Martial artists learn to break things slowly

Cecil's Columns/Staff Reports
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Having done a lot of breaking - boards, concrete, rocks, bricks and so forth on my way to black belt (got it, too), Here it is: it’s physics, but not just physics.

The way I explain this to my students is, the force you hit with is proportional to the mass (your hand, foot, whatever) and the speed at impact. You cannot do much about the mass of your hand, but you can learn to hit faster, and thus harder. kinetic energy = (mass * velocity * velocity ) / 2 so there is more to be gained by getting faster than there is by making your hand bigger. (you get faster by getting stronger, so more pushups!)

So much for physics. Another very important aspect of this is that your hand (or foot, or head) has to be conditioned over time to do this. This is covered by Wolff’s Law, (another google exercise, or just look here: Wolff's law - Wikipedia ). Briefly, bones that are made to work hard get more adept at working hard. This is why your hand hurts from breaking one board and stepping up to five or more right away. Bones will adapt to the load put upon them, but it takes time to for them to do it.

Here is the thing I spent an entire sunday afternoon on once: When learning to break concrete pavers (2 inch thick by 8 inch wide by 16 inch long), my instructors were trying to get me to hit the paver off-center, rather than at center of span. Why? This really is just physics, but what is the explanation for hitting off center if this is correct?

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Welcome to the SDMB, dmartin.

A link to the column you’re commenting on is appreciated. This is the one: How do martial artists break boards and bricks using only their bare hands? - The Straight Dope

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I’m not one of the physicist and structural engineers around here, but I might have some useful info. I just reread some parts of Rex Roberts’ Your Engineered House, where he lays out the strengths and characteristics of concrete.

Pavers are usually made in molds, pretty side down. That will tell you, in a minute, which side to strike.

Concrete is made of aggregate (sand,) Portland cement (a fine powder,) and water. Each grain of sand becomes coated with many particles of cement, then the water makes the whole thing get sticky. After that, the chemical reaction called curing gives it its final strength.

Most concrete, says Roberts, is mixed too wet, in order to make it easier to mix and handle. It’s strong, but could be stronger if it had to be pushed, not poured, into place. The excess water rinses some of the cement particles off the the sand at the bottom, and by the time it begins to set, there’s more cement at the top than at the bottom. So, in a sidewalk or footing pad, the best tensile strength is at the top, where you don’t need it.

In your molded paver, that’s reversed because it’s inverted to show the pretty side (the weaker side.) You’ll want to strike the ugly side.

If you slowly break a stick across your knee, you’ll see the side on your knee is compressed. The other side is stretched until it fractures, when it tensile strength is defeated.

Now, to your question. When you say you were encouraged to strike the block off center, did you mean toward one supported side, or toward one long edge? If the first, I don’t understand why. If the second, I can understand why, but I can’t clearly explain it.

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One more thing. Curing takes weeks to reach full strength. If there’s a block-maker near you, and you can arrange to get some incompletely cured pavers, you might have an easier time of it. That would be cheating of course, and would probably violate a karateka’s code of ethics