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The physics behind the most annoying thing that could ever happen to you: A paper cut

MICHEL MARTIN, HOST:

A very different topic now - A, let me ask you this.

A MARTÍNEZ, HOST:

Yeah.

MARTIN: What do you do when you get a paper cut?

MARTÍNEZ: Oh, I - some really vile Spanish curse words (laughter).

MARTIN: OK. Well, maybe next time, you could channel Kaare Jensen, who teaches physics at the Technical University of Denmark.

KAARE JENSEN: I got a paper cut at home. I thought, this cannot be. Now I have to understand what's going on.

MARTIN: Jensen coped by designing a study, A.

MARTÍNEZ: All right, so what question did he try to answer?

MARTIN: Well, why do some types of paper cut, while others don't?

JENSEN: We consider trying to recruit subjects, but it's unethical, and it's really hard. No one volunteers.

MARTIN: So he and his colleagues got different kinds of paper - book paper, photo paper, Post-it notes - and they found a substitute for skin.

JENSEN: We built a little robot, a little ninja machine that can do the cutting. The paper can then cut into a block of ballistics gelatin, which has the same mechanical properties as your finger or the skin.

MARTÍNEZ: Yeah, that's the stuff they make the dummies out of on "MythBusters."

MARTIN: Right, right.

MARTÍNEZ: Yeah.

MARTIN: So unlike metal materials like a kitchen knife, where you want the edge to be as thin as you can get it, there's a point where paper gets too thin to cut. It's too weak. It just buckles.

MARTÍNEZ: Yeah. For example, you can't chop onions with tissue paper.

MARTIN: Of course, but you can dry the tears you cried from chopping the onions.

MARTÍNEZ: (Laughter) Yeah. So what did they find?

MARTIN: Well, they published their study in the journal Physical Review E.

JENSEN: The most hazardous paper is 65 microns thick, and that corresponds roughly to some newspapers, but also dot-matrix printer paper, which is, like, an old-fashioned type of printer. Some scientific journals, like Nature and Science - they are also quite hazardous to handle.

MARTÍNEZ: (Laughter) Old fashioned-y (ph) printers.

MARTIN: I know. Thank you for that. So the researchers also found cuts are more likely when the paper is at an angle to the skin, not straight on.

MARTÍNEZ: And they designed a paper knife that actually really works.

MARTIN: Yes.

JENSEN: We made a product that we call the paper machete. It's a little 3D-printed handle that will house scraps of paper, and we actually showed that a simple paper blade can cut into produce - fruit, chicken, apple.

MARTIN: But the paper blade only cuts once, and then it has to be replaced.

MARTÍNEZ: Unlike our fingers, Michel, which stand the test of time.

MARTIN: So true.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.

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