How karate chops break concrete blocks

How can a doughy mortal hand, vulnerable to hangnails, over-heated bags of popcorn, and razor-sharp pieces of potato chip wedged between the keys of an ordinary computer keyboard, go through concrete blocks and wooden boards?

A quick look at the internet or at the musical training montage sequence from an eighties martial arts movie makes it obvious that shattering concrete or wood with bare hands is possible. In fact, some readers have acquaintances who, due to extensive training or extensive drunkenness, have tried it out themselves and succeeded. Of course, those readers also have friends who, due to extensive drunkenness, have fallen down and hurt themselves badly against wood or concrete. The main question is, what makes the difference between success and injury? What factors go into hitting a piece of concrete and splitting the concrete in two, instead of having the hand that struck it explode in a shower of red goo like an over-ripe tomato?

To begin with, it's important to strike at the right point. The best point is not just at the narrow midpoint of the block. A wide blow throws away a lot of strength, since it's only necessary to crack the block in one place to break it. It's also important to strike quickly at the surface of the block. Most blows are part connective smack and part push. This delivers the most damage when fighting flesh, but helps protect concrete or wood. Concrete and wood have a good mix of rigidity and elasticity. The materials will bend, and even flex back like a rubber band would, but the limits of their malleability are much lower. Bending and snapping back can do more damage to them than it can to things that flex easier. By making the blow fast and pulling back, the striker hits the block hardest and allows the material to do the maximum amount of bending. A follow-through push will keep the material from snapping back, and snapping itself.

All fine and good, but it's rare to see someone punching a building in half, or kicking cracks into the pavement, even if their form is perfect. Look at the picture below.

How karate chops break concrete blocksS

Between each of the concrete slabs are little dividers. It's rare to find dividers between layers of sidewalk. This is what separates the cracks from the splats. Striking through a piece of concrete is hard, but punching through a piece of concrete lying on the ground is pretty much impossible. The material has to be able to give to crack. The same property is observable in lighter, more flexible materials. Hold a piece of paper by both sides in the air and a knife, applied to the middle, will cut right through it. Put it on a smooth concrete floor, and it will be much, much harder to cut the piece of paper using the same knife.

Like everything else in life, breaking is just a primitive, degenerate form of bending. The paper can't bend, and so it doesn't give. A concrete block works the same way.

So remember, if you must punch a concrete block (you mustn't), make sure that you do it (don't) to a block (of styrofoam at most) that is held up in the air (by someone who will pull it away if you do something dumb like actually try to break it with your hand) and not down on the ground (like you will be, rolling back and forth in agony and that won't be my fault, damnit. I warned you).

Via The Straight Dope and Discover.