How do knives and other sharp instruments work?



How does a sharp object break through other objects without initial force(unlike, for example, bullets etc)

In: Technology

Who says there’s no intial force? You *do* excert some force against the object you’re cutting, it’s just a lot less. The reason is that to cut something you need pressure, which is proportional to both force and applied area. So the sharper the knife the smaller the cutting edge area is, and the less force is needed to cut.

You do need *some* initial force, you can press a razor blade against your skin gently and it won’t cut you.

Blades work by focusing the applied force onto a *very* small area, magnifying a small force into a very large one.

You push down on the knife with 3 pounds of force, but the knife edge has only 1/10,000th of an inch of surface contact with the potato you’re slicing. Right at the cutting edge, that 3 pounds magnifies to a crushing 30,000 PSI and shears through the chemical bonds holding the potato together.

I think this may be a good analogy:

Imagine you have a big piece of square plywood and you go to a body of water.

First you lay down the plywood on the water “normally” and try to push it down as hard as you can. But it is really hard, because the surface area is very large.

Then you turn the plywood on the side and it goes below water very easy, because the surface area is very small.

When something has large surface area, you need a lot of force to penetrate and with small surface area, less force.

(and as others have mentioned, there are always some force involved in cutting)

Take a rubber mallet and give yourself a light tap on the hand with it. It hurts a little bit, right? Now take an ordinary hammer with the same weight, and give yourself a light tap on the hand with it. It hurts more. Why? Because although the force being applied is the same, the hammer’s head applies the force over a smaller area.

That’s basically what sharp blades do. They apply force to an extremely small area. So small that the material just gives way under the extreme force.