system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes kevlar better against bullets than steel (or any metal)?
gpt
There are two ways to stop a moving object: you either have something hard and heavy enough to stop it over a short distance, or you have something flexible with high tensile strength that "catches" it and slows it down over a longer distance.

In the case of the hard and heavy, this is what you typically see on vehicles because, well, it's heavy. The thickness of the material is usually porportionate to it's effectiveness. The projectile will impact the side of the armor and shatter/deform because the hardness and mass of the armor is higher than that of the projectile. Obviously there is a sliding scale here, as the heavier a projectile and the faster it moves, the harder it is to slow it. This is why tank rounds are designed the way they are: very high velocity, dense (hence depleted uranium) and a small cross section (discarding sabot). The intent is to overwhelm the armor by concentrating a significant amount of energy in a small area.

Kevlar, on the other hand, takes a different approach. Instead of trying to resist and stop the moving projectile at the surface, Kevlar will "give" and flex under the impact. But because Kevlar has such a high tensile strength, it doesn't break as it gives. It effectively acts as a kind of 'catcher's mitt' to slow the bullet to a stop. This is why even if your armor stops a bullet, it hurts like a fucker because your body is still being hit very hard, it's just dissipating that energy over a larger area.

As you can imagine, however, the threshold for the effectiveness of Kevlar is much lower than that of steel or composite armor, which is why kevlar vests are usually only rated for certain kinds of handgun rounds, and only resist rifle bullet impacts with the addition of ceramic inserts and below a certain caliber/velocity.