Why/how does cuing ribbon curl when you run the sharp end of a pair of scissors along it? Would any other tool or material ever have this effect?

Question

*CURLING RIBBON, sorry. I was delirious when I posted this.

This has probably baffled me *since* I was five. What is this sorcery?!?

In: Physics

Answer 1

ELI5 answer: If you tighten a ribbon at a sharp angle against a scissor blade, and then loosen the ribbon, the ribbon seems to ‘remember’ the bend that you put in it. By running the edge of the scissors along the ribbon, you’re putting a small bend at every point of the ribbon. All together, this continuous bend turns into a curl.

Slightly less ELI5 answer: The physical reason the ribbon remembers a sharp bend is because of [plastic deformation](https://en.wikipedia.org/wiki/Deformation_%28engineering%29#Plastic_deformation). When a polymer (or any other sufficiently pliable material) undergoes enough stress, the molecules permanently rearrange themselves into their new shape. By pulling the ribbon against a sharp blade, you stretch the molecules on the outside of the ribbon more than the blade-facing side of the ribbon. If stretched hard enough, the molecules on the outside plastically deform and permanently elongate. Since the outside is now longer than the inside, the ribbon maintains the bend even when the scissors are removed.

Hope this helps!