Dig through the JUnQ

Here you find all contributions made by external authors to JUnQ. This includes peer-reviewed articles and editorial board reviewed open questions.

Jul 242017
 
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Put a raw egg on a flat table, and give it a good spin with two fingers. The egg spins, however, rather slowly because the liquid inside poorly exchanges momentum with the outside shell. Thus, when you spin the egg by applying force to the shell, most of the inside resists the motion and the friction forces between the inside (immobile) and the shell (mobile) will be slowing down the egg’s speed. But if you consider a cooked egg, proteins contained inside the egg are now forming a solid phase that is tightly joint to the shell. In that case, there are no friction forces in the “egg system” and the movement is not slowed down.

 

In this case now, the only existing friction forces are those between egg and the support (the table) and the air (which can be reasonably neglected). If your initial momentum transfer is strong enough, you observe a strange phenomenon: the spinning egg starts to rotate upright.

The physical concept used to explain this phenomenon is inertia. Spinning ice skaters can reduce their moment of inertia by pulling in their arms, allowing them to spin faster. You can also sit on a swivel chair and spin on yourself. Extend your arms horizontally and you will slow down.  The same is happening with the egg. Friction forces tend to slow down the egg, and decrease overall energy. To save energy, like the skater, the egg stands up and the momentum of inertia consequently decreases.

To spin upright, the egg needs some energy, exactly as one needs some energy to get up in the morning, fighting against gravity. The necessary energy is provided by the rotation itself, and the change of orientation of the egg will only happen if the spinning is fast enough.

 

— Adrien THUROTTE

 

Video movement of interia

Bou-Rabee, N. M., J. E. Marsden, and L. N. Romero, A geometric treatment of Jellett’s egg, Angew. Math. Mech. (ZAMM) 85, (2005), 618-642

Jul 102017
 
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Ordinary glass as it is used for windows can exhibit exceptional behaviors and even shred a rifle bullet to pieces, furthermore it can help to make car windows safer and to understand the inner processes in volcanos.

Key to all these fascinating properties are the so-called Prince Rupert’s Drops. These structures are solidified drops of glass, which are produced by letting a drop of molten glass fall into a bucket of water. The sudden shock caused by the massive temperature drop on the surface of the glass basically locks in the outer shape of the drop, preserving main body and tail. (Fig. 1) [1,2,3].

Fig. 1: Prince Rupert’s Drops. The thick main body with the long, thin tail is well visible (Copyright: public domain). [1]

This object has now very unique properties: If the main body of the droplet is hit by a hammer, it practically never breaks. It even withstands a direct hit of a rifle bullet, whereas the bullet can be completely shred to pieces. All this the main body of the glass droplet can stand without breaking. However, if there is too much force applied to the fragile tail of the Prince Rupert’s Drop, or if it is even just nicked, the whole drop explodes into tiny pieces of glass that can spread over several meters. [4,5]

To understand how this fascinating behavior is created, we have to have a closer look at how the drop is created. Everything starts with a drop of hot, molten glass, suddenly getting in contact with water. As mentioned, the outer layer of the drop immediately solidifies and locks in the characteristic drop or tear shape. The thin tail is created when the drop detaches from its origin (e.g. the glass rod) and starts falling and also gets locked into its shape when touching the water for the first time. While the outer layers are now already solid glass, the interior of the drop is still a hot liquid (Fig. 2 top). Consequently, this glass contracts while cooling down and starts pulling the solid outer layers inward, stressing them just like an arch bridge is stressed (compressive strain) and thereby stabilizes the structure. Along the axis of the drop, however, the strain is not compressive but tensile, because the shrinking material tries to pull along the tail.

Fig.2: Mechanism of creation of a Prince Rupert’s Drop. The hot, liquid interior of the drop compresses against the already hardened outer shell. The result is a highly strained structure (Copyright: CC-BY JUnQ). [4,5]

These stresses make the round shaped main body of the drop extremely resistant to external disruptions, whereas the tail constitutes a weak spot (Fig. 2 bottom). If the latter is now damaged in any way, the energy stored in the mechanical stress can be released and a mechanical failure front runs through the material, destroying more and more of it until the main body is shredded into dust. This process can happen with a speed of around 1600m/s, just like an explosion, and it usually only ends with the pulverization of the whole drop. Thus, this is the secret of the Prince Rupert’s Drop; it is always experiencing extreme internal stress that makes the convex part so extremely stable (like an arch bridge) that even rifle bullets shatter on them.

Finally, in its cooled state, the drop represents a system that exhibits extreme internal stresses. These stresses make the round shaped main body of the drop extremely resistant to external disruptions, whereas the tail constitutes a weak spot (Fig. 2 bottom). If the latter is now damaged in any way, the energy stored in the mechanical stress can be released and a mechanical failure front runs through the material, destroying more and more of it until the main body is shred to dust. This process can happen with a speed of around 1600m/s, just like an explosion, and it usually only ends with the pulverization of the whole drop. Thus, this is the secret of the Prince Rupert’s Drop; it is always experiencing extreme internal stress that makes the convex part so extremely stable (like an arch bridge) that even rifle bullets shatter on them.

So, we can ask whether this effect can be useful for anything. The answer is yes, indeed. Exactly the same principle is used in tempered glass like it is used e.g. in car windows. This glass does not shatter into sharp shards, but instead produces relatively smooth and small pieces and therefore is less harmful for the passengers of the car in case of an accident. Currently, Prince Robert’s Drops are even researched to understand better the quick cooling of volcanic lava under certain circumstances and therefore the inner processes within a volcano. Thus, all in all, these fascinating objects are full of wonders.

— Kai Litzius

Apr 152017
 
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The pursuit of the unobserved and the unfathomable in scientific research often affords the scientist glimpses of unrivaled visual experiences. The Princeton University Art of Science exhibition provides an avenue where scientists have the opportunity to present their images obtained during their research. The exhibition helps to spread awareness of the scientific technique and the artistic brilliance that research is replete with, to artists as well as to the common demographic. The exhibition attempts to forge a strong connection between Art and Science. The exchange with artists reveals a different way for scientists to visualize and contemplate their own research.

Read more about the exhibition here: Princeton Art of Science Exhibition

Apr 152017
 
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Tatjana Daenzer

JUnQ, 7, 1, XIV–XV, 2017

Is it possible to know everything in every discipline? Surely not, especially not in modern times in which it is increasingly important to have experts of an explicit field of knowledge. We all remember some real whiz kids from our school years but only a very few of us can be outstanding experts in widely varied fields. Just imagine the time you would need to learn all of it.

Read the full article here: Scholars Then and Now

Apr 142017
 
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Soham Roy

JUnQ, 7, 1, X–XIII, 2017

Science fair demonstrations are something that I always look forward to. I was there this other day at one such fair for gifted youngsters. I was demonstrating an experiment on densities. The experiment was quite a familiar one. The one where liquids with different densities do not mix. And where liquids with a lower value of density stay on top of liquids with larger densities, as distinct layers. To make it more vivid and interesting for the kids, I added a different color to each layer. A young boy came up to me after the demonstration and said…“It would be so boring if we did not invent colors to begin with”. His observation struck me and got me thinking. With our academic training in Science, we take a lot of stuff for granted. We rarely stop to wonder at the beauty and artistry inherent in the everyday experiments that we do and in the things that are around us.

Read the full article here: A Tale of Art and Science

Apr 142017
 
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Tyler Thrasher is an artist using many different techniques to express himself. He is a musician, a painter, an illustrator, a photographer and, not least, to some extent a scientist. For one of his current projects, he grows crystal clusters on collected, inanimate objects, like dead insects and skulls. By transforming deceased creatures into something beautiful, often mystical, he attempts to follow the approach of alchemists. Nevertheless, his art builds on “hard science” and follows the physical rules of crystallization. His results offer a different, inspiring view on a well-known method and teach not only science but also the inherent beauty of their studied objects.

Find the Interview here: Insect Alchemy

Apr 142017
 
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Physicist or Comedian? Action or science? Science journalist Dr. Sascha Ott provides during his talks and shows impressive evidence that knowledge and humor do not necessarily have to be contrasts.
Dr. Ott started studying physics in 1991, but soon figured out that journalism appeared to be more attractive to him. Eventually, he became a profound science journalist and started to perform his own science talks and shows.

Find the Interview here: On Air and on Stage

Apr 142017
 
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Dr. Eckart von Hirschhausen

Mit freundlicher Genehmigung von Dr. Eckart von Hirschhausen und Spektrum.

JUnQ, 7, 1, III, 2017

Eine Stichprobengroesse von N = 1, da straeubt sich der Wissenschaftsjournalist. Aber da das N in diesem Fall nicht N. N., also noch zu benennen ist, sondern mir bekannt, weil ich es selbst bin, berichte ich heute ueber einen kleinen Selbstversuch im Hirnscanner. Ich habe mich zweimal in die Roehre gelegt, vor und nach dem Sommer. Dazwischen habe ich Tanzstunden genommen. Ich wollte wissen: Wie plastisch ist mein Gehirn?

Lesen Sie den ganzen Artikel hier: Vom Kopf in die Beine und zurueck