How does a bullet or knife kill somebody?
Like most people my age, I've seen thousands of dramatized murders on TV and in the movies. My question is, what mechanism in real life causes a person to die after a bullet is shot into them or a knife is stuck into them? Is it loss of blood, or shock, or organ damage, or what? How does that alien piece of metal kill a person?
It depends (in the case of bullets) on how big the bullet is, how fast it's going, and where it hits you. Speed (muzzle velocity) is the most important factor; we learn from physics that kinetic energy, i.e., destructive power, increases arithmetically with mass, but geometrically with velocity. Thus you have more to fear from a rifle than from a handgun. Slow, small caliber bullets, and knives, too, for that matter, rarely kill anybody immediately, unless they sever a major artery or pierce the brain, and even then death often takes several minutes. In most such cases, death results from blood loss, brain damage, or (in long drawn-out cases) from infections such as gangrene resulting from contaminants borne into the body by the bullet or knife. An abdominal wound can result in mortal infection from fecal matter seeping out of the intestines.
Large bullets, and small bullets that travel very fast, such as those from an M-16 rifle, can kill almost instantly, mainly by reducing the region of impact literally to hamburger. They also generate something known as "hydrostatic shock." The body is composed largely of water and as such may be viewed as a hydraulic system. Liquid being noncompressible, the shock caused by the high-velocity entry of a large projectile (don't you love this technobabble?) is transmitted throughout the body, causing widespread organ damage and disruption of nervous functions. Even a wound to an arm or leg can be fatal in some instances.
There are numerous variations on the above, most which are undoubtedly familiar to readers of detective stories. Hollow-nosed bullets, for instance, flatten on impact and bulldoze their way through the body, making death almost certain, since the massive damage they cause is virtually irreparable. I could go on, but you get the basic idea.