system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does it become much easier to tear plastic or similar materials once you have a small cut from the edge (or anywhere on its surface), but painfully difficult to tear them without any cuts?
gpt
Stress concentration.

Let's imagine a sheet of paper, gripped evenly on both ends and pulled apart. Not hard enough to tear, yet, but it's in tension. Held in front of you, pulling left and right.

Let's look at the middle of this paper, a line running up the center - this line should be right between your hands as you stretch the paper sideways.

At any point on this line, the paper is experiencing maybe 1 pound per inch. That's not enough to tear it, but it's a lot.

Now we introduce a small snip at the edge of the paper - cutting the end of the line. It still hasn't torn. Imagine what the paper at that snip is doing - it's pulling apart now, so obviously it is no longer bearing that 1 pound per inch. Instead, the stress it would be experiencing is passed on to the paper right where the cut ended. The stretchier our material, the more spread out this stress is. Paper's not very stretchy, so you get a lot of stress concentrated right where your cut stopped. Way more than the original one pound per inch. At this point, the paper probably starts to tear where the stress is greatest. As the tear grows, this point of high stress is pushed along with it.