There was a video on tonight’s Most Outrageous Moments that actually interested me.
Imagine a standard pair of glass doors in an office building lobby. It’s flanked by two large rectangular windows (longer than they are wide), and are separated from the floor by two much thinner windows. All these elements are separated by the usual metal frames.
In this video, two workmen were pushing seven foot high spool of wire up a flight of stairs facing the doors. As you might expect from this buildup, they lost control of it, and the spool rolled down about five or six stairs, across about three feet of lobby floor, and right into the glass doors, breaking them entirely.
What I found startling, though, was the fact that both the large rectangular windows ALSO shattered. I’m sure I’m missing something, but for the life of me, I can’t see why they’d have done that, especially since the smaller windows did not break.
I thought about that, but wouldn’t that have required the glass to “stand up to” the spool long enough to flex the rest of it? And wouldn’t that have also broken the smaller windows?
Well, from this angle, you can see the doors and the windows breaking. Here’s the view from the outside, and this one offers a closer view from the inside.
I think there’s probably enough information there to say why all the windows shattered like that.
I would say that the impact flexes the whole glass wall. The big panels had a long vertical dimension, and so were flexed a lot, and consequently broke. The big panels also span the point of impact of the spool, which would have been the point that flexed the most. The lower panels are only very short in the vertical dimension and so would be stiffer in that dimension, and would have been flexed less. And were correspondingly further from the point of impact and maximum deflection.
I’m sure the shockwave from the impact wasn’t as dramatic as in the first Matrix, but it probably existed. The larger windows must have allowed for more flex, causing them to break, but the smaller ones didn’t ripple enough. I’m sure I got the vague physics explanation of this completely wrong, but think of dropping a pane of glass, then dropping a glass shard. The pane of glass will probably break, but the smaller chunk would take a further fall to cause it to shatter.
I Like the show; but I always get annoyed and pissed at the very fake overdubbing of the “off screen” person “supposedly” recording the video.
You know that guy who can clearly be heard without any extraneous outdoor types sounds…“come on honey, you can do it…OWWW”
IT’S THE SAME DAMN VOICE FROM TWO VIDEOS AGO!!!
“watch out mom, it’s really slippery…woooaahh!”
JUST SHUT UP WITH THE FAKE VOICE OVERS…I CAN TELL IT’s FAKE and your taking away from the effect!!
oh…umm…sorry…
I’m pretty sure the lower windows break also.
They just don’t fall out of their frames.
Looking at the first video, you can see the lower window on the right change in transparency after the spool hits.
The spool wasn’t traveling fast enough to impart a shockwave into the glass or its structure. The wave traveling through the material would have been a plain ol’ acoustic wave. By definition, an acoustic wave travels at the bulk speed of sound in a given material (as opposed to faster than that speed in the case of a shockwave). Therefore, the fastest the wave can transmit information into the glass is at sonic velocity, and the wave is traveling at the same speed as the glass is breaking*.
Think of the water coming out of a hose and hitting a barrier. Since the water is traveling at a subsonic velocity, information from the the water molecules in front can transmit that a wall is coming up, and a decay of pressure will occur from some point up stream until the stagnation point.
*Had the wave been supersonic (a shockwave) in concrete, and the pressure was less than that for brittle failure of the material, we’d see a rarefaction wave develop at surface interface (either a material boundary or free surface) This rarefaction wave travels back towards the point of impact, and we would see what’s known as a spallation where these two waves meet. It is this concept that allows bunker busters to work so efficiently; they don’t have to go through the material before the back end starts failing (about 85% of the way through for a projectile traveling Mach 1 into concrete).
For a commercial, I would presume they’re using thinner/more smashy glass (along with break-away doors to let the not-likely-very-heavy-spool pass through), along with explosive squibs to shatter the entire office-front simultaneously.
The bulk speed of sound in glass is around 4,000 m/s. The detonation speed of detacord is around 8,000 m/s; not enough to make a perceptible difference when we’re only dealing with two meters of propagation.
