Rotate vs revolve

Rotate versus revolve, from http://www.worldwidewords.org/nl/uifj.htm
by MICHAEL QUINION

Q From Brian Miller, Australia: A loosely organised group of eccentric friends and wine lovers meets each week. The question arose, does a lazy Susan revolve or rotate? What about the plates on it?

A That’s an interesting question, which lacks a simple answer. If anybody’s not sure about a lazy Susan, by the way, it’s a device on a table which turns to give easy access to plates and condiments.

... The two words are used so interchangeably in the sense of spinning round that for most purposes they’re synonyms and they’re treated as such in thesauruses. To take an example, does a wheel rotate or revolve? Most people would say it can do either.

If you’re arguing from etymology (always risky), it can only rotate, since that term is from the Latin verb rotare, to turn in a circle, whose root is rota, a wheel. But you might argue that it revolves, because that verb is from the Latin volvere, to roll (in this case, the re- prefix implies repetition of the action) and a wheeled vehicle certainly does roll along.

Strictly speaking, there is a difference, which is most noticeable in the terminology of astronomers. For them, the earth rotates every 24 hours but takes a year to revolve around the sun. The rule about which verb to use is based on the position of the axis of rotation. If the body turns on an axis within itself it rotates but if the axis is outside it revolves. Following this definition, a wheel can only rotate (hooray for etymology).

The strict answer to the question, therefore, is that the lazy Susan rotates. However, because the plates on it orbit or circle around an axis outside themselves, they revolve. Do not insist on this careful distinction during the later stages of a dinner party or the lazy Susan may become a spinning projectile aimed at you.

As I say, the rule is rarely observed outside science and the two words have been hopelessly muddled for centuries. A revolving door actually rotates; a rotating shaft makes revolutions. You might argue that a revolver ought to be a rotator but it depends whether you are thinking of the cartridges or the cylinder that holds them.

La trottola

Da "Semplicemente fisica. Fraintendimenti, bugie, buchi neri nell'apprendimento scolastico della fisica", di  Giovanni Tonzig, Maggioli Editore, 2010 - 227 pagine

Space Oddity

Assembly line

"The assembly line was invented 100 years ago. It’s time to invent the disassembly line", Steven Cherry is telling at

http://spectrum.ieee.org/podcast/at-work/innovation/the-future-of-the-assembly-line

in a conversation with David Nye,  professor of American history at the University of Southern Denmark.

Deep Space Beacon

Pulsed gamma rays from the Vela pulsar  from photons detected by Fermi's Large Area Telescope. The Vela pulsar is the brightest persistent source of gamma rays in the sky. The bluer colour in the latter part of the pulse indicates the presence of gamma rays with energies exceeding a billion electron volts (1 GeV). For comparison, visible light has energies between two and three electron volts. Red indicates gamma rays with energies less than 300 million electron volts (MeV); green, gamma rays between 300 MeV and 1 GeV; and blue shows gamma rays greater than 1 GeV. The image frame is 30 degrees across. The background, which shows diffuse gamma-ray emission from the Milky Way, is about 15 times brighter here than it actually is.
Source Goddard Space Flight Center
Author Roger Romani (Stanford University) (Lead), Lucas Guillemot (CENBG), Francis Reddy (SPSYS)

n.23 - Joe e Moe

Un cuneo di massa M è in quiete su una superficie orizzontale priva di attrito. Un blocco di massa m è posto sul cuneo. Non c'è attrito tra il blocco e il cuneo. Il sistema è lasciato libero da fermo. Calcolate: a) l'accelerazione del cuneo, 2) le componenti orizzontale e verticale dell'accelerazione del blocco, e verificate il limite M che tende a infinito.

A wedge with mass M rests on a frictionless horizontal tabletop. A block with mass m is placed on the wedge. There is no friction between the block and the wedge. The system is released from rest. Calculate 1) the acceleration of the wedge, 2) the horizontal and vertical components of the acceleration of the block, check the limit when M  --- > infinite. 

Risolviamo il problema con Joe e Moe, ossia col moto relativo.

Consideriamo due osservatori: Joe è fermo sul piano orizzontale e Moe è fermo sul cuneo.

Joe vede che il blocco m si muove sul cuneo  scendendo verso destra  e che il cuneo si muove verso sinistra  con una accelerazione A. Moe, che è sul cuneo, non vede il cuneo muoversi, ma vede la massa m  che si muove soggetta al peso, alla normale N del cuneo e alla forza  fittizia mA verso destra.

Scriviamo l’equazione per l’accelerazione  del blocco m, PARALLELA al piano inclinato,  che fornisce  l’accelerazione lungo il piano inclinato  vista da Moe, e poi scriviamo l’equazione  d’equilibrio lungo la PERPENDICOLARE al piano inclinato:

GAMMA RAYS and Gamma-Ray Burst

Gamma rays have the smallest wavelengths and the most energy of any wave in the electromagnetic spectrum. ... http://missionscience.nasa.gov/ems/12_gammarays.html
Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the brightest electromagnetic events known to occur in the universe.[1] Bursts can last from ten milliseconds to several minutes. The initial burst is usually followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, microwave and radio).[2] http://en.wikipedia.org/wiki/Gamma-ray_burst...
On April 27, 2013, NASA's Fermi and Swift satellites detected a strong signal from the brightest gamma-ray burst in decades. Because this was relatively close, it was thousands of times brighter than the typical gamma-ray bursts that are seen by Swift every few days. Scientists are now scrambling to learn more....
http://edition.cnn.com/2013/05/06/opinion/urry-gamma-ray-burst/index.html?sr=sharebar_twitter

Cilindro e oscillazioni

Baricole

Da "Storia e leggenda del vetro" di Paolo Mazzoldi
http://www.edscuola.it/archivio/lre/storia_del_vetro.pdf

rendimento