Wikipedia:Reference desk/Archives/Science/2017 February 15

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February 15

Diabetes type 2 - mechanism of the problem

What is the problem of diabetes type 2? I've said it because the pancreas don't secret enough insulin, and my teacher said it's not because of secretion of the pancreas since the pancreas secretes normal insulin levels but the problem is with the cells that they receptors for the insulin are covered by fats. He added that diabetes type 1 has to do with the fact that the pancreas don't secret enough insulin but in diabetes type 2 the pancreas does secrets enough insulin. Then I've asked him again, why diabetic type 2- patients used to take insulin if they have enough insulin in their body and he answered me that it is given to the patient when the diabetes is progressive after some years that the pancreas is already out of function. Now all this information is really against what I was told in the past by another teacher (that said what I did). What's true? 93.126.88.30 (talk) 00:11, 15 February 2017 (UTC)

There are many variations of diabetes. See brittle diabetes and insulin resistance, for example. StuRat (talk) 02:24, 15 February 2017 (UTC)

Our article Diabetes mellitus says that your teacher is correct-- type 1 is due to a bad pancreas, while type 2 starts with the cells' bad insulin reception but can eventually involve the pancreas going bad as well. Loraof (talk) 02:27, 15 February 2017 (UTC)

Yes, the article answers the question quite well. Type 2 diabetes begins with insulin resistance, where cells fail to respond properly to insulin. Now, as the disease progresses this can lead to pancreatic dysfunction, as the pancreas attempts to produce more insulin to compensate for this insulin resistance, and some type 2 diabetics can eventually require insulin administration. I get the feeling the original questioner is not a native English speaker. Wikipedia is available in many languages; if you have trouble understanding the English article, see if there are articles in your native language(s). --47.138.163.230 (talk) 03:49, 15 February 2017 (UTC)

• The best person to discuss this with is an endocrinologist. My general practitioner put me on insulin due to my A1C, and I put on 30 lbs. I changed to a different doctor who sent me to an endocrinoogist, who said that not only was the insulin a bad idea, but that it would eventually cripple my pancreas. He put me on various non-insulin medicines, and I lost the gained weight, and then some. I had bariatric surgery Jan 20, an am totally off all DMII meds (I had been taking three pills and a shot a day), and have lost another 30 lbs. So obviously my pancreas has not shut down yet. Seek professional advice. μηδείς (talk) 22:29, 16 February 2017 (UTC)

See also here. Count Iblis (talk) 00:13, 17 February 2017 (UTC)

By which mechanism sex is considered a risk factor for arteriosclerosis?

I listened today to a lecture in which the lecturer has said that there are 6 risk factors for arteriosclerosis: 1. food. 2. smoking 3. high blood pressure 4. obesity. 5. age 6. sex. I really don't have an idea on what it's based, because after googling I didn't find source for that. But my question is about sex. What does it mean that it's a risk factor and by which mechanism sex can be a risk factor for arteriosclerosis? (Maybe he wanted to say that they are general risk factor for life rather than for arteriosclerosis. It's not clear) 93.126.88.30 (talk) 00:25, 15 February 2017 (UTC)

Apparently, men are significantly more susceptible to it than women are. My source for that is this article.[1] Google the subject and there should be plenty of entries. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:30, 15 February 2017 (UTC)

• (edit conflict) Right, here "sex" means the sex of the person, not how much sex s/he has. Our article arteriosclerosis does not mention risk factors, but the related article Atherosclerosis#Risk factors does mention being male as a risk factor, with a citation. Loraof (talk) 02:21, 15 February 2017 (UTC)

long tangent not relevant to the OP
The following discussion has been closed. Please do not modify it.
This might be why "gender" is used more and more instead of "sex". ←Baseball Bugs What's up, Doc? carrots→ 02:24, 15 February 2017 (UTC) OSHA form: Q:"number of employees, broken down by sex?" A: "None (booze is more of a problem for us.)" -Arch dude (talk) 06:11, 15 February 2017 (UTC) No, thats wrong. It's because gender has social components, while sex (the property, not the act) is biological (if not always binary). --Stephan Schulz (talk) 14:14, 15 February 2017 (UTC) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ - Yes, thanks. The worst part is when people who should know better use "gender" as a sort of euphemism for "sex". SemanticMantis (talk) 15:27, 15 February 2017 (UTC) That's precisely the point. Whether Schulz likes it or not, "gender" is often used as a synonym for "sex". In addition to the "broken down by sex" joke, there's this old one-liner with many variant punch lines, where someone's filling out a form, and where it says "sex" they respond with something like "3 times a week." ←Baseball Bugs What's up, Doc? carrots→ 16:11, 15 February 2017 (UTC) Wiktionary's article "Gender" points out that the meaning "sex" goes back hundreds of years, but the use with that meaning has been declining, especially in academia, for the past 50 years. Our article Sex and gender distinction says The sex and gender distinction is not universal. In ordinary speech, sex and gender are often used interchangeably.[3][4] Some dictionaries and academic disciplines give them different definitions while others do not. Loraof (talk) 16:40, 15 February 2017 (UTC) The use of "gender" to mean sex, or even social roles correlated with sex, used to be seen as a sort of humorous intentional error, the word "gender" being properly restricted to grammar. I think what's happened in recent decades (much less than fifty years, maybe twenty or thirty) is that non-grammatical meanings have become accepted as fully legitimate. At the same time, "gender" in the non-grammatical sense may also have shifted away from biology and towards social roles. --Trovatore (talk) 16:49, 15 February 2017 (UTC) According to Etymology online [2] the "gender as grammatical construct" sense is older, but the "gender as synonym for sex" is pretty damned old too. The grammatical meaning dates from the "late 14th century" while the gender-as-sex meaning dates from the early 15th century. It does say that the preference for using gender to mean "male or female" sex dates to the 20th century, but the meaning is almost as old as the grammatical one. Basically it only almost always meant that, at least in equal measure, to the word "sex". --Jayron32 19:05, 15 February 2017 (UTC) But it was a disfavored meaning; that's my point. It wasn't considered precise speech. --Trovatore (talk) 19:15, 15 February 2017 (UTC) It was an old meaning of gender: "A class of things or beings distinguished by having certain characteristics in common; (as a mass noun) these regarded collectively; kind, sort." (OED) from the fourteenth century that was used occasionally for the class of males or the class of females as in " His heyres of the masculine gender" (1474) and "Has thou oght writen there Of the femynyn gendere" from the Wakefield Mystery Plays (1460?). In 1945, the American Journal of Psychology defined gender as "the socialized obverse of sex". Dbfirs 20:36, 15 February 2017 (UTC) Perhaps. But it was considered jocular, imprecise, or otherwise inferior in the mid-to-late 20th century. --Trovatore (talk) 20:48, 15 February 2017 (UTC) Was that because psychologists and sociologists began using it with a different meaning? Dbfirs 21:19, 15 February 2017 (UTC) I suppose it's conceivable. I seem to recall that William Safire wrote in opposition to the non-grammatical usage. If someone could find his essay/chapter/whatever, it might give some insight. But mostly it just always struck me as something Gomer Pyle would say; definitely not scientific prestige usage. --Trovatore (talk) 22:11, 15 February 2017 (UTC) @Trovatore:What? Where? I was taught in middle school, in the late 20th c., that sex is biological and gender, though overlapping in usage, is a social concept. In essence, the "socialized obverse" from Dbfirs source. All my continued education, high school through PhD has supported that same general distinction, though detailed specifics of the definitions may vary. This is certainly true in biosciences, but also in my experience with sociology, psychology and the humanities. ('gens, gentis', from the Latin, type or kind, especially of people, underlies gender, but also genus, gentile, generate, genetics, etc. That itself comes from a PIE root *gene- "to produce, give birth, beget,") Granted, I was not around in the mid 20th c, but you should provide some sources for your repeated claim of "humorous intentional error", etc. I agree it is not proper to use "gender" to mean "biological sex" in a contemporary scientific context, but I really don't know what you're getting at for the rest. SemanticMantis (talk) 22:14, 15 February 2017 (UTC) Well, this was the way I heard it anyway. There's a hint of it in the EO link that Jayron gave, though it doesn't put a date on it. As I say, I think there's something by Safire somewhere, but I don't have it to hand. --Trovatore (talk) 22:19, 15 February 2017 (UTC) I don't have access to the Saffire opinion, but the OED says: " In the 20th cent., as sex came increasingly to mean sexual intercourse (see sex n.1 4b), gender began to replace it (in early use euphemistically) as the usual word for the biological grouping of males and females. It is now often merged with or coloured by sense 3b" [and 3b, with cites from 1945 onwards, is "Psychol. and Sociol. (orig. U.S.). The state of being male or female as expressed by social or cultural distinctions and differences, rather than biological ones; the collective attributes or traits associated with a particular sex, or determined as a result of one's sex. Also: a (male or female) group characterized in this way."] Dbfirs 22:30, 15 February 2017 (UTC)

• Viagra does cause delayed back ache. Perhaps these phenomena are related? μηδείς (talk) 22:19, 16 February 2017 (UTC)
  • Maybe related to this oldie: "If you don't swallow it quickly, you'll get a stiff neck." ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:40, 18 February 2017 (UTC)

Our article Testosterone and the cardiovascular system explains one potential issue - the sex hormone testosterone is linked to increased incidence of arteriosclerosis.

The body makes testosterone in response to the arousal stage of a sexual encounter. I'm not sure if that's enough to increase your risk of atherosclerosis.

Most importantly, men usually have higher circulating testosterone levels than women all the time, regardless of their level of sexual activity; it's important to development and maintenance of Secondary sexual characteristics such as beard growth.

As the our article Testosterone and the cardiovascular system shows, this explains the higher incidence of arteriosclerosis and heart disease in men.

So, the answer to your question is being of the male sex definitely increases your risk of atherosclerosis because testosterone is linked to that disease process. The level of testosterone increases during sex due to how arousal triggers increased testosterone output but I don't know having more sex increases your prone-ness to atherosclerosis. loupgarous (talk) 01:36, 19 February 2017 (UTC)

Feynman Lectures. Lecture 48. Ch.48-4 . Refractive index < 1, phase speed > c [3]

...

In other words, for the slowest modulation, the slowest beats, there is a definite speed at which they travel which is not the same as the phase speed of the waves—what a mysterious thing!

— Feynman • Leighton • Sands, The Feynman Lectures on Physics, Volume I

What does he mean by "slowest"? Very low frequency of modulating wave?

A part from words Let us see if we can understand why to the end of the chapter is also obscure.

Now because the phase velocity, the velocity of the nodes of these two waves, is not precisely the same, something new happens. -- He considers x-rays. X-rays have a speed $v_{\text{x-rays}}=c/n$. How could it be that phase velocities are different? Even in the glass (or carbon) two x-ray waves must propagate with equal speed.

$\definecolor {myorange}{rgb}{0,0.38,0.52}\color {myorange}dk/d\omega =1/c+a/\omega ^{2}c$ -- When I express the formula (48.14) like this: $\omega ={\tfrac {1}{2}}(ck\pm {\sqrt {(ck)^{2}+4a}})$ and differentiate, I get: ${\tfrac {d}{dk}}\omega ={\tfrac {d}{dk}}{\tfrac {1}{2}}(ck\pm {\sqrt {(ck)^{2}+4a}})={\tfrac {1}{2}}(c\pm {\tfrac {c^{2}k}{\sqrt {(ck)^{2}+4a}}})$. But Feynman got ${\tfrac {c}{1+a/\omega ^{2}}}$. Why? Username160611000000 (talk) 07:52, 15 February 2017 (UTC)

• Group velocity might help you. Tigraan^{Click here to contact me} 13:58, 15 February 2017 (UTC)
  • The images in the article show group speed ≠ phase speed. This case is not explained by Feynman yet.
  • I assume next: with frequency $\omega _{2}=8\pi$ (Fig. 48–1) the refractive index will be $n_{2}=1-{\tfrac {Nq_{e}^{2}}{2\epsilon _{o}m(\omega _{_{2}})^{2}}}=1-{\tfrac {a}{(\omega _{_{2}})^{2}}}$. So the phase speed will be $v_{p;2}=c/n_{2}$.
    And by analogy for $\omega _{1}=10\pi$ we have $v_{p;1}=c/n_{1}<v_{p;2}$.
    Is it that what Feynman meant?
    Username160611000000 (talk) 18:00, 15 February 2017 (UTC)
  • WRT formula :
    
    ${\tfrac {1}{2}}(c\pm {\tfrac {c^{2}k}{\sqrt {(ck)^{2}+4a}}})={\tfrac {1}{2}}(c\pm {\tfrac {c^{2}({\tfrac {w}{c}}-{\tfrac {a}{wc}})}{\sqrt {(c({\tfrac {w}{c}}-{\tfrac {a}{wc}}))^{2}+4a}}})={\tfrac {1}{2}}(c\pm {\tfrac {cw-{\tfrac {ca}{w}}}{\sqrt {(w-{\tfrac {a}{w}})^{2}+4a}}})=$
    $={\tfrac {1}{2}}(c\pm {\tfrac {cw-{\tfrac {ca}{w}}}{\sqrt {w^{2}+{\tfrac {a^{2}}{w^{2}}}-2a+4a}}})={\tfrac {1}{2}}(c\pm {\tfrac {cw-{\tfrac {ca}{w}}}{\sqrt {w^{2}+{\tfrac {a^{2}}{w^{2}}}+2a}}})={\tfrac {1}{2}}(c\pm {\tfrac {cw-{\tfrac {ca}{w}}}{\sqrt {(w+{\tfrac {a}{w}})^{2}}}})=$
    $={\tfrac {1}{2}}({\tfrac {cw+{\tfrac {ca}{w}}\pm cw\mp {\tfrac {ca}{w}}}{w+{\tfrac {a}{w}}}})={\begin{cases}{\tfrac {cw}{w+{\tfrac {a}{w}}}}={\tfrac {c}{1+{\tfrac {a}{w^{2}}}}}\\{\tfrac {\tfrac {ca}{w}}{w+{\tfrac {a}{w}}}}={\tfrac {c}{{\tfrac {w^{2}}{a}}+1}}\end{cases}}$
    
    It's strange, but second answer doesn't fit ...
    Username160611000000 (talk) 20:16, 15 February 2017 (UTC)

What type of person makes all the factory parts and programs them to work?

In a factory, everything seems to be automated. What type of person invents all the factory parts and programs them to work properly to create lots of things in a relatively short period of time? What skills and knowledge are needed? Are there any books about this matter? 166.216.159.7 (talk) 15:42, 15 February 2017 (UTC)

Designing machines is done by engineers. --Jayron32 16:26, 15 February 2017 (UTC)

Also, to expand on that, no one single person would be expected to design an entire factory all by themselves. Hundreds of engineers working in teams would do that. --Jayron32 16:29, 15 February 2017 (UTC)

See Manufacturing engineering and Industrial engineering. Loraof (talk) 16:31, 15 February 2017 (UTC)

Well, also it should be noted that, depending on what the factory is making, there would be a dozen or more engineering sub-disciplines involved. After all, the robot itself may be designed by a mechanical engineer, while wiring would be done by an electrical engineer. If (for example) it was a chemicals factory, there would be chemical engineers to handle designing reaction vessels. It goes on and on. No one person would be expected to do all of those jobs, it would require many dozens of engineers from a wide variety of disciplines. --Jayron32 16:47, 15 February 2017 (UTC)

• Design of things is still the design of things, whether they're made by hand, by manually-controlled machine or by fully-automated robot. Many of the design issues, either the aesthetic design of them or their engineering design is still much the same.

Manufacture by automated machine relies on the CAD / CAM / CNC cycle. One might use CAD to draw something that's then made entirely by hand, one might begin to design for robot manufacture by using a pencil and paper, but somewhere along the route an automated manufacture involved moving into the CAM / CNC world.

The first CNC began with punched cards and punched tape, first for the Jacquard loom and later for milling machines. These are powered, manually controlled machines, where their hand controls are replaced by power controls under the direction of the tape's instructions. G-code (from the 1960s) was an important step for this (it's still around today) where very simple instructions are given to a dumb machine in the form "move this control so far, move that control a little". Writing G-code was long recognised to be a long and tiresome task, using much the same skills as a manual machinist used for manual machining.

CAD has been around for as long as computers were available. It could be said to begin in the 1940s, but this was theoretical modelling of complex engineering (nuclear engineering, aerodynamics and fluid dynamics for gas turbines) rather than an interactive "design" process. CAD, as we'd know it now, begins in the late 1970s with graphical terminals and minicomputers.

CAM joins up the design and control aspects of this: it embodies the machinist's knowledge of how to use a machine to make a shape. CAM becomes increasingly sophisticated and automated: from an early beginning in checking the low-level and error-prone G-code for validity, or generating batch machining tasks (e.g. remove 0.5" of material in repeated 0.025" steps, starting from the same place), through to modern 3D printing. 3D printing can specify a solid shape by CAD, but the CAM not only works out how to move the print head to make the shape but can also transform the hand-drawn solid automatically into a hollow honeycomb, saving material and print time.

Factories are also about more than machining. I spent much of my career making production line machines to assemble or test assemblies, particularly in car factories. These didn't look like "robots", or even robot arms, they mostly resembled a conveyor belt with workstations for each task. An operator might manually place a gear on an axle, some screws are automatically fed from a magazine screwdriver and driven home, then a power press drives a bearing into place. Finally a rotating shaft measures the force needed to turn the assembly and if this is within the expected limits, to check that everything turns freely. Skills for this (a good job market) ranges from general mechanical engineering design, specific production engineering (there are college degrees in that speciality), electrical work, PLC programming and (my part) general computer programming for connecting user interfaces (see SCADA) to machinery and usually doing some statistical processing for tasks like SPC. Some projects were purely for testing, such as engine test cells and dynamometers.

There is a massive literature and training organisation around these industries. Andy Dingley (talk) 18:42, 15 February 2017 (UTC)

Its almost never only one person. Naturally its not obviouse to outsiders how many problems need to be solved to build a working, reliable factory. Experts on automation may seem like the real core of designing a factory but they only have basic knownledge of for example electrical installations, architecture, work security, workflow or finances.

Ofcorse, if you plan a multimillion investment like a factory you want to make shure for example some machine gets placed on a proper solid floor that can withstand all the forces and also doesnt get you in trouble if some oil leaks on to it. Just imagine some goverment inspector who wants ot check out your "factory" slipping and braking his arm on some oily patch next to a machine you just put there thinking what could go wrong. You have to think of everything and ofcourse thats even a challenge for a team of experts, let alone one who is specialized on Automation. --Kharon (talk) 02:05, 17 February 2017 (UTC)

I worked in the body panel press shop at Pressed Steel Fisher in Swindon for one job. The steel sheets (maybe 6' square) are greased before they go into the presses. The floor was of wooden blocks. As usual, I was wearing a suit and tie - the few concessions were that the tie was a clip-on, I wore Doc Martens and I kept a large boilersuit handy if I actually had to climb into a machine. In Swindon though, this wasn't enough - that plant was filthy, in a clean sort of way. The grease from the sheets got everywhere, including the wooden floor. I had to nip out at lunchtime and buy myself a set of hobnailed boots just to stay upright. I also purchased a picnic set - clean plastic table, clean chairs and a picnic umbrella, which I set up as a computer workstation alongside the machine for whilst I was in there. Naturally I set the umbrella up too - I'd paid for it, I was having it!

I still preferred it to the BMC A-Series engine cylinder head machining line. They were made from a particularly graphitic grade of cast iron, which threw a haze of conductive black dust into the air. That got everywhere and caused no end of short circuits. Our computers had to be IP66 waterproof to survive. Andy Dingley (talk) 15:06, 17 February 2017 (UTC)

Walking on the moon

If I applied the same forces with my feet as I do on earth, but was skipping (like the astronauts did) on the moon's surface, how high off the surface would I be jumping? --31.92.250.145 (talk) 18:17, 15 February 2017 (UTC)

It would depend on how high you could jump on earth. ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:34, 15 February 2017 (UTC)

• Quick approximation is that the lunar gravity is 1/6th that of Earth. You're also going to jump less well encumbered by a space suit, and carrying the extra weight of a space suit, rather than just running shorts.

But (comparing your performance on Earth whilst carrying the weight of a spacesuit) you could jump to six times the height you could reach on Earth. Andy Dingley (talk) 18:44, 15 February 2017 (UTC)

The relevant equation is $E=mgh$, where $E$ is the amount of energy you put into the jump, $m$ is your mass (plus that of anything you're carrying), $g$ is the gravity of the planet or moon you are on, and $h$ is the height you jump. This can be re-arranged to $h={\frac {E}{gm}}$. Assuming you can apply the same amount of energy into the jump (it will be over shorter period), then, as, Bugs said, the height you jump is inversely proportional to the gravity. LongHairedFop (talk) 19:33, 15 February 2017 (UTC)

Yeah but the astronuts didnt seem to be jumping that high: only say 150cm. They should be diong a few feet at least.Also, why are the in slow motion?31.92.250.145 (talk) 22:07, 15 February 2017 (UTC)

My guess would be that they had been trained to jump slowly and gently, to avoid going too far or too fast. Even in low gravity, inertial mass and momentum remain unchanged, so collision injury is just as serious. The apparent "slow motion" is a direct result of acceleration from the moon's gravity being only one sixth of that on Earth. Dbfirs 22:22, 15 February 2017 (UTC)

They also had to be careful, because if they were to trip and fall, they could damage the suit or the equipment pack, which could be fatal. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:32, 15 February 2017 (UTC)

I seem to remember at least one of the moonwalkers falling over with no apparent ill effects. Anyway, why do you have to bounce around in low gravity? And what was the mass of the spacesuits etc compared to their unsuited body mass?31.92.250.145 (talk) 22:56, 15 February 2017 (UTC)

There are videos of astronauts falling (see this). In photos, Buzz Aldrin has dirt near one knee, but I think that he said that he didn't remember falling down. As far as jumping on the Moon, the space suit is restrictive. It also added 200 pounds on Earth, but if an astronaut weighed 175 pounds on Earth, that would be like (175+200)/6 = 62.5 pounds on Earth. If the spacesuit were completly flexible (it wasn't) then you should be able to jump nearly three times as high (with the added weight) as you can on Earth. The reason that they appear to be moving in slow motion is because of the acceleration of gravity and his first law of motion, F=ma, and the equation for how far a body falls under gravity. If you work through the math, it takes about 2.5 longer for something to fall the same distance on the Moon as on Earth. The same thing applies when something is going up. And bouncing around was the best way to move. During the Apollo 11 moonwalk, Aldrin demonstrated several methods of moving. Bubba73 ^{You talkin' to me?} 00:07, 16 February 2017 (UTC)

Some time in the last 50 years I have read [sorry, no specific reference yet found] that the skipping or 'galumping' gait adopted by the lunar astronauts was what they quickly found to be the surest method of progression in the conditions. The lower gravity means both that a given muscular effort causes a greater rise, and that foot/ground friction is much reduced. Many of us will have experienced the advantage of 'galumping' when proceeding downhill at speed (somewhat comparable to the lower gravity situation), and also of the adjustments necessary when walking on slippery ice; the combination of the two is out of most people's experience. Our article Apollo 11, under 'Lunar surface operations', includes the following text – note the last sentence:

"Armstrong said that moving in the lunar gravity, one-sixth of Earth's, was "even perhaps easier than the simulations ... It's absolutely no trouble to walk around."^[36] Aldrin joined him on the surface and tested methods for moving around, including two-footed kangaroo hops. The PLSS backpack created a tendency to tip backwards, but neither astronaut had serious problems maintaining balance. Loping became the preferred method of movement."

The best gaits to use in various gravity strengths (and on various surfaces) is a difficult matter to research, although experiments (as preparation for possible human exploration of Mars, whose gravity is about 1/3 ours) have been done by suspending subjects by sprung wires, sometimes from an overhead travelling framework – relevant videos I have seen may be findable on the usual sites, here are a couple new to me:

https://www.youtube.com/watch?v=LvnDIDqcfGI

https://www.youtube.com/watch?v=5MraTpKa7a8

As yet there has not (to my knowledge) been a conclusive finding for Martian surface gravity (approximately 1/3 of ours). The poster formerly known as 87.81.230.195} 90.203.118.169 (talk) 04:11, 16 February 2017 (UTC)

The lunar gait was described as hopping or kangaroo hopping, rather than skipping. The point is that under the low gravity they found it easy to hop with both legs together, but that as the suit limited movement of each leg going past the other, it was much easier to hop with both together.

When falling over they tried to either fall forwards onto one knee (under the low gravity this would be low risk) or else to fall backwards. The PLSS rear pack was much more robust than the front equipment and cameras, also it left the astronaut lying on top of the PLSS rather than under it, making it easier to get up again. Andy Dingley (talk) 10:08, 16 February 2017 (UTC)

One light controlled by two switches

Sometimes there are two switches in different locations for one light. When you turn off the light with one, you can turn it back on with the other. Is there a diagram somewhere that shows how this is done? Is it somewhere at Multiway switching? I can't imagine how it could work. Many thanks, Anna Frodesiak (talk) 23:12, 15 February 2017 (UTC)

Yes, Multiway switching#Traveler system is the usual way. Andy Dingley (talk) 23:15, 15 February 2017 (UTC)

Thank you, Andy! I saw that and didn't get it, then stared at it for longer and now I totally get it. Cheers. :) Anna Frodesiak (talk) 23:27, 15 February 2017 (UTC)

(asside) When I was a child, my uncle, an engineer showed me how two switches do this. He challenged me to figure out how to do it with three switches. I couldn't do it. The third switch has to be a different type. Bubba73 ^{You talkin' to me?} 19:20, 16 February 2017 (UTC)