the mass, Luke
One of the "mysteries" surrounding how this is all possible, is all due a common mistake made when looking at mass.
Force, momentum, and energy transfer are all directly proportional to mass. And since a martial artist will have the same body mass before, during and after it strikes, you'd think it would be the same figure in the equations used to work this stuff out, right?
It's not the total mass of the martial artist that matters. It's the amount of mass used. You can't use your entire body mass for a strike, but you can use as much as possible - not just the arm, for example. Snapping your hips, pushing with your legs - it all contributes.
"This explains why boxers are seldom knocked unconscious by jabs - where little more than the mass of the arm contributes to the punch - but are frequently knocked out by hook punches where the entire mass of the body is thrown behind the punch," Chananie explains. "The same principle of using the entire body mass to deliver a blow applies in breaking techniques as well."
So now you know. Maybe don't try this at home, though.
[Sources for Chananie's study: Bardosi, Z., “Kintematical Movement Evaluation of the Straight-line Karate Techniques.” Proceedings of the Eighth International Symposium of the Society of Biomechanicsin Sports, July 3–9, 1990, Prague, Czechoslovakia, 23-30 (1990); Bloomfield, Louis A., How Things Work: the Physics of Everyday Life. New York: John Wiley & Sons, Inc. (1977); Walker, Jearl D., “Karate Strikes.” American Journal of Physics 43, 845-849 (1975); Wilk, S.R. et al., “The Physics of Karate.” American Journal of Physics 51, 783-790 (1983)]