How does a knife cut?

Most people would feel under pressure when faced with a knife, and that's why they work so well.

Asked by: Nicholas Vlastos, email

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Imagine squashing a block of butter with the palm of your hand. The butter has to move out of the way, so it is squeezed sideways as it gets flattened. Now use the edge of your hand, like a slow-motion karate chop. The edge has much less surface area, so the pressure is higher and it is easier to push your hand through. Also, because the edge is narrower, you don’t need to push so much of the butter out of the way. Instead of flattening the whole block, you leave a narrow slot.

The blade of a knife is like a very thin hand. It applies huge pressure at the edge to push the material apart. When you try to cut a tomato, a blunt knife crushes a wide band of plant cells underneath it but a sharp knife will cut through a single line of cells, separating the long-chain cellulose molecules in the cell wall.

The sharpest knives are those with the thinnest edges. Obsidian (a form of glass) can be sharpened to an edge just three atoms across. Obsidian scalpels are sometimes used in surgery and have been found to produce less scarring, but a narrow edge is more delicate and the obsidian blades become blunt more quickly than surgical steel.


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