How Does a Knife Cut?

How Does a Knife Cut?

The answer to that deceptively simple question is surprisingly complex. How does a simple wedge of metal slice into another object, breaking through its structural integrity, to cut that object up? Essentially, any time that a knife, ax or blade of any kind does this, the blade can cut because of two different reasons.

Reason 1

When a knife is pressed or chopped straight through an object, that object is essentially being pinched in half. You might ask why a sharp knife is more effective at this than a dull knife. This is a matter of physics--a transfer of kinetic energy, to be specific. When a knife is sharp, its edge is narrower than the edge of a dull knife. This means the point of contact between the knife and the object to be cut is much smaller. The weight of the knife combined with the force you use pressing down is exerted over a much smaller point of contact, magnifying that force dramatically so that the sharp knife cuts through the object more cleanly and with less force than a dull one.

Reason 2

Reason 1 immediately raises the question, why does sawing with a knife cut better if all that the knife does when it cuts is pinch something in half? That's because pinching is only half of the cutting process. When you sharpen a knife, you work out the nicks and imperfections in a perfectly straight, or nonserrated, knife. If you were to look at the blade of that knife through a microscope, you would see that the sharpening process only serves to raise more metal burs along the knife's edge. These burs are in fact microscopic naturally forming serrations. They form teeth just like those of a saw. The sharper the knife, the more serrations there are. When a knife cuts through an object, these microscopic serrations rip and tear apart the object as the knife passes, whether it is drawn straight up and down or in a sawing motion. In this way, the material of the object is forced aside for the passage of the knife.