 Printer-friendly
version |
29 August 2003
E-Bulletin Backscatter
In the Dark on Blackouts
Editor's Note: The dramatic image we posted last week showing the recent blackouts from space apparently was a doctored photo. However, we included a web link (http://www.noaanews.noaa.gov/stories/s2015.htm) to a page that showed some less dramatic, but genuine images showing the blackout from space. We regret the error and wish to thank Joseph DiVerdi for bringing this to our attention.
Sheldon Greaves
"Who Wants to be a Mole-ionaire?"
Sheldon:
http://www.billcarroll.org/whowants.htm
This is a fun game for testing your knowledge of chemistry. It's patterned after the popular TV show. I did well at it until I stopped doing mental arithmetic and started guessing.
Norm Stanley
Spontaneously Disintegrating Glass
Hi Sheldon,
The note from Johan Benson sounds suspiciously like unrelieved strain in glass. The glass cup was made with a handle which could build in enormous strain unless properly relieved during manufacture by adequate annealing. Even if the glass was annealed, the two connection points for the handle have the potential to cause strain if subjected to temperature stresses.
Then, one day, the glass is yet again exposed to cold temperatures and warms back up to room temperature. One strain too many, I fear. By shattering into many pieces, the "strain in the glass" has been relieved and all is well-- except, of course, for the many pieces...
Ely Silk
Sheldon,
It is always nice to have at least couple of theories to explain any such event. This encourages a more detailed exploration rather than allowing one to succumb to the temptation of thinking one has found THE solution. In this case, the cause might be due to:
1. Thermal expansion as the
glass cup changed temperature.
2. Resonance from the TV (which sometimes produces high frequency sound
waves).
3. Vibration, animal, or insect causing the cup to fall.
4. The story being fabricated.
One place to start is by examining the fragment evidence carefully and to try to reconstruct where the fracture first occurred. One might also try to measure the frequency and amplitude of any sounds coming from the TV. Ideally one would try to reproduce the results with a similar cup, although it might be hard to duplicate the stress that sometimes exists within glass objects.
This is not the first letter the bulletin has published in which someone asked about unusual phenomena. I love hearing about this kind of thing but frequently find that the authors do not provide enough detail for my liking. In most cases, questions like this should lead to a physical investigation and eventually an experiment, as opposed to an answer provided by an expert.
Peter Baum
Sheldon,
In my travels to Turkey over the years I believe I have come to understand a similar local phenomenon. A very common Turkish custom is to have an "Evil-Eye" hanging prominently in most every store, home, taxi mirror and on a lot of car bumpers. An evil-eye is usually a poured piece of blue glass with a smaller pour of white glass in the middle followed by an even smaller pour of black glass. Sometimes the white and black are replaced by glued plastic pieces, but the blue glass remains a staple. They come in a wide variety of sizes from charm bracelet dangles to 12" diameter wall hangings and are available for sale almost everywhere. I usually bring several back as gifts when I travel. Anyway, the local legend says that if an evil person enters your shop, home, etc. the evil-eye will spontaneously shatter alerting the owner to his or her presence. They swear by these things and everybody has seen it happen or heard about it happening to a friend.
|
Turkish Evil-Eye. |
The key to the cause of the uninitiated destruction lies in the resulting fragments. Like your glass explosion the fragments are small and the explosion forceful and seemingly spontaneous.
If we examine how evil-eyes are made it is easy to understand the apparent random nature of the explosion, and it is an explosion. The eyes are created by vendors, they simply melt down some cheap glass, add some color and pour them out like cookies on the cold pavement. The relatively cool pavement causes the bottom of the glass to cool much quicker that the interior. To make matters worse they have a tendency to splash the tops with water to speed up the cooling time and in so doing so increase their daily profits.
Whenever you unevenly cool the exterior of a volume of glass you cause tremendous stress. The outside literally shrinks around the liquid core holding it in compression. This traps a high pressure core of glass under a very thin skin of "tempered" glass resulting in great surface tension. Almost any little thing can cause this surface layer to break and the resulting crack propagates at tremendous speed across the remaining surface; an explosion usually results with a report. This is how windshields and other "tempered" glass is made.
The phenomenon was actually discovered back in the 17th century by glassblowers of the time. Prince Rupert witnessed this in a Bavarian glass shop and brought some glass drops back to England to impress the King. Since then they have been known as "Prince Rupert Drops," a quick web search will show you how easy it is to make them at home. The process simply involves dripping hot liquid glass from a pipette or glass mixing rod into water. The result looks like a teardrop with a long thin tail, but be careful, they explode violently when the tail is snapped or the surface is scratched. The resulting drops are extremely hard and can actually be hit with a hammer and nothing happens. But like I said, all you need to do is snap off a piece of the almost invisible tail and the chain reaction begins Ð if I'm not mistaken the destruction travels through the glass at the speed of sound in glass. All it takes is a small break in the surface layer, no matter how small.
Back to your cup, I'd assume the quenching of your cup during the manufacturing process was a little too fast and you ended up with a Prince Rupert's Tear in the shape of a drinking vessel. A change in barometric pressure, uneven warming after a cool drink, almost anything could have set it off. Be glad you were not drinking from it at the time.
By the way, I have several evil-eyes hanging in my home and one more at work as a decoration, I'm still waiting for the first one to break. I guess all my friends (and bosses) are pretty nice.
John Witzel
Sharks!
Dear Sheldon:
I liked Forrest M.Mims III article on sharks (see "The Sharks of Summer"). It is rare when we can get such detailed and objective descriptions of shark behavior. I thought that he added a great deal to our knowledge of small sharks just from that one article.
James Farr
Mind of a Theorist: Mass and Energy
Dear George,
I tried in earnest to follow your agrument about m(v). I can see my way through the algebra. What I can't figure out, is why you can represent the added velocity of the rest frame and the center of mass frame as -V and not +V. I must be dense, but can you make this clear to me?
Robert Hirsch
George replies:
Sure: The one frame is seen to be moving in the opposite direction from the other frame; i.e. each frame is a rest to itself and the other is moving through, that is why there is a -V. Hope that clears it up. This points out how important it is to visualize the physics; there is no inherent mathematical reason to choose one frame over the other, it is a physical intuition that guides us to the correct answer.
George Hrabovsky
Editor's Note: Due to scheduling issues, readers who were looking for the second part of Mark Streitman's article this week will find it in next week's (5 September) issue of the E-Bulletin. -SG
