Like what difference does the cutting motion do compared to just pushing down a knife ?
In: Physics
well not really. Some chefs will even scream at you “DON’T SAW THE MEAT!”
The usual technique is push forward down, retreat, forward down again, retreat etc until it’s successfully sliced. Letting the entire blade length and the sharpness do all the work. But not zigzagzigzag sawing motion all the way down.
Having to do that exposes the lack of maintenance on the knife, and also shows your bad technique and accumulated bad habits, which is why you had to do “back and forth motion”. And that’s why some chefs might scream at you.
Point to note, this is only for meats. Blades with serrated edges (bread knives) are logically accepted as sawing motion knives.
It doesn’t need to be pulled back and forth. But the sawing motion can often make cutting easier.
There’s two basic versions of what’s going on when you cut something.
If there’s sawing motion going on, you’re sawing the object. On a microscopic level, a sharp knife still has tiny teeth. Google image search is your friend here.
Just pushing straight down also works, in that case the cutting is done by concentrating force is a really tiny area and overcoming the tensile (resistance to being stretched) strength of the material.
Which version of cutting is dominant or most useful depends on what you’re cutting.
Picture some cheese, an onion, and bread.
Cheese has very low tensile strength, and comparatively high compression strength, pushing straight through is easy and gets you a nice slice. Now look at stuff designed for cutting cheese. Often just a thin wire.
Bread is higher in tensile strength, low in compression. If you press straight down, you’ll crush the loaf before cutting it. Look at bread knives: they’re saws.
An onion is in between those two extremes. If you just want roughly chopped onion, slamming the knife into it is fine. But for thin uniform slices, the knife needs to slide a bit.
You could just press down harder. Look at print shops that have a hydrolic cutter that can slice straight down through a hundred sheets at once.
When we cut foods for the most part we are slicing through things that are softer than the blade. Back and forth motion will cause some bunching and that unevenness gives the blade a areas that are higher and more vulnerable. Lay a towel on a table and pretend your hand is a knife, one horizontal move and you have ripples and wrinkles.
Something that nobody here has mentioned is that the sawing/slicing motion while pushing results in the effective cutting angle of the knife edge being much less than the actual cutting angle of the edge.
The short answer is; a sufficiently sharp knife does not need to be.
So either your knife is blunt or the material you are cutting is too solid for a knife to cut without becoming blunt as it cuts, so you use a serrated knife which has points, then you need to draw the knife back and forth as you slice as the little teeth will break the surface of whatever you’re cutting as you go.
Even if sawing is not necessary, it’s usually a good idea to cut with a sliding action to avoid cutting entirely on one plane of the blade which will ensure it becomes unevenly blunt.
It doesn’t, it’s just that most knives you encounter are blunt, so need to be used like a saw. Try a really high quality sharpened knife and you can cut straight down.
Pulling the knife back and forth uses a sawing motion, which creates friction and helps the blade slice through materials more effectively than just pushing down.
Pushing a knife through something is chopping, knives made for chopping are heavy so the blade does the work (think cleaver)
Sliding your knife across something is cutting, that’s what kitchen knives are meant to do. It’s now sawing, but ever so slight movement of the blade from first contact to cut completion
Not ELI5 but reason is:
* The pushing down motion only relatively compresses the item you’re knifing
* The cutting/slicing motion causes greater shear deformation in the item you’re cutting, acting to turn the small microscopic cubes underneath the knife edge into parallelepiped (3d parallellogram) shaped objects
* Shear deformation has been mathematically shown to produce 1.73x the distortion energy compared to pure compressive deformation, based on theories by von Mises.
* High distortion energy leads to a breaking of the material’s intermolecular bonds.
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