Originally published at: https://boingboing.net/2020/04/27/sharp-knives-are-better-at-sli.html
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Wait, sharp knives cut better than dull ones?? Is this new?
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Best way to cut tomatoes, use a serrated knife: a bread knife or a steak knife
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Came here to say this, thanks for getting to it sooner.
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If your knives are sharp, then there’s no need for serrations. But even with sharp knives the secret is to draw the blade, rather than just push. My family doesn’t get this and dinner time is a cacophony of their knives suddenly breaking through the food and crashing into the plate.
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Is the point that a sharp knife has micro serrations grabbing at tomato skin, vs. on a dull one the serrations are curled over, presenting a smooth surface that doesn’t catch the skin? The idea of using a steel on a knife is to get those serrations pointing inline again.
If so, what about a fractured glass blade that is so smooth as to have no micro serrations? Or would it be more used not to saw at the tomato, but to push straight through it?
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This angle grinder has a lot of friction for cutting tomatoes.
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I read a piece just this week by a well-known chef that said to use a serrated knife because tomato skins WILL eventually blunt a plain-edged blade, no matter how sharp. Now you know why, and now you know why Tribonet is just plain wrong to advise using a plain-edged sharp knife. If I could remember where I read it I sure would provide the citation.
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My grandmother told me to always use a serrated knife for tomatoes, and my mother has always done as well. Husband had never heard this, and was skeptical…until the first time he tried it. Instant convert!
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First thing I learned in the restaurant trade was that people get cut with dull knives. Keep 'em sharp.
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That’s interesting, because I do find a sharp knife works fine on toms, but then I have a Sheffield steel set of knives that are lifetime guaranteed and have never needed sharpening (no exaggeration) and they work fine on toms. I think they were advertised as diamond sharpened or something and they do not have a perfectly plain blade - it is not serrated but is not perfectly plain, either. They are 25 years old and still as sharp.
But the plain knife that needs sharpening from time to time, has never worked properly on things like tomatoes. We do have a couple of properly serrated (miniature saw-teeth) knives that do get used on toms, anyway.
For a tribology site, they are being sloppy. They are either leaving out a lot of information, or they are misrepresenting some basic physics. It’s not about friction.
The most important fact is that friction does not change with surface area. The classic example is that it takes the same force to push a brick laying down as it does a brick standing on end. Going from a sharp to a dull knife doesn’t change the friction. Same materials & same normal force = same friction force. Unless they have evidence that the coefficient of friction is non-linear, which they don’t bother mentioning.
The article says something about a jagged edge cuts better, but then talks about obsidian blades, which don’t have jagged edges, also cutting better. A jagged edge does help, but it’s not changing the friction. The little pointy bits, if it is jagged, apply an even higher pressure on the skin than a sharp knife alone and results in a macroscopic mechanical tearing. Not friction.
Sharper blades cut better because they apply more stress. They cut even better when moved because, and this always blows my mind, they effectively get sharper. I’ll explain this bid o’ crazy if anyone asks.
How come I’m ranting? Because I’ve had to take graduate classes in both tribology and designing cutting tools. And I’ve spent the last 25 years designing tools for friction cutting soft, wet, stuff (aka ultrasonic surgical blades).
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I thought the same at first, but I think you and they are both mixing macro- and micro-scale terms and abstractions without explaining which you’re using when. Friction is a macro-scale abstraction, and does not depend on surface area in contact. there is no fundamental physical force of friction, as you know.
Coefficient of friction absolutely does depend, extremely heavily, on microscale surface structure, including roughness, which depending on your definition of “contact” does affect the amount of surface area in contact between two objects. You’re changing the surface energy (different surface structures have different numbers of broken bonds and different energy bands/available quantum states) and also how much of the total cutting edge surface interacts with the surface of the thing to be cut.
Yeah, they describe a sharp knife cutting better than a dull one as “counterintuitive,” when it’s, like, the most intuitive thing someone could think about a knife…
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Calling all 80’s kids!
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I don’t think @Dioptase1 is conflating different things as much as it was a little ambiguous what they were referring to.
They raise the issue of tearing. Is that still called “friction?” At a certain point, we’re talking about shear forces rather than frictional ones. Yes, surface roughness is a key factor in determining mu, but at a certain point the best macroscopic descriptor is no longer friction and you’re dragging a spike through material on a microscopic scale. Once you have a running puncture and are applying shear, then friction is less of a factor than the pathways for soft material failure.
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I’ve also found that using a spoon with the hollow side up is better than using it with the round side up!
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Fair enough, I could have phrased that better, fundamentally I think we’re in agreement though. We invent different words and abstractions for different size scales because we find them useful, even though fundamentally physics doesn’t care. Mix the abstractions up and it can be really hard to disentangle the confusion, even for people who know exactly what they mean.
It’s even worse when the same words can have different meanings in different contexts, or when the jargon meaning doesn’t quite match up to the colloquial one. In this case, the article talks about surface area of a knife blade at the micro scale making it easier to cut a tomato. Then they go on to talk about it at the macro level on the sides of the blade in the context of making hard to cut cheese with a dull knife.
Everything they wrote is true, but there’s no reason not to distinguish the two with more precise words.
That image in the post though - lol. Also helps to have the blade down when attempting to cut tomatoes.
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Came here to say the same thing. It’s why you can get serrated peelers to peel things like tomatoes too.
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