Like what difference does the cutting motion do compared to just pushing down a knife ?
In: Physics
You don’t have to saw a knife back and forth if it is properly sharpened. You can just press down and forward in one motion
Moving it forward or backward while pressing down also changes the effective slope of the blade. Think of it like walking straight up a hill vs using switchbacks. By going at an angle, the slope is shallower or “sharper”.
Knives aren’t perfectly smooth. If you look at a knife edge under a microscope, you’ll notice it’s serrated, like a saw blade. The sharper the knife, the smaller the ‘teeth’, but they’re always there. In short, the reason you use a sawing motion is because knives are still saws, just on a smaller scale.
That said, if you’re finding you need to be sawing through everything, consider sharpening your knife. A proper whetstone gives the best results, but even a pull through sharpener can massively help reduce the amount of effort you need to expend in order to cut things.
Depending on the material you cut and the sharpness of the knife, sawing may not be necessary, but it’s usually helpful for the following reasons:
1. Sharpness of the knife: if the knife isn’t sharp, the imperfections in the edge turn it into a saw when you use sawing action, helping with the cut.
2. Debris accumulation: as you cut, some food gets stuck to the edge and prevent effective cutting as you go deeper. Sawing action moves this debris away and gets an effective cut
3. Fiber stretching: Sawing action exposes new to-cut areas and stretches the fibers of the item being cut, making it easier to cut
Push cutting is a thing, but it requires a much finer edge, and is actually pretty a common test of high sharpness post-sharpening. Chisels and razors, for example, almost exclusively push cut, but they also require a much higher level of sharpening than a typical kitchen knife.
Whetstones leave grooves in the edge where each abrasive particle scrapes off a bit of metal. Finer stones have smaller particles and leave narrower grooves, but anything less than the finest polishing will still leave some level of serration behind at a microscopic scale.
Poorly maintained knives may not be sharp enough to push cut anymore (if they ever were), but the edge between these serrations is protected from that abuse, and will often cut just fine. “Sawing” exposes the food surface to more of these still-sharp troughs, and can improve the apparent cutting performance of such a blade.
tl;dr: sharpen your knives
Not sure if this is a thing everywhere, but here in Ireland, “blunt knives” (regular cutlery knives at homes or restaurants), tend to only be serrated on the left side, and smooth on the right side, so if you’re cutting something dense like meat, you can just tilt the knife to the left slightly, so the serrations “bite” better and it’ll cut most things with very little force.
If you look at a knife edge under magnification, you will see microscopic “teeth” on the edge of the blade. These teeth diminish the finer the grit the knife is sharpened at. A knife with a rougher finish will cut with a sawing action very well. If you polish it to a fine grit and remove these teeth, the knife will “push cut” better (meaning just pushing the blade through the material rather than sawing)
Imagine a razor blade when pushed along the skin it cuts hair but the side to side motion pulls the skin as opposed to pushing it and it cuts.
Depends on the knife. If it has one sharp edge, use it both directions. If it has teeth. there is a difference. They are not symmetrical. Some are made to be pulled, and others are pushed. An extreme example is a chainsaw. It only works in one direction.
there was an animation about razor blades not cutting the skin when moving the blade along the skin, but doing left and right movement would cut the skin (sawing motion) because of the way the pressure is applied, it stretches the material you are cutting instead of compressing it, which happens when only pushing.
that said, like some other people mentioned, if the blade is Sharp enough for the material it can cut through just by applying pressure, but yeah,
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