How and why do spark plug pieces break car windows so easily?
Because the little metal gizmo on the end of the spark plug is concentrating all the force you apply to the window into one little teeny spot on the window. The glass, which maybe could handle that kind of pressure across its entire surface, shatters when the same amount of pressure is condensed into one spot. An analogy: Squeeze an egg, barehanded. (Over the sink, preferably.) Try to apply equal pressure all over the egg’s surface. You have to squeeze surprisingly hard to get it to break. Now try the same thing, while wearing a ring. The ring will concentrate the force you’re applying, and you wind up with goop in your hands. Another analogy: You can safely lie on a bed of nails — heck, they have ’em out at Ripley’s museums and the like — because they evenly distribute your weight across lots of nails, and the force applied to any one nail isn’t enough for it to break or even bruise your skin. But you certainly wouldn’t want your entire weight to rest on one nail. As the article you
I suppose even a rock doesn’t have the overall density of a spark plug, unless it happened to be a lead rock, or iridium. Exactly. Ceramics can be HARD. Knifemakers have experimented with ceramic blades that go off the metal hardness scale. Boker sold one for a while, maybe still does. From what I hear, when the blade finally gets dull, you certainly can’t sharpen it with a wetstone. Just about anything metal in an automobile engine could be made of ceramic, and auto manufacturers have experimented with this. Ceramic tiles have also been used for the heat shields of spaceship re-entry modules. Someone more technical can expound on these fascinating tidbits and correct my loose terminology, if interested…
Spark plug pieces have hard sharp points, structurally quite similar to the window punches used by emergency rescure personnel to shatter tempered glass windows. Read a bit more about emergency window removal here.
