Why does this sine wave like pattern appear in cut plastic?

Question

I was opening a package that was plastic wrapped from the super market with the prong of a fork, and this pattern (in the picture) emerged:

I started by poking the prong of the fork (right side of the picture), and slid it along the package (left in the pic). This sine wave like pattern appeared, and I am very intrigued into why this happened. I tried it again with a different pack, but the result just looked random, much like I have seen before.

Is there any reason this sort of pattern would develop? I wasn't trying to create a pattern when I opened it. The very first 'oscillation' from the right looks like it has a longer 'wavelength' than the rest of the pattern, which makes me suspect some property of the plastic caused this particular pattern to emerge.

I have tried looking for an answer online, but I can't seem to find anything when I search for information on patterns induced by cutting.

Is there a simple explanation for this phenomenon? (I apologise if this is an ill fitted question; my motivation for asking is really only curiosity because I found this quite surprising).

Answer 1

When you cut the plastic film using the prong of a fork, it probably slightly stretched the film as it was being cut. The stretching, though, was only along the edge. The only way the stretched portion can be stretched but remain attached to the unstretched portion is for it to deform into a curve that lets the edge be longer than the straight edge that would have resulted if the cut were done with an extremely sharp knife. Because the stretched portion is very narrow, the curve of the edge can't depart much (in distance) from a straight line. The amount of stretch appears to be over 2x, so a lot of extra length needs to be packed into a short space. The most efficient way to do that is something resembling a sine curve. If you make a similar cut with a fresh razor blade, I'd guess that you will just see a straight cut edge with no ripples.