system
Given the following post, respond as if you're commenting on Reddit:
human
Why are most bulletproof vests vulnerable to knives?

I know that a lot of soft, Kevlar-based vests are vulnerable to stabbing, but I forget the exact reasons why. I know that they stop bullets by distributing the force across a relatively large area, and that a lot of it is dependent on the high tensile strength of aramids, but I don't know why this makes them vulnerable to low-velocity stab attacks. I was also wondering why, if they tend to be vulnerable to knives and the like, they are not vulnerable to high-velocity 'sharp' projectiles like shrapnel/fragmentation.
gpt
Kevlar is a fabric, which spreads the impact of a bullet across a larger area. The blade of a knife can slip between the threads of the fabric.

A fragmenting bullet is still lead, which is soft, and warps before passing through the weave. A lead knife probably would be stopped by kevlar, but a steel knife wouldn't be.