|MadSci Network: Physics|
Not very often! What we see from TV and movies is nowhere close to what happens in reality. Glass windows (large enough to jump through) present a few difficulties for those wishing to go through them. When normal glass breaks, the failure is critical. Once enough pressure is applied to the surface, the glass will fracture at a weak point or defect. This crack will then quickly and rapidly grow to very large distances. Often a crack will propagate all the way to the edge of glass. The reason is that in the amorphous glass structure, there are no dislocations or grain boundaries to stop the crack. From the initial point of impact/failure there will probably be several cracks running outwards. Each piece of broken glass will possess razor sharp edges along the cracks. Quickly, as the person is attempting to go through the glass, the pieces will begin to fall as they are no longer supported. In even a modest sized window, the glass pieces will in fact be quite heavy. The combination of extremely sharp edges and acceleration from gravity will prove extremely dangerous to any person attempting this. Now, you might ask, "What if the person is moving fast enough to get through the window before the broken shards have a chance to fall much?" This wouldn't work for a similar reason. The edges of the cracks will still be razor sharp. So a person moving towards and through them at a high rate of speed will be severely cut as their body pushes the various shards out of the way. It seems very likely to me that should the glass be large enough, that the attempt could potentially prove to be fatal. It almost certainly would require a trip to the emergency room for extensive trauma. Safety glass presents a slightly different phenomena. First, the difference between normal glass and safety glass comes down to one thing (well, largely). Glass fractures from tension. In safety glass, the surface of the glass is actually held under compressive stress by the interior of the glass. This is done by heating the glass and then rapidly cooling the exterior surface of the glass. Then as the interior slowly (relatively) cools it places compressive stress on the outside layer. This has two effects. First, before the glass can fail the compressive force must be overcome. In effect this makes the glass able to withstand several times the force of ordinary glass. Secondly, when the glass does fail the compressive stress already present in the glass cause it break into lots of tiny pieces. This is commonly known as dicing. The glass can also further be laminated with a plastic to help prevent the small pieces from going anywhere. So for safety glass, you get two benefits. First it will be much harder to break the window in the first place, and second there will not be terrible, huge shards raining down upon you when it does break. Does this mean it's safe to jump through? No. The small "diced" glass pieces will still possess sharp edges capable of causing deep cuts. It's just much more likely not to be lethal or as serious. Hence you can imagine that among the most important places to use safety glass is in automobile windows. So how does it work in the movies? Well, often they're not using glass at all. Instead they will use either plastics, other silicone compounds, or even sugar (yes, a giant sheet of clear sugar). All this also assumes that there's nothing for our unfortunate person to hit after making it through the glass. But that's another question. There is a great collection of "Bad Movie Physics" at : http://intuitor.com/moviephysics/mpmain.html The information on glass largely mirrors (ha!) my own, but they tackle many other areas too.
Try the links in the MadSci Library for more information on Physics.