why are smooth edges better for some cutting, but serrated for others?

e.g. for butter cheese cucumber a smooth edge is best, for bread, wood, meat a serrated edge seems best (just had to make the kids sandwiches and changed knives many times)

What you’re basically describing is chopping vs. sawing.

My cheese knife is serrated and my steak knives aren’t. Go figure.

OK I’ll bite. Why is chopping sometimes better than sawing and vice versa?

Try sawing something crisp like a cucumber with a serrated knife. It’s a great way to make cuke-slaw. Now try it on a thawed, uncooked chicken breast. Lay down a dropcloth first.

You want a single, sharp edge that will cleanly slice through the material, with the absolute minimum of damage to the nearby material. It’s even more imperative in real chopping when you’re using the blade as a wedge, like splitting cordwood. If it’s dull or uneven you’ll mash your veggies rather than getting nice little chunks.

On the other hand, for something like bread, wood, or cooked meat that’s either hard or doesn’t shred very easily (or both), the serrations really act like a lot of tiny little blades. This helps scrape out material (like sawing wood) and get through the hard, thick parts (like a crust of bread or an orange rind).

Butter and cheese actually are somewhat different. They barely need to be cut at all. You’re really using the knife as more of a scoop there, and partly to get through the rind of some cheeses.

I agree with your description but not sure about the precise mechanism. A razor blade will cut through all the materials best, so I think there must be a practical reason why sometimes serrated is preferred. In wood it is to clear a gap so the saw doesnt stick. This might be important in soem foods

I suspect that a serrated edge has the edge over an average semi-blunt knife due to its tearing action. Thinking about it now, a knife that is flat along its edge exerts little pressure as the force is distributed along a long surface whereas a serration exerts more force on its individual points allowing it to enter the material.

and ginsu knives work (I’ve heard) because off the slight bumps (serrations?) on the side that don’t let food stick to the knife because it creates little pockets of air.

It’s not actually true that a razor blade will cut through all materials best. Take a good, crusty, Italian bread for example. Put a perfectly perpendicular force on the razor blade against the bread. You’ll get through the crust okay, but you’ll end up only squeezing the majority of the bread, with some localized cutting of the bread bubbles here and there. Start rocking the blade or swiping it, and the real cutting begins. Why? Well, you’re putting extreme localized forces at each of the cutpoints. It has the illusion of a nice, simultaneous cut, but you’re really making thousands upon thousands of small cuts at much greater force at discrete, instanteneous moments in time. Consider the amplification effect of a blade (or anything). There’s a mechanical advantage by which the input force (your strength) is multiplied into the area of the blade surface. When you apply a perfectly perpendicular force, your strength is divided into the entire length of the blade. As soon as you start rocking or slicing, your strength is only divided into the contact points as discrete moments in time. Those forces can be thousands of time stronger than the force for the whole length of the blade.

Of course why not use a 10" chefs knife to slice bread rather than a serrated break knife? First consider a chefs knive versus a razor blade, and then understand the mechaninical advantage that fulcrums give. You’re able to change the angle of the razor blade during your slicing motions much more effectively than a long knife, and so you get apparently faster cutting. But then, are you really? It’s take forever to get through a whole loaf. The serrations on a serrated knife provide all of these force points at an angle that’s not perpendicular to the blade, i.e., you don’t have to twist and rock in order to slice. Also with such a knife your application of force is different; rather than only pushing down, you’re also drawing back and forth.

You can slice with a serrated knife, but it takes more force because the majority of the sharp points aren’t perpendicular to the blade edge. It’s like using a dull knife, and is not as safe as using a sharpened, non-serrated knife. Even using a chefs knife with certain items requires a very light, brief sawing/rocking motion to get through (for example) the outer skin. It only takes a very slight break, and in all likelihood most people that chop don’t even realize that they’re briefly applying such a force.