Body armor is worn by security guards, body guards, policemen, army men and many other people who act as protective force. It is so strong that it stops a bullet. When a fast moving bullet hits a person, the bullet transfers its momentum to that person. We know that momentum is conserved which means it cannot be lost. Thus, the momentum transferred to the person is the push that knocks him over/injures him. Armors help push the bullet away instead of penetrating the wearer.
Body armors are commonly made of ceramic called alumina (Aluminium oxide – Al2O3). It is an electrical insulator and has good resistance to weathering. Body armors could be hard or soft. Hard ones are much more secure and cumbersome too compared to the soft ones. Soft armor is more flexible to wear and used when there is no high risk of attack.
So what is inside the armor which helps deflect the bullet? It is actually a strong net made out of a very strong material called Kevlar fiber. It is about five times stronger than a piece of steel of the same weight. This fiber is very densely woven to form a net. As the tethers (vertical and horizontal strands forming the net) are strong, they disperse the incoming impact energy throughout the net and deflect the bullet. Denser the net, more energy it can absorb. The more tightly it is woven, less is the impact force felt on the wearer. To accomplish a denser and tight weave, the fibers are twisted before weaving. Then the net is coated with resin and sandwiched between two layers of plastic film.
The body armors may not be a 100% protective covering but can always be an effective one.