# Why don't potato peelers need sharpening?

Why don't potato peelers need sharpening, when all other knives need regular attention to avoid becoming blunt?...
13 March 2011

## Question

Why don't potato peelers need sharpening?

We put this to Tony Atkins, Emeritus Professor of Engineering at the University of Reading... Diana - It turns out that it doesn't need sharpening, simply because it doesn't need to be sharp...

Tony - There is something called the critical crack opening displacement which means how much have you got to stretch the end of the crack before the crack will carry on propagating? If you're using a knife to get rid of the skin or the peel, then you have to wedge open the material at the end of the blade, at least as much as this property called the crack opening displacement. I was surprised to discover this magic displacement that you have to achieve for potato is actually much bigger than you would have to do for meat or cheese. Now the implication of that is, that to cut meat certainly and cheese, you really need something very sharp whereas with potato, because this property value is big anyway, you can say that, "Well, why bother to have something which is sharp?"

Diana - A potato peeler doesn't need sharpening because it still works well even when it's blunt. But there's also something about the angle of cutting which makes using a less sharp tool even easier.

Tony - What is much more interesting about cutting is the whole business of why, if you take a knife, however sharp it is, you can cut, but it is so much easier if you introduce some horizontal reciprocating motion. That turns out to be a very, very interesting problem that I've solved and it goes like this: If you're say, cutting something that requires work to do it, which is force times displacement, and you say, "Okay, well if I put a bit of work in sideways, clearly I won't require as much work pressing down."

That is true, but when you do the sums, you get a strange non-linear coupling between the forces. Meaning that the slightest horizontal movement reduces the vertical force considerably and that's why it's so noticeable. What this also goes on to is that it means that the overall forces required to cut are less - if you have this slice and push it together, and that means that you don't damage the surfaces that you cut.

That can be commercially very important if you think of buying salads or something containing melon in a supermarket. In the normal way of cutting them, you damage lots of cells adjacent to the surface that you've cut and these weep out liquid. So this shelf life is improved if you do the cutting properly.