Wikipedia:Reference desk/Archives/Science/2014 December 21

= December 21 =

First date similarity to job interview
Is there scientific basis when people say that a first date is exactly the same or similar to a job interview
 * In what ways are they supposed to be the same? (It can't be every possible way--I don't think many people submit a resume before going on a first date.) --173.49.11.192 (talk) 01:04, 21 December 2014 (UTC)
 * (e/c)It doesn't sound like the sort of thing scientists would spend much time on. Not much grant money in it. Typing first date interview in Google, brought up a bunch of hits with some explanations, such as this one. Matt Deres (talk) 01:05, 21 December 2014 (UTC)

Strength of 3D Printer Plastic
They printed a wrench on the International Space Station. For the printer and source material they use for that printer that they have there, what's the spec on the shear stress a wrench made with it is likely to be able to take before it bends and breaks? 75.75.42.89 (talk) 02:30, 21 December 2014 (UTC)


 * I can't give you numbers, but I can say that the printer, designed by Made in Space inc., "uses an extrusion based method that layers hot liquefied ABS plastic to build a defined object". So they're in the envelope of what acrylonitrile butadiene styrene can do. -- Finlay McWalterᚠTalk 02:59, 21 December 2014 (UTC)


 * Yeah - but this is more of a test than an effort to make a useful tool. They actually plan to ship the wrench back to earth as soon as possible to test it's strength compared to one made on an identical machine here on earth.
 * But there are 3D printing technologies that produce really strong nylon parts - and laser-assisted metal sintering can produce metal parts in several useful metals. These are early days for zero-g 3D printing and because most (all?) normal 3D printers rely on gravity, it'll probably take many iterations of this technology before they can use it "for real".
 * In many respects, 3D printing in space is actually easier than here on earth. The need for "support materials" in earthly 3D printing is reduced or even eliminated in zero-g - and without the effects of convection on heat flow, it may be easier to control some kinds of materials.
 * SteveBaker (talk) 07:28, 21 December 2014 (UTC)

The article describes it as a "ratcheting socket wrench". Do we know if it functions as such, or does it just have the exterior resemblance of of a socket wrench? If it truly ratchets, do we know how many parts it is comprised of, what assembly was required, and whether any surfaces need post-printing finishing? A typical ratcheting socket wrench is composed of many parts, including bearing and springs, although I could imagine an ersatz, non-reversible (or perhaps double-sided, reverse by flipping), plastic wrench with only two pieces which snap together. -- ToE 12:59, 21 December 2014 (UTC)


 * See for example this article, which says the wrench consists of "...a sequence of 21 prints..." that resulted in "...a working socket wrench complete with ratchet action...". As an aside, there are a few simple ratchet wrenches over at Thingiverse that shows ways to make one with fewer parts. WegianWarrior (talk) 13:08, 21 December 2014 (UTC)


 * Thanks WegianWarrior. It's nice to hear that the thing actually ratchets.  FWIW, it's not clear that that tool itself took 21 prints.  Instead, it appears to have been amongst the items manufactured during a sequence of 21 prints.  "It also marks the end of our first experiment – a sequence of 21 prints that together make up the first tools and objects ever manufactured off the surface of the Earth." -- ToE 17:50, 21 December 2014 (UTC)


 * Made in Space's blog entry gives some more details: "The ratchet was designed as one print with moveable parts without any support material. The parts and mechanisms of the ratchet had to be enclosed to prevent pieces from floating in the microgravity environment." and "The ratchet took 4 hours to print ...". It also has a detailed image of the a copy printed on earth, showing that the handle is labeled "3 in-lb"-- ToE 12:34, 22 December 2014 (UTC)


 * I wonder how enclosed interlocking/ratcheting parts were made separately without support material without getting stuck together during creation. 75.75.42.89 (talk) 13:03, 22 December 2014 (UTC)


 * My impression is that it was made from a number of separate parts that snap together somehow. This is entirely feasible.  Also, you may not need much in the way of support materials in zero g.  If I was tasked with making it in one piece - I'd have a very thin piece that would be extruded just to prevent the parts from drifting around during printing that could maybe be designed to simply snap the first time any significant amount of force is applied to the ratchet, leaving all of the moving parts free thereafter.  With that approach, there would be no loose parts and no  need to actually assemble it.   But I have no way to know whether that's what they actually did.  I think it's a shame that they haven't published the files they used for printing it.  I'm sure a LOT of 3D printer owners would love to take a shot at making it.  SteveBaker (talk) 18:29, 23 December 2014 (UTC)


 * I see, kind of like how plastic model car/airplane/whatever pieces are held to a plastic frame in the kit and meant to be disconnected. 75.75.42.89 (talk) 22:14, 23 December 2014 (UTC)

Physics in the movie Gravity
I just finished watching the movie Gravity. I was surprised that it actually portrayed the physics relatively realistically compared to other Hollywood films, but there was one particular scene that I found a bit odd. When Sandra Bullock's and George Clooney's characters arrive at the International Space Station using Clooney's MMU, their velocity relative to the station is too great and they have only a limited amount of fuel left to both correct their course and slow down enough to grab onto the external handles. Because of the bumpy landing neither character get a permanent hold, but Bullock's foot is tangled in some ropes while she grasps a tether that keeps Clooney suspended away from her. If I didn't describe that very well it looks something like this:

0[ISS]0-8[Bullock]8--C[Clooney]C

Now at this point with both the ropes and the tether pulled tight and all of the objects at rest with respect to each other, as I see it there is no force on Clooney or Bullock and she should be able to reel his tether in and climb up the cables to the ISS. But instead of that Clooney makes some bizarre statement about him pulling her with him and ends up letting go and flying off into space. Is there any possible force that would have been causing him to drag her and then push him off into space once he let go? 139.195.41.154 (talk) 11:57, 21 December 2014 (UTC)
 * If the tether is pulled taut, there will be some pent-up potential energy in the tension of the tether, like a rubber band pulled taut. Being at "rest" may mean that all forces are balanced, but it doesn't mean that all energy is nil.  Just like a stretched rubber band can be motionless and under balanced forces, when one end is let go, the other end flies off in the other direction.  Clooney's character was screwed either way: If the line holding Bullock to the ISS broke, the both of them would be pushed out into space.  If the line between Clooney and Bullock broke, Clooney drifts out into space, and Bullock drifts towards the ISS.  He's a goner either way, but by letting his end of the rope go, he saves her.   -- Jayron 32 14:14, 21 December 2014 (UTC)
 * I also thought that scene was a terrible exception to a very good movie. It looked like Clooney had reached the end of his tether, was motionless in space, but some malignant ongoing force was hell-bent on pulling him away as a plot device.  It should be apparent that once Clooney was truly motionless relative to Bullock his inertia could not add additional tension to the tether between them, and once Bullock was truly motionless relative to the shuttle the tension on her tether couldn't increase.  Now there are a hundred ways you can try to salvage the physics by saying that X wasn't really motionless, the cables were about to slip, the tether was unravelling like the rope in a Western cliffhanger, there was a leak in the jetpack, there were extra dimensions of spin and revolution that weren't immediately obvious, whatever... but the scene just didn't carry over that message to me as a viewer.  It didn't feel right. Wnt (talk) 16:28, 21 December 2014 (UTC)
 * Forgetting for the moment the truly appalling acting, that film was marred by continual glaring physics errors. To move from a Hubble orbit to an ISS orbit to a Tiangong orbit will take enormous amounts of delta-V, certainly not possible the way it was depicted. MMU from Hubble to ISS? Please. For a film claiming to realistically depict spaceflight, it was deeply disappointing. Fgf10 (talk) 19:40, 21 December 2014 (UTC)
 * That is not a physics error, it's a "geography" error. They are simply pretending that those satellites are all in similar orbits. --65.94.50.4 (talk) 02:01, 22 December 2014 (UTC)
 * I don't know that they ever claimed 100%. They had to take more than a little dramatic licence to make it happen. Mingmingla (talk) 23:56, 21 December 2014 (UTC)
 * I could excuse this because we can't rule out (well, at least not with certainty) that one or more satellites might have been moved prior to the events depicted. Of course, that would be unlikely to actually be done... Wnt (talk) 01:13, 22 December 2014 (UTC)
 * This "strange" force is called tidal force and can be quite strong is the rope is long enough. Ruslik_ Zero 20:50, 21 December 2014 (UTC)
 * It's not at all difficult to calculate. Assume a tether 650 m long. Picked out of the air as one ten thousanth the distance to Earth center. Inverse square law, the g differential is 0.0002 which means 100 kg George is being pulled by 20 grams force. Yeah, I know grams are not force. So, if he's strong enough to pick up a pencil on Earth, he's strong enough to pull himself up and join Sandra. And if the cable is a lot less than half a mile, then it's a lot less force. Grade for the movie as lesson in orbital mechanics, B. That is, 'way above average, given the miserably low standards of Hollywood in this regard. Jim.henderson (talk) 21:12, 21 December 2014 (UTC)


 * It's been a while since I saw the movie, but the way I remember that scene was that there wasn't enough strength in the ropes to bring both of the two characters to a stop (relative to the station); they were starting to part or untangle or whatever, and the two people were still moving. If so, there is no problem; Clooney lets go and the ropes are strong enough to stop one person.  Does the scene really show them coming to a stop, or are people being fooled by perspective or something? --65.94.50.4 (talk) 02:07, 22 December 2014 (UTC)


 * Here's the scene. I think this counts as fair use from our perspective, if not the Youtube uploader's. There's a moment early on at which the rope holding Bullock and Clooney jerks and they seem to come to rest relative to the station. Seemingly they should then have bounced back and started drifting toward each other. But later shots (especially the long shots) seem to show them both drifting away from the station again, at a constant distance from each other, with the rope between them still taut for some reason. When Clooney lets go he immediately "falls", the rope goes slack, and Bullock starts moving rather rapidly toward the station. It's not a tidal force—there's only about 6m of rope between them, and ~20m from Bullock to the station, and anyway the rope is oriented tangentially to Earth, not radially, so the tidal force would bring them together, not apart. There's not enough rotation for it to be a centrifugal force. And of course no gravitational/fictitious force could pull one person toward the station and the other away. Maybe it was magnets. -- BenRG (talk) 04:17, 22 December 2014 (UTC)


 * Or was there any rotation involved (both of them revolving around ISS)? Was it supposed to be the centrifugal force that was pulling JC away ? (In that case, the rope should have wound round the ISS). I don't recollect the mention of rotation in the scene, though - WikiCheng | Talk 12:09, 22 December 2014 (UTC)


 * With particular reference to the OP's question: . And some other sources representing how the film became a bit of a whipping-boy for physicists, engineers, astronauts and other relevant experts in the wake of its original theatrical release:, , , , .  But for my money, it was comedian Louis C.K. who struck on one of the more glaring oddities that make it hard to suspend disbelief and follow the narrative: starting at 0:47.  S n o w  talk 23:25, 22 December 2014 (UTC)


 * These are all valid complaints. But watching the movie, I found myself tending to rationalize them all away with the same thing: I kept thinking that this had to be fairly far in the future.  The various old hardware hanging around was more for historic interest than cutting edge science, propellant had become much cheaper (I was thinking of an orbital skyhook, not a true space elevator but capable of assisting loads to greatly reduce cost), and, yes, even reluctant astronauts were being pulled into service.  (Though the endless parade of people succumbing to the much more mundane "light grenade" lure of Mount Everest does admittedly cast doubt on the idea that we'd ever run out of people looking to be put at hazard) Wnt (talk) 21:39, 23 December 2014 (UTC)

Not quite a robot, so what do you call it?
I recently bought a kiddie Build-Your-Own-Robot kit on a whim, thinking it would amuse my cats. This so-called "robot" is made up of a hollow sphere that pops into a little square frame with a dome on top. Within the sphere is a simple motor with a weight attached. The motor causes the sphere to spin, and the weight keeps it off-balance so that it can turn and move around obstacles. Basically it just skitters around the floor bumping into things. It certainly isn't a robot, but is there a word in science/robotics for something like this?146.235.130.59 (talk) 22:52, 21 December 2014 (UTC)


 * Why isn't it a robot? It's certainly not a complicated one, but it behaves just as our wikipedia article on robots indicates that it should. Mingmingla (talk) 23:54, 21 December 2014 (UTC)


 * I wouldn't call it a robot simply because there is no way to give it data or receive data from it. It's just a motor in a plastic shell. It can't see, hear, make decisions, or be programmed to perform a task. It's like the engine in a car or a clock, just a series of moving parts. Clocks and car engines aren't robots, surely?146.235.130.59 (talk) 13:35, 22 December 2014 (UTC)


 * It sounds like a Marvelous Toy. ←Baseball Bugs What's up, Doc? carrots→ 03:41, 22 December 2014 (UTC)


 * Robots are usually presumed to do some sort of work, the word comes from the Slavic robotiti to work as in slave labor. μηδείς (talk) 04:39, 22 December 2014 (UTC)


 * Like entertain cats? Nil Einne (talk) 14:00, 22 December 2014 (UTC)


 * I think the def of a robot they are using here is that it must change position on it's own, without control by a person. Compare it to Roomba, an only slightly more sophisticated robot.  (talk) 15:16, 22 December 2014 (UTC)


 * Good point, StuRat. So at what point does something stop being a wind-up toy and start being a robot? The specific "robot" I am talking about is just a motor in a plastic case. The motor alone wouldn't be called a robot, neither would the case. If I take out the motor and turn it on it just sits there spinng, and the case is just an empty piece of plastic. Put the two of them together and neither one of them is changed at all. The motor just spins, the case is just a case, but together they give the illusion of intelligent movement. Is that enough to call it a robot?146.235.130.59 (talk) 19:41, 22 December 2014 (UTC)


 * Still similar to a Roomba. It has a chip in it that tells it to change direction when it runs into something.  If you took that out it would just run into a wall and stay there.  So, probably not enough to call it a robot at that point (a powered-wheel lawn mower would do that much, if you disabled the dead man switch). StuRat (talk) 01:58, 23 December 2014 (UTC)


 * How do we feel about using "automaton" in this context? {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 20:14, 22 December 2014 (UTC)


 * Well now I just feel stupid for not thinking of that before. This toy certainly seems to fit the description, in that it's just a mechanically animated object that gives the illusion of life/intelligence.146.235.130.59 (talk) 13:41, 23 December 2014 (UTC)


 * We're making a mistake by looking for a Platonic ideal of the meaning of the concept "robot". Concepts are used and defined in context.  Obviously a cat toy is not a "robot" displacing unskilled laborers from factories.  A toy salesman, however, might find the cachet of a robotic cat toy pleasing.  The etymology has been given.  We can't provide  the metaphysical Answer.  For closely related words, google "robot synonym". μηδείς (talk) 02:55, 23 December 2014 (UTC)
 * There is certainly a divide between the technical meaning (something like "A computer that can take physical actions in the world") and the general public's idea of something mechanical that seems to move around by itself. But when you come across things like Robot combat where the machines are generally just radio controlled by humans - or your cat toy, it's clear that the common-use term has diverged from the technical meaning.   People who build things like 3D printers describe the moving parts of those things as "robots" - but to casual users of the term, they are no more robots than a microwave oven would be.   So language changes.   "Computer" used to mean "Person who does calculations for a living" - but not anymore.   Right now, if you want to describe machines of this nature accurately, you need to start layering on more description.  "Autonomous robot" might describe the Roomba vacuum cleaner - but exclude the cat toy.  But then we know that the Roomba doesn't do much other than driving in random directions until it hits something, then turning through a random angle and trying again.   That's something that mechanical childrens' toys have been doing for at least a century - so no computers required.  (My favorite toy in the late 1950's was a "Tricky Tommy Tractor"  which did more or less what the Roomba does with just a single motor and a differential-drive gear).   On the other hand my Neato Robotics vacuum cleaner has a laser sensor that maps out the shape of your rooms, intelligence to plan a route through that environment and a stack of algorithms it uses as strategies for getting out of a jam, a memory of where it got stuck and had to be rescued - with a sense of "danger" to avoid those places in the future.  It even automatically returns to its charging station to 'feed' when it's battery gets low.   The Neato comes close to behaving like a live animal - and certainly lays claim to the name "robot" in every sense of that word.   However, a regular inkjet printer also meets most of the criteria to be a robot - and yet I can't think of anyone who'd describe that way.
 * It all comes down to the steady evolution of language...as annoying as that can sometimes be. SteveBaker (talk) 18:04, 23 December 2014 (UTC)


 * Yes, of course, Steve. The alarm clock that wakes me up each morning is a robot, as is the train I take to work. This is what's called special pleading. μηδείς (talk) 01:43, 24 December 2014 (UTC)


 * I think being able to change it's location is key to the definition of "robot", excluding the printer. However, I take "robotic" to mean "similar to a robot", in that it can move, but not necessarily change location, so a "robotic arm" is correct, but you wouldn't call the arm a "robot" by itself, since it can't change it's location.  So, you could call a printer a "robotic printer", but that would be redundant, unless used in a context where there's also some type of non-robotic printers (printmakers ?).  Then there's the word "bot", which to me means automated software, with no physical movement involved. StuRat (talk) 18:16, 23 December 2014 (UTC)


 * ...and "robo" as in robocall...where, again, no physical action is involved, but a sense of an automatic machine doing the work of a person is present. We also use the word "robotic" in the sense of being poorly capable...a bad actor may speak his lines 'robotically' - or someone might dance the Robot (dance) by moving in a rough, jerky manner.  Yet we might also say that a sniper picked off the enemy with robot-like precision...which is kinda opposite to dancing robotically.  This word is clearly changing meaning too fast for dictionaries and hard definitions to keep up.  So we just hold on and see where it takes us...I think this word is in for a rough ride before it finally settles into a new set of meanings.  SteveBaker (talk) 21:34, 23 December 2014 (UTC)


 * I think that meaning of robotic is "gets the job done, but inelegantly". Or, as might be said of a musician "technically proficient, but lacking 'heart' ".  We are finally starting to get some robots that don't move and speak in a jerky motion, so that might eventually make that "inelegantly" part fade away. StuRat (talk) 00:34, 24 December 2014 (UTC)


 * Note that in South Africa, "robot" can refer to a robot policeman, i.e. a traffic light. As I hold to the view that the nation of South Africa is as entitled to add words to English as music promoters, I regard this as a fair usage anywhere. Wnt (talk) 05:26, 24 December 2014 (UTC)