Spurred by a wave of recent Web videos showing the bottom of a dropped hovering dramatically in midair , physicists have provided new insights into this phenomenon, from the existence of shock waves in the falling Slinky, to a remarkably universal "levitation" time for a Slinky on other planets or moons despite their different gravitational fields.
In February 2000, the late science writer Martin Gardner posed a simple question intended for physics students, but also triggering a new round of papers and videos on the much-studied toy. Gardner wrote: "If you hold one end of a Slinky, letting it hang down and then drop it, what happens?"
"It turns out the bottom stays suspended, levitating in air for some period in time," said Shimon Kolkowitz, a physics graduate student at Harvard University in Cambridge, Mass. As an undergraduate at Stanford University in Palo Alto, Calif. in 2007, Kolkowitz wrote a paper now posted online for a class taught by his professor, physics Nobel Laureate Robert Laughlin.
And recently, Bill Unruh, a physics professor at the University of British Columbia, in Vancouver, heard some colleagues in the faculty lounge discussing a video of the levitating Slinky. As a result, Unruh, a world expert in black hole radiation, became captivated with Slinky physics.
Making calculations over a couple of days, Unruh wrote and posted a paper on the falling Slinky at the website arXiv.
Inspired by Gardner's riddle and earlier Slinky studies while putting together his paper, Kolkowitz calculated that the bottom of his metal Slinky would remain suspended for approximately three-tenths of a second. And only recently he made a surprising realization: the levitation time of the toy would be exactly the same if it were dropped on the moon, Jupiter or Mars, even with their vastly different gravitational fields.
Unruh found that the falling Slinky creates a shock wave through the toy, analogous to the blast wave of a bomb or a sonic boom created by aircraft.
What in the world is going on?
"A Slinky is a simple spring, with the unique attribute that the spring in its natural resting state has all the coils touching one another," Unruh said.
"It's what's called a pretensioned spring," Kolkowitz added. "If you just leave it sitting on a desk on its side it'll actually be fully compressed."
Held from midair, the Slinky stretches out, quickly reaching a condition known as "equilibrium." in which the downward force of gravity is balanced by the upward tension of the coils above it. When the top is released, the bottom stays suspended. The top of the Slinky collapses, so that the coils slam into each other. That collapse travels down as a wave through the Slinky. The bottom coils remain at rest until the top crashes into them.
And that's the key to understanding how the bottom of the Slinky remains suspended in midair for a short while.
"The bottom part of the Slinky hasn't deformed in any way," Kolkowitz explained. "Until that compression reaches the very bottom it won't move."
This levitation time -- approximately 0.3 seconds for Kolkowitz's own Slinky -- would be the same on any planet or moon. Gravity and tension of the spring effectively cancel each other out.
Kolkowitz said that one way of understanding this is that on the moon, the weaker gravitational field wouldn't stretch the Slinky as much, so the spring would compress more gently towards the bottom when dropped, taking the same 0.3 seconds to travel there. On Jupiter, the stronger gravitational field would stretch the suspended Slinky to a greater degree, so that the spring would have a larger distance to compress. But the more stretched-out top would snap back faster toward the bottom, resulting in the same levitation time.
As Kolkowitz pointed out, however, the Slinky's center of mass -- which shifts, but is always located somewhere in between the top and bottom of the toy -- still accelerates according to gravity all the way down to the ground from the moment it's released. So there's no violation of any of Newton's laws or Galileo's observations about falling objects.
The levitation time would only increase with a heavier Slinky and decrease if the coils were stiffer. The spring's mass and stiffness, Kolkowitz said, are the only two factors that affect the duration of levitation.
Kolkowitz pointed out this levitation effect would occur when any other spring or other elastic, nonrigid object is dropped -- and no object is completely rigid. "It's just that the Slinky is an especially easy system" in which to observe the effect, he said.
Another way to think about the levitation problem is that "the wave velocity in that Slinky is all that matters," Kolkowitz said. The wave velocity dictates "the length of time it takes information to reach the bottom of the Slinky," he said. Once that wave slams into the bottom, the bottom no longer levitates.
In his analysis, Unruh observed that the collision of the upper part of the Slinky with the motionless lower coils is an example of a shock wave, analogous to a sonic boom that occurs in aircraft traveling faster than the speed of sound. Moreover, the wave that moves through the toy travels parallel to the compression of the Slinky, making it a "longitudinal" wave, the same type of wave as a sound wave. The normal speed of this wave in a Slinky is best measured by how many loops per second the wave passes through, about 50-100 loops per second for a typical Slinky, depending on such things as the thickness of the coils.
But in a falling Slinky, the coils crash into each other, creating a shock wave.
According to Unruh, the velocity of the shock wave, when it reaches the bottom, is notably higher than the normal velocity of the Slinky wave, breaking a sort of "sound barrier" in the Slinky.
"This behavior of shock waves is typical," he wrote in an email to Inside Science. "The blast wave of a bomb gets to you faster than the sound of a bomb would if it were very small."
A shock wave is simply a statement that something in a physical system changes abruptly, in this case, the velocity of the lower coils in the Slinky.
"There is a lot of interesting physics in a very, very simple system," said Unruh.
Kolkowitz said that this is an easy experiment for anyone to duplicate: use a stopwatch to time the fall when a friend drops a Slinky. This technique depends on the reflexes of the person running the stopwatch and therefore could introduce some error.
Filming the falling Slinky with a video camera that captures a known number of frames per second and then counting the number of frames in which the bottom of the Slinky stays still would allow experimenters to more accurately calculate how long the Slinky's bottom stays suspended.
"It's just such an easy experiment to do and it's kind of fun," Kolkowitz said.
Though Kolkowitz doesn't use Slinky experiments in his quantum physics work, he said the surprising insights on the levitating Slinky shows how studying and measuring even everyday objects can provide results that are "counterintuitive and not what you expect."
O singura doza de bautura dietetica acidulata, consumata zilnic, este de ajuns pentru a creste riscul aparitiilor unor afectiuni cardiace si hepatice, având totodata capacitatea de a favoriza declansarea diabetului.
Cercetatorii de la Facultatea de Medicina Miller din cadrul Universitatii Miami, împreuna cu cei de la Centrul Medical al Universitatii California, au efectuat un studiu pe 2.564 de participanti pe care i-au monitorizat timp de 10 ani. Ei sustin ca cei care beau astfel de bauturi sunt cu 43% mai predispusi sa dezvolte boli vasculare, atac de cord sau atac cerebral, comparativ cu cei care nu consuma bauturi carbogazoase.
Studii anterioare efectuate asupra bauturilor racoritoare, care contin cantitati substantiale de îndulcitori artificiali, au demonstrat ca acestea pot contribui la dezvoltarea acelorasi boli hepatice care apar, de regula, în urma consumului excesiv de alcool.
Bauturile acidulate dietetice sunt promovate ca o alternativa la cele care contin zahar, fiind considerate mai sanatoase pentru ca au mai putine calorii. Însa unele cerecetari pun la îndoiala aceste afirmatii, sugerând ca bauturile în cauza pot da dependenta.
Cu toate acestea, modul în care bauturile racoritoare favorizeaza aparitia afectiunilor vasculare este înca necunoscut, motiv pentru care oamenii de stiinta intentioneaza sa continue cercetarea.
Nel vecchio West si appendevano davanti al saloon i manifesti di brutti ceffi con su scritto: “Wanted”. Oggi, in Germania, alla polizia basta scrivere un post su Facebook per mobilitare centinaia di migliaia di informatori pronti a dare una mano nelle indagini. Infatti, nella città di Hannover questo esperimento di crowd-investigation ha già portato all'arresto di 8 malviventi. Ma, come spiega The Next Web, la decisione di trasferire la caccia all'uomo sui social network potrebbe avere alcuni lati negativi.
La cittadina tedesca, che conta circa 500mila abitanti, ha dato il via a questo progetto nel marzo 2011. Da quel giorno, la pagina Facebook del commissariato tedesco ha raggiunto quasi 100mila fan, la maggior parte dei quali (il 70%) ha meno di 35 anni. Un esercito di informatori via Web che segue le segnalazioni da parte delle autorità.
Infatti, in Germania la polizia ricorre alle indagini pubbliche quando ha la necessità di individuare una persona che abbia compiuto un crimine grave. Si parla di rapine, danni alla persona, abuso sessuale e incendio doloso: tutti casi in cui le autorità possono diffondere informazioni sui principali sospettati e sperare in una segnalazione che faccia scattare le manette.
Ma l'authority per la protezione della privacy della Bassa Sassonia non ha visto di buon occhio l'iniziativa del commissariato di Hannover, e lo scorso gennaio ha ordinato di sospendere il servizio per due settimane. La ragione ufficiale è che le informazioni su un'indagine di polizia – anche se pubblica – non possono essere diffuse su Facebook, perché i server del social network risiedono in territorio straniero.
Nonostante la batosta, il segretario degli Interni Uwe Schünemann è intervenuto di persona per sistemare la cosa. La soluzione? Le informazioni protette dalla legge tedesca rimangono nei computer del commissariato di Hannover, e sui server di Menlo Park vanno a finire solo dei pratici link postati sulla pagina Facebook. Davvero machiavellico.
La soluzione di Schünemann ha fatto un favore non solo ai 100mila fan del commissariato di Hannover, ma anche al resto delle autorità tedesche. Molte altre città stanno pensando di adottare lo stesso modello, e presto la caccia all'uomo su Facebook potrebbe estendersi a tutto il paese. Eppure, l'idea di dare in pasto alle fauci del social network l' identikit di un presunto criminale non entusiasma più di tanto.
Di fatto, potrebbe bastare un semplice errore di persona per rovinare la vita a qualcuno o esporlo a un linciaggio. Inoltre, Facebook è popolato da una grande quantità di troll pronti a spammare alla prima occasione. Lo dimostra il fatto che il commissariato di Hannover ha dovuto cancellare più di 500 post e bannare 100 utenti che avevano ironizzato su un caso di omicidio.
The microneedles painlessly pierce the top layer of skin, then gradually deliver the medication within them by harmlessly dissolving into the patient's bloodstream. As an added bonus, once everything is complete, there are no bio-hazardous used needles to dispose of. Now, bioengineers from Massachusetts' Tufts University have developed what they claim is an even better type of microneedle, which is made from silk.
With some types of existing microneedles, the harsh conditions required for their production can destroy the sensitive biochemicals that they were supposed to deliver. It can also be difficult to fine-tune the rate at which they deliver their medication, plus infections can sometimes occur where they enter the skin. According to the Tufts scientists, their silk microneedles address all of these problems.
The team started with aluminum molding masters that contained arrays of microneedles, each needle measuring 500 micrometers in height, with tips less than 10 micrometers in diameter. An elastomer was cast over those masters to create a negative mold, then a drug-laden silk protein was cast over that mold. Once the silk was dry, it was removed from the mold, then further processed using water vapor and various other means.
The whole procedure was conducted under ambient pressure and temperature, and resulted in biocompatible, dissolving silk microneedles impregnated with the large-molecule drug, horseradish peroxidase.
Using methods such as varying the silk protein's drying time, the researchers were able to alter its structure, which in turn allowed them to precisely control its rate of drug release. It was also found that adding tetracycline to the protein inhibited the growth of Staphylococcus aureus bacteria at the application site on the skin.
As with other types of microneedles, the silk needles can be shipped and stored without refrigeration.
Colin Delehanty si Sheldon Neill, doi americani pasionati de fotografie si calatorii, au decis sa promoveze unicitatea Parcul National Yosemite, unul dintre cele mai frumoase si mai apreciate habitate naturale din America de Nord, prin intermediul unui video inedit.
Parcul se întinde pe o suprafata de 308 mii de hectare si are un relief foarte variat si o biodiversitate remarcabila, gazduind nenumarate specii de plante si animale. Desi anual parcul este vizitat de peste 3,7 milioane de oameni, sistemul de managment are grija ca acestia sa nu dauneze ecosistemului.
Proiectul reprezinta o prima colaborare între cei doi fotografi, care au efectuat mai multe calatorii la Yosemite. Filmarea a fost facuta folosind tehnica time-lapse, care expune tranzitia dintre diferite momente ale zilei si anotimpuri.
Creatorii spera ca montajul, care arata frumusetea acestui loc, sa ne faca sa constientizam ce am putea pierde daca încalzirea globala continua sa evolueze în acest ritm fiindca nu luam masurile necesare. În prezent, arborii mari din parc, care au un rol esential în ecosistem, au început sa dispara, ghetarii se topesc în ritm alarmant, iar descarcarile electrice provoaca din ce în ce mai multe incendii.
Library.nu non è più accessibile e la sua collezione di 400mila ebook piratati è stata messa offline. L'operazione è avvenuta martedì, in seguito all'azione congiunta di diverse case editrici internazionali e le relative associazioni di rappresentanza, cui si è unita anche l' Associazione italiana editori insieme a giganti del libro come HarperCollins, Oxford University Press e Macmillan. L'ingiunzione è partita da un tribunale di Monaco contro due siti, Library.nu e il connesso iFile.it. La libreria illegale è accusata di aver messo a disposizione migliaia di link a file pdf contenenti ebook coperti da copyright, liberamente scaricabili attraverso iFile.it. L'ordine è partito dopo 7 mesi di indagine privata gestita dalla tedesca Börsenverein des Deutschen Buchhandels e la International Publishers Association.
Library.nu raccoglieva opere di narrativa e libri di testo, 4mila dei quali erano italiani. Si calcola che il sito, da quando è stato messo online nel dicembre del 2010, abbia generato 8 milioni di euro grazie ai banner pubblicitari e gli accessi premium, una formula che ricorda quella di Megaupload a sua volta chiuso con l'accusa di aver reso disponibili contenuti coperti da diritto d'autore. Un giro di affari completamente generato dalla diffusione di materiali coperto da copyright. Stando a quanto dichiarato dalla Aie i file depositati su iFile.it saranno progressivamente cancellati. "L'industria internazionale del libro ha dimostrato che continua a resistere contro la criminalità organizzata che attenta al copyright. Non tollereremo scrocconi che realizzano profitti ingiustificati, privando gli autori e gli editori di quanto loro dovuto. Questo è un passo importante verso un commercio più trasparente, onesto e leale dei contenuti digitali in Internet" è stato il perentorio commento di Jens Bammel, Segretario Generale dell’associazione internazionale degli editori.
Secondo l' Huffington Post, Library.nu avrebbe avuto la sua base in Ucraina, ma il suo dominio sarebbe stato registrato nell'isola di Niue nel Pacifico. Un portavoce di iFile.it, rimasto anonimo, avrebbe inoltre dichiarato l'estraneità della piattaforma di filehosting con Library.nu. Secondo l'accusa, al contrario, i due siti sarebbero stati strutturalmente connessi per offrire un servizio congiunto di ricerca e download. L'indagine sarebbe inoltre arrivata a una svolta grazie a una leggerezza dei proprietari dei domini. Library.nu offriva la possibilità di effettuare donazioni tramite PayPal il quale inviava agli utenti la ricevuta recante il vero nome degli intestatari dell'account sul servizio di money transfer in Rete, Fidel Nunez e Irina Ivanova, gli stessi nominativi in possesso della Irish Companies Registration Office presso la quale era stata registrata l'azienda che gestiva iFile.it. E la connessione è stata immediata.
Scientists are now trying to use plasmonic nanoparticles in cancer therapy whereby light energy is converted into heat in order to kill cancer cells. The advantage of such treatment is that it does not cause side effects that are common to chemotherapy. Mingyong Han at the A*STAR Institute of Materials Research and Engineering and co-workers have now developed gold plasmonic nanocrosses that are particularly suited to eliminating cancer cells in cancer therapy. The team demonstrated the usefulness of these nanocrosses by using them to kill human lung cancer cells.In general, metallic nanostructures have a particular frequency at which light excites electrons close to their surface. The collective movement of electrons, or resonance, in the metal is what converts the light energy into heat. The wavelength at which the resonance occurs is strongly dependent on the size and shape of the nanostructures.
For biomedical applications, the nanostructures should be effective no matter which direction they are illuminated from. Furthermore, the nanostructures should be efficient in absorbing near- to mid-infrared wavelengths because tissue is transparent to the light of these wavelengths.
Based on these requirements, the researchers decided to make gold nanocrosses (see image). In normal synthesis, however, gold would usually grow into the shape of the nanorods. To fabricate nanocrosses, the researchers added copper ions to the growth solution. The incorporation of small amounts of copper caused a twinning of the gold’s crystal structure, which in turn led to the growth of side arms from the crystal facets. “The unique cross-shaped gold structure enables multi-directional excitation to achieve a strong plasmonic resonance in the near- and mid-infrared region. This greatly lowers the laser power required for photothermal cancer therapy compared to nanorods,” says Han.
The researchers tested the performance of their gold nanocrosses by modifying their surfaces and binding them to human lung cancer cells. When irradiated with near-infrared laser light of relatively modest powers of 4.2 W/cm2 for 30 seconds, all cancer cells were killed. The researchers are now planning to test the effectiveness of the gold nanocrosses on animal models in future experiments.
Other applications of the gold nanocrosses are also possible, including photothermal imaging, in which small amounts of light are converted into local heat, or the sterilization of surfaces. “In our current research, we are studying gold nanocrosses for the photothermal destruction of superbugs on biofilms,” says Han.
The "passive optical diode" is made from two tiny silicon rings measuring 10 microns in diameter, or about one-tenth the width of a human hair. Unlike other optical diodes, it does not require external assistance to transmit signals and can be readily integrated into computer chips.
The diode is capable of "nonreciprocal transmission," meaning it transmits signals in only one direction, making it capable of information processing, said Minghao Qi (pronounced Chee), an associate professor of electrical and computer engineering at Purdue University.
"This one-way transmission is the most fundamental part of a logic circuit, so our diodes open the door to optical information processing," said Qi, working with a team also led by Andrew Weiner, Purdue's Scifres Family Distinguished Professor of Electrical and Computer Engineering.
The diodes are described in a paper to be published online Dec. 22 in the journal Science. The paper was written by graduate students Li Fan, Jian Wang, Leo Varghese, Hao Shen and Ben Niu, research associate Yi Xuan, and Weiner and Qi.
Although fiberoptic cables are instrumental in transmitting large quantities of data across oceans and continents, information processing is slowed and the data are susceptible to cyberattack when optical signals must be translated into electronic signals for use in computers, and vice versa.
"This translation requires expensive equipment," Wang said. "What you'd rather be able to do is plug the fiber directly into computers with no translation needed, and then you get a lot of bandwidth and security."
Electronic diodes constitute critical junctions in transistors and help enable integrated circuits to switch on and off and to process information. The new optical diodes are compatible with industry manufacturing processes for complementary metal-oxide-semiconductors, or CMOS, used to produce computer chips, Fan said.
"These diodes are very compact, and they have other attributes that make them attractive as a potential component for future photonic information processing chips," she said.
The new optical diodes could make for faster and more secure information processing by eliminating the need for this translation. The devices, which are nearly ready for commercialization, also could lead to faster, more powerful supercomputers by using them to connect numerous processors together.
"The major factor limiting supercomputers today is the speed and bandwidth of communication between the individual superchips in the system," Varghese said. "Our optical diode may be a component in optical interconnect systems that could eliminate such a bottleneck."
Infrared light from a laser at telecommunication wavelength goes through an optical fiber and is guided by a microstructure called a waveguide. It then passes sequentially through two silicon rings and undergoes "nonlinear interaction" while inside the tiny rings. Depending on which ring the light enters first, it will either pass in the forward direction or be dissipated in the backward direction, making for one-way transmission. The rings can be tuned by heating them using a "microheater," which changes the wavelengths at which they transmit, making it possible to handle a broad frequency range.
Cercetatorii de la Universitatea din Pennsylvania au conceput un sistem cu ajutorul caruia quadrocopterele de mici dimensiuni pot zbura în formatii complexe, prefigurând un viitor în care asemenea mini-roboti vor fi folositi pe câmpurile de lupta.
Mini-robotii zburatori au fost conceputi de compania KMel Robotics, iar sistemul de coordonare care permite celor 16 quadrocoptere (masinarii de zbor cu patru elice) sa faca manevre complexe a fost dezvoltat de o echipa de cercetatori de la Universitatea din Pennsylvania, din cadrul laboratorului GRASP (General Robotics, Automation, Sensing, and Perception).
Fiecare robotel are senzori sofisticati care îi permit sa se pozitioneze cu precizie, astfel ca mini-robotii zburatori se pot deplasa în grupuri, evitând obstacolele si având capacitatea de a schimba formatia de zbor în spatiul 3D.
Iata imaginile spectaculoase cu mini-roboteii zburând în formatie:
Anche le app trafugano i nostri dati. Ecco quali sono!
Google spiava gli utenti Apple. Č la pesante denuncia del quotidiano Wall Street Journal. Ma in che senso? Secondo il Wsj, l'attività di chiunque navigasse in Rete con un browser Safari (disponibile di default su Mac, iPhone e iPad) era tracciata dal motore di ricerca di Mountain View.
Queste informazioni erano contenute nei cookies, brevi file testuali memorizzati nei dispositivi degli utenti, ma senza il loro consenso. La pratica dei cookies, infatti, è usata comunemente nel Web, ma Safari di default la aggira, non permettendo il tracciamento del proprio girovagare online e garantendo maggiore privacy. Google, con poche righe di codice e qualche escamotage, sarebbe riuscito invece a ingannare il browser e a conoscere le abitudini degli utenti Apple (come già succede con altri browser).
A cosa servono queste informazioni? In sostanza, sono fondamentali per il marketing: in loro assenza, BigG non riuscirebbe a proporre pubblciità mirata. Contattati dal quotidiano Usa, a Mountain View hanno risposto che nei cookies non era conservata alcuna informazione personale, ma si sono premurati di bloccare subito questa procedura.
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