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From practical experience, it's obvious that a sharp non-serrated knife will cut items with more ease if the user attempts a sawing motion.

The intuitive reasoning for how a non-serrated knife cuts - the blade is sharpened to just several atoms thick on the edge and whatever is being cut simply can't hold up to forces applied across such a small area - obviously breaks down here. (Experimentation!)

The most logical explanation that occurs to me is that the knife is imperfectly sharpened, leading to micro-serrations that assist cutting. To utilize these imperfections, one must use a sawing action while slicing.

Can anyone explain what's going on here?

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    $\begingroup$ I don't think knives are sharpened to a few atoms thick, but the sawing likely depends on the material. Nobody saws butter for example. Stringy fibers in meat (appetizing, right?), cut better when stretched (by the back and forth) while being cut similar to how a taut string cuts more easily than a lose one by a scissor. Sawing is also a more natural human motion. Ease of movement should be taken into account too. $\endgroup$ – userLTK Nov 20 '17 at 5:46
  • $\begingroup$ Possibly relevant question: physics.stackexchange.com/q/134119/26969 $\endgroup$ – Floris Nov 20 '17 at 5:47
  • $\begingroup$ I'll add that nobody saws the skin off an apple, just straight pressure with a flat blade while steak is usually cut with serrated blade. The difference is the structure of the material being cut. $\endgroup$ – userLTK Nov 20 '17 at 5:51
  • $\begingroup$ @userLTK It was something I'd heard. Some definitely are sharpened that much, though maybe not a common kitchen knife. Also, I'm not referring to actual "sawing"; it's just the term that best describes it. What else do you call it when you wiggle the knife in the plane of the blade a bit? $\endgroup$ – CoilKid Nov 20 '17 at 6:02
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    $\begingroup$ Your logic is correct. There are two sharpening techniques, German using a sharpening steel that makes micro-serrations requiring a sawing action to engage, and Japanese using super fine sharpener stones (up to 8000 grit) to make the blade razor sharp that does not require a sawing action. $\endgroup$ – safesphere Nov 20 '17 at 7:24
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Depending on the type of knife and the material that is being cut, "sawing" may or may not be helpful. This question and associated answers talk more about the shearing action of knives, chisels etc; but we all have experienced that cutting a tomato, for example, is best done with a slicing action.

You are right that there is a component of "imperfection" in the blade that comes into play. Specifically, the micro-serrations at the edge of the knife produce points of greater stress concentration, and this will locally cut through the material. By applying a slicing action, the sharp "point" on the blade can move to multiple points in the material being cut, and so initiate failure. You can consider this "tearing", or just splitting at the tip of the serration.

The other thing to consider is that the slicing acction exposes "fresh" cutting surface during the cut: while debris might accumulate on the face of the knife, effectively dulling it, when you use a sawing action you actively remove the "clogged" knife edge from the cut and replace it with a fresh cutting surface.

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