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When cutting something with a knife, have you ever felt that it cuts much better if you push or pull rather than just press down? This isn't just a feeling. It's a feeling that actually has scientific basis .
▷ Pressing vs. Pushing/Pulling — The Difference in Cutting Force When you simply press down on the blade, the object experiences a ' compressive force ' that tears or breaks. This requires a lot of force and often does not cut cleanly.
However, when you push or pull the knife , shear force is added to the compressive force, which dramatically improves cutting efficiency .
This shear force is a force that twists an object sideways and is particularly effective on textured structures such as fibers or tissues .
Also, from a microscopic point of view , the blade is not completely smooth, but has microscopic serrations and protrusions.
These protrusions act like saws to cut through the grain of the object during the 'push/pull' motion.
If you cut the cake by pressing down on it, the cross-section will look like the cake is clumped together, but if you push or pull the knife long enough, the cross-section will look clean.
So, even when cutting cake or soft ingredients, if you move the knife in a long motion , you can see that the cross-section is cut smoothly and cleanly . The reason that cake cutters are particularly long compared to other knives is because they are intended to be cut by moving them in a long motion. However, as the distance the knife moves increases, there is a greater risk of accidentally cutting another part of the body (your hand), so always be careful .
▷ Shear force from curvature — making it easy for beginners to cut So why are the kitchen knives we commonly use slightly curved?
The curvature of the blade naturally induces a “push/pull effect” (pushing the knife or food) even when the user simply presses down from top to bottom to cut.
Thanks to this , even beginners can easily cut by applying natural shear force without any special skills. = It cuts well.
Curved farming tools, represented by the sickle > All weapons based on this,
It was born from the wisdom of “easily inducing shear force” even for beginners.
The subtle curvature of the sickle and cleaver is the result of a design that allows even beginners to easily achieve the shearing effect.
▷ Wait, isn't the Chinese food straight? On the other hand, the Chinese kitchen knife (a large Chinese kitchen knife) takes a completely different approach.
This knife works like an axe, with the blade creating a slit in the material and the thick body cutting through the slit .
So even if the blade is slightly dull, the cutting power is still maintained. This is because the cutting method is centered on 'compression force' . (More precisely, the expression 'center of inertia' is correct.)
For this reason, Chinese knives are designed to be heavy , and are often used in a way that feels close to pressing down with some force to cut.
The unique “tang tang tang” sound of chopping boards on Chinese food is also a unique characteristic of Chinese food that was born from this different approach. ㅎㅎㅎ
Of course, there are experts who sharpen their Chinese knives to cut the radish thinly. There are many experts who quietly cut their swords while viewing the tang tang tang sound as “too much force”, so be careful not to make hasty generalizations!
Although the thickness of Chinese knives varies depending on their intended use, they are usually two to three times thicker than regular kitchen knives.

Among them, there are knives that have a curved blade, thus incorporating the advantages of a regular kitchen knife.

▷ Curvature of weapons - The emergence of cavalry and the curve The main force on ancient battlefields was mostly infantry .

Spartan, Athenian, and even Roman heavy infantry, these infantry mostly used short straight swords such as the gladius .

At that time, it was closer to a secondary weapon that supported the spear and shield rather than a primary weapon. Therefore, it was a secondary tool that was taken out and used only when it was close enough to engage in a fight with a spear.

It is clear why a straight sword was chosen here.
Because it was easy to mass-produce, easy to store and use in group combat, and advantageous for straight-line stabbing and suppression.
Technically, the straight sword was a “comfortable and reliable” choice in many ways.
The standard weapon of the Roman infantry. You can see the gladius, a close-range 'straight sword' focused on stabbing.
Meanwhile, in cavalry-centered societies such as Central Asia and Mongolia, the story is different.
The swords most commonly used in this area were mostly curved swords .
Cavalry must charge quickly on horseback and cut down their opponents.
At this time, with a straight sword, the sword would absorb the shock and the energy would be directly transmitted to the hand, or the cutting force would easily be dispersed. It would have been easy for the attack to rely on compressive force rather than the shear force mentioned above. (So the cavalry lance, a “weapon that concentrates momentum into one point,” can also be seen as a type of division.)
On the other hand, the curved sword naturally forms a 'push or pull' trajectory as the sword makes contact with the target . As a result, shear forces are generated literally “by a passing slash”, resulting in highly effective cutting .
The representative light cavalry unit, the Hussars, holding up their swords
This type of curve naturally spread across Europe as the mainstream shifted from infantry to cavalry.
In Poland and Hungary, it was called Szabla, in France and England, it was called Sabre, and in English, it was called Saber.
These are words that literally mean 'cavalry' .
In some creative works, it appears as the name of a knight class, but to be exact, it is a word that means “cavalry sword.”
The actual weapon isn't even a cavalry sword ㄷㄷㄷ

Since the Asian region had a "cutting-centered" sword culture from the beginning, infantry also developed using swords, so straight swords were rare.

Even in modern times, the shear efficiency of the curve is still valid. A representative example is the jungle machete, especially the famous Nepalese curved sword 'Kukri'.
When cutting through tough jungle vines or tree branches, a simple swing of the blade or branch will push the curved blade or branch, naturally applying shear force to cut the object .
Especially when cutting the fibers of tough jungle plants, compression alone is not effective, so a curvature that seems particularly exaggerated has developed.
On the other hand , weapons with short blades, such as daggers, are difficult to push or pull sufficiently , so they are made with little or no curvature or with curvature only at the tip. They often evolved into a stabbing-focused design . There are also curved knives (karambits) that resemble animal claws, but they are often used for fighting demonstrations, collections, and decoration rather than actual combat.
In creative works, you can often see scenes where daggers or pocket knives are swinging around in a big way, but this does not accurately reflect the characteristics of daggers. If you are trying to show off the amateurism of your opponent, it may be an accurate scene, but if you consider realistic weapon use, you need to keep in mind that daggers are weapons specialized for stabbing .
I'm not saying you shouldn't swing a dagger at all, but it's not very effective at cutting, and even if it does, it leaves a very shallow wound.
If you just hit hard, it will slide along the curved blade and “features a jungle where shear force is maximized”
▷ 'Beki', practiced countless times in Kendo — Why draw a curve?
Beginners who are new to kendo tend to think that if they just strike hard with the sword, they will be cut.
But in reality, if you just press like that, the blade will often get caught on the target or stop mid-sword. Even if it's a famous sword.
So in kendo, you practice repeatedly striking along a trajectory that draws an 'arc', that is, a curve .
This is not simply for show or formality, but to ensure that the sword's trajectory naturally includes 'pushing' and 'pulling' movements .
That is, the shear force is maximized through curved motion rather than straight motion, and it is made to cut properly.
In actual kendo training, the depth and trajectory of the cut are controlled.
Focus on learning the balance so that the knife doesn't cut too shallow or too deep.
For example, in 'cutting rice stalks', if the knife does not draw an ideal arc, the cut is stopped, If you pull too hard, the knife may slip and the cut may appear to curve up or down instead of going straight.
Straw Exhaust — If the sword isn't pushed hard enough, it will stop, and if it's pushed too hard, it won't cut all the way through or will have a weird trajectory.
▷ Bonus: The Sharpest Blade, Single Molecule Cutter - Obsidian Today, a variety of knives are emerging that maximize cutting power through nano-processes and advanced materials.
But theoretically, the sharpest blade is made from a natural material: obsidian .
Obsidian is a natural glass formed by volcanic activity that, when split in a specific way, forms extremely thin, almost single-molecule level, sharp edges .
These blades are only a few tens of nanometers thick, and can cut sharper than modern razor blades or even surgical scalpels. They have even been used in some surgeries. It's glass-based so it doesn't cause any inflammatory reactions.
(Left) Electron microscope comparison of an obsidian blade and (right) a modern metal mass

Of course, its practicality is poor because its durability is too weak (it can be broken by hand), but it is interesting that even natural materials have reached the level of maximum cutting power.

So, the Aztec obsidian weapons were not symbols of an old era, but in some ways, they could be seen as cutting-edge technology comparable to modern sharpness.

▷ The sword is curved not just because it looks cool
The curved swords we often see in games, such as the curve of a Japanese sword or a Joseon sword, a Middle Eastern-style curved blade, or a saber-like weapon…
This curvature wasn't created just because it 'looked cool'.
It contains the logic of battle accumulated to survive in an actual battlefield.
And it contains a scientifically proven structural solution to improve cutting power.

Of course, there are swords that are strangely curved for aesthetic reasons… but basically, swords that are “for cutting” naturally acquire curvature over the course of evolution .

Some say that the final evolution of the sword is the "Dao", a short but more curvatured Joseon Hwando compared to the Japanese sword.
By the way, did you know that in games (especially 3D), these curved swords are harder to find than you might think ? LOL
Straight weapons are relatively easy to 3D model, texture, rig, and animate.
Curved weapons make sword trajectory and collision detection calculations more complex, and rotational trajectory animations also become more difficult .
Especially in older game engines, the direction of the cut and motion sync are important, but the problem of collision range or hit effects being out of sync with distance occurred more frequently with curved lines. In the beginning, there was even a time when it was “an object of avoidance” !
Even in recent games, it is common to see swords like Japanese swords being modeled as curves, but when actually handling collisions internally, they are processed “just like straight swords.”
The circumstances of adults are really... ㅋㅋㅋㅋ
Hwang Seong-gyeong, who appears in SolKal6, also appeared in the past with a straight sword instead of a straight sword. (Screenshot by Sophitia)
Even in the latest version, the effect contained in the curved sword can be seen as a compromise, expressed as a “straight line”.

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