Steel and ceramic armor plating function the same way as the iron suits worn by medieval knights: they are so hard and dense that they deflect or redirect the energy of a bullet or other weapon. The armor material pushes out on the bullet with the same force (or nearly the same force) with which the bullet pushes in, so the armor is not penetrated. The harder and denser the material, the better its ballistic properties.

Aramid fiber armor plating, such as Kevlar or Twaron, react differently to bullets. Aramids are not hard like steel or ceramic, so instead of deflecting or redirecting the bullet’s energy, aramids absorb and disperse the energy from the point of impact over a wide area. A bullet pushes on a horizontal length of fibers, and the bullet’s energy pulls on every interlaced vertical fiber. Provided the fibers are woven and layered together in sufficient mass, they’ll absorb the bullet’s inertial energy.

Ballistic glass also gains its bullet-resistant quality by its thickness, or mass. Ballistic glass consists of standard automotive glass inter-layered with a polycarbonate material in a process called lamination. Bullets fired at sufficiently thick ballistic glass will pierce the outside layer, but the mass of the glass will absorb the bullet's energy and stop it before it enters the vehicle.