The Science of Sword Cuts
Cut and thrust. Those two moves are the quintessence of any sword fight. There will be many swords, techniques, and schools of fencing. But at the end of the day, it all comes down to those two.
Cut and thrust.
The lunge is simpler. You move your arm forward and line up the sharp tip towards the opponent.
There may be variations in weight distribution and accuracy, but the move is so simple it can be done efficiently by a novice.
When it comes to cutting, however, the story changes. Giving a good cut —clean, efficient, deadly— is difficult, and sometimes takes years of training.
Today we’ll take a look at the science behind it.
It’s All about Friction
What does it mean to cut?
How does the edge of a sword act when cutting an object?
We can speak of “cutting” when the blade of the sword slides through the object. This means that the steel creates a cut in the object, rather than just piercing it.
But what if we look at the cut on a molecular level?
If we do that we will see that the action of “cutting” is not a cut itself. Rather, the blade works by tearing the object.
When the steel comes into contact with the object, the friction caused by the blade tears the material, causing the cut. As it moves over the object, the friction increases.
The more friction, the more effective the cut.
As a general rule, the smaller the surface area, the greater the friction generated. A blunt blade has a huge surface to cover, so the amount of friction generated is minuscule. Therefore, it cuts less.
On the other hand, a very sharp blade will concentrate all the energy on a small area. This allows it to generate an immense amount of friction as it rubs against the object, which leads to a better cut.
In short, when we cut something with a sharp blade we are doing nothing more than tearing the object at a microscopic level. It’s not much different than cutting a piece of meat with a serrated knife.
The difference is that the finer the blade, the smaller the tearing, and therefore the cleaner the cut.
The Science behind Katana’s Advantage
As we have just explained, cutting is a matter of the friction generated by the steel when it enters into contact with the object.
However, when it comes to cutting with a katana there are other factors that also come into play.
These factors are: shape and weight.
With a Little Curvature, Please
The main advantage of the katana when cutting is in its curvature. This allows the blade to concentrate all the energy generated by the strike in one direction, which improves alignment.
The alignment is decisive when it comes to giving a good blow. A bad alignment will completely ruin the blow— it can even damage the edge of your sword!
Good alignment, on the other hand, allows the blade to concentrate all the energy of the blow in one place, which leads to friction, which leads to an incredible cut.
With a straight sword, there is a greater chance that even small deviations will lead to a badly lined cut, as the blade provides no intrinsic advantage when cutting.
A katana, on the other hand, will correct its alignment when striking. This will reduce the chances of a misaligned stroke.
The self-correcting effect is even increased if the katana has Bo-Hi. This feature increases the balance of the sword and makes it easier to deliver a well-aligned blow. It even acts as feedback, emitting a characteristic sound —that very famous whoosh— when the blow is well aligned.
Thick vs. Thin
The second feature that gives the Katana an advantage when it comes to cutting is its thickness.
Katanas are thick swords. Pretty thick.
In fact, you would need to combine the blade of two conventional swords to achieve the thickness of a katana.
What is the advantage of this?
If you have ever wielded a conventional sword, you may have noticed that it tends to vibrate. If you have tried to hit something hard you may even have experienced how the sword transforms in your hands into a jelly that does not stop shaking.
Even if you are cutting a light object, the slightest misalignment will make the sword shake.
The thickness of the katana, on the other hand, prevents it from vibrating. Whether you are wielding it or hitting something, the blade always remains rigid.
This allows the katana to absorb the blows better, while making clean cuts more easily.