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Cutting your finger

  1. Jan 16, 2006 #1
    We're all familiar to the fact that when you for example put two fingers together, they aren't really physically touching each other, it's the electromagnetic forces of the atoms from the different fingers resisting each other... which in turn translates to our nerves and finally our brain in the sensation of "touch".
    So... if you have a very sharp knife, and cut your finger with it, your skin will burst open and blood will rush out etc. What I wonder is:
    What happens in the atomic/subatomic level when the atoms of the blade of the knife comes near the atoms of the finger, which finally destroys atomic bonds in the finger resulting in the skin bursting open and blood rushing out?
  2. jcsd
  3. Jan 16, 2006 #2


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    Staff: Mentor

    The pressure of the knife against your skin.
  4. Jan 16, 2006 #3
    That doesn't answer the question. I've always wondered about the mechanics of sharpness (what makes something exceptionally sharp) and cutting too.
  5. Jan 16, 2006 #4


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    The surface of your finger, not all the atoms are bonded, kind of like sand. There's little rocks, the atoms inside the rocks are connected, but the sand is not one big rock. The pressure of the knife on the sand allows the knife to move through.
  6. Jan 17, 2006 #5


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    I think what makes something sharp is just that the surface area of its edge is very small, which means that for a given force on the object, the pressure where it touches your skin will be much greater (pressure being force/area). Think of someone stepping on your foot with the heel of your sneaker vs. with the heel of a high-heeled shoe, the weight is the same but the surface area is smaller in the second case, so the pressure on your skin where the heel pushes on it is greater.
  7. Jan 17, 2006 #6
    Oh, good point. :)

    Which would mean that, since it is dependent of pressure, that density also plays a role making it more difficult to cut through dense materials such as Titanium.
  8. Jan 17, 2006 #7
    yes, and let me tweak that a bit...

    differing materials have molecules that are more or less tightly bound to each other. if they're very tightly bound (examples might be titanium, as you mentioned, or diamond), it takes a greater force to make them separate.

    a dull knife may cut your skin because, though the pressure on the knife isn't very strong but the area it's applied to (the knife-edge) is very narrow, so the pressure (force per area) is still quite high.

    a sharper blade has a narrower width across which the force is applied (more kg/cm^2) so it cuts more easily.

    another example, varying only one parameter at a time, might be a box-cutting knife that you might use to slice up cardboard boxes. when the blade is new and very sharp, the force per area at the cutting edge is extremely high, letting you slice the boxes up easily.

    as the blade wears and becomes "blunter", the force per area drops considerably, making the force required to cut the cardboard much higher.

    hope that's a useful "macro" view of the issue for you.
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