How Can A Bullet-proof Vest Stop A Bullet?
Here's an experiment: take the small coil springs from a dozen or so retractable pens and roll them together in a heap until they are thoroughly tangled and entwined. Now try to pull them apart from end to end. You should find them extremely difficult to pull apart this way, as anyone who has ever tried to untangle a 'Slinky' toy will know. Individually, those little coil springs offer only little resistance and can be completely stretched out very easily. But together they seem to acquire extra strength from each other, and it becomes increasingly difficult to stretch any of them. When they are tangled together, one has to stretch all of them in order to stretch any one of them. What this experiment gives you is an analogous image of what happens inside a 'bullet-proof' vest.
A bullet fired from a gun has kinetic energy and momentum due to its mass and the velocity at which it travels. That bullet carries out its function by delivering its load of kinetic energy completely to its target. When it strikes the target transfer of energy is achieved as the bullet stops moving; the more quickly the bullet stops, the more rapidly the energy is transferred. This is the principle behind the 'knock down power' of any bullet-cartridge combination. A bullet-proof vest accepts the energy from the bullet and dissipates it so that only a small portion is passed on to the actual target, the person who is wearing the vest. That small portion of energy will probably still be enough to knock the wearer flat on his or her backside, it still hurts a lot, and will almost certainly leave a very unpleasant bruise at the point of impact. But if the vest has done its job, the bullet has not penetrated, and the person wearing it gets to walk away essentially unharmed.
The secret to this is in the material used inside the vest. Believe it or not, a bullet-proof vest is filled with nothing more than several loose layers of a light plastic fabric. But not just any plastic will do the job. This application calls for plastic fibers of exceptionally high tensile strength, fibers that it takes a great deal of energy to stretch even the tiniest amount (not fibers that will stretch a lot before they break...). In this case, those fibers are made of a polyarylamide plastic known familiarly as 'Kevlar'. Kevlar is the proprietary name for the material; it is becoming more common to refer to the material generally as polyarylamide. Fibers of Kevlar don't stretch very readily when put under tension. In fact, this material is even harder to stretch than steel! But it weighs a great deal less than an equivalent value of steel fibers would weigh.
