Wikipedia:Reference desk/Archives/Science/2017 June 4

= June 4 =

Electra 10-E
How did a full fuselage fuel tank on the Electra 10-E (as installed, for example, on Amelia Earhart's Electra) affect the CG position? Because in my (simulated) experience, it makes the plane unbelievably nose-heavy (to the point where the elevator forces are impossibly high and I have to use elevator trim as the primary pitch control) -- but in World Flight: On the Earhart Trail, Ann Pellegreno wrote that it makes the plane dangerously tail-heavy (to the point where she had to use full nose-down trim AND hold the stick forward to keep it from pitching up into a flat spin), and in Last Flight, Amelia made no mention of CG problems at all (either because she had none, or maybe because she considered these to be a minor annoyance not worth mentioning). So which is it? 2601:646:8E01:7E0B:68C1:51CC:3DC8:E141 (talk) 00:25, 4 June 2017 (UTC)


 * Anyone? 2601:646:8E01:7E0B:C0E3:5FFE:A7B7:468A (talk) 02:59, 7 June 2017 (UTC)


 * Just a thought, but can that tank be installed in different positions on the plane ? StuRat (talk) 03:06, 7 June 2017 (UTC)


 * It's a possibility, but in this case I'm specifically asking about an installation identical to that in Amelia's plane. 2601:646:8E01:7E0B:C0E3:5FFE:A7B7:468A (talk) 04:52, 7 June 2017 (UTC)


 * How do you know that your simulated version had it installed in the same place that she did ? StuRat (talk) 20:42, 7 June 2017 (UTC)


 * I don't (and I doubt that it does -- it's from First Worst Class Simulations (included with their Around the World in 80 Flights mission pack), so it's probably anything but accurate), but I'm asking about her plane, not the virtual plane on my computer. 2601:646:8E01:7E0B:F821:2693:F38:C173 (talk) 01:03, 8 June 2017 (UTC)
 * I don't know Pellegreno or her book but your comment about what she has written makes her description sound amateurish. If an aircraft's CG position is too far aft it is longitudinally unstable which makes the aircraft tiring to fly because of the need to be constantly correcting the attitude. (This is highly undesirable on a long-duration flight.) Long before the aircraft pitches uncontrollably upwards into a stall, its longitudinal instability will cause it to alternate between too fast and too slow, necessitating frequent corrective action by the pilot.
 * I found the FAA Type Certificate Data Sheet for the Electra 10-B (Navy R-30-1). It specifies four fuel tanks, all installed in the wing. It is most likely Earhart's aircraft was specially modified to have a long-range fuel tank in the fuselage. Whether it is likely to cause the aircraft to be unstable, nose-heavy or neither depends entirely on where the CG of the fuel in the tank lies relative to the approved CG range for the aircraft. Any competent aeronautical engineer would strive to install the tank within the approved CG envelope for the aircraft so that the presence of this fuel has no adverse effect on longitudinal trim.
 * You have not commented on where you assume the CG of the fuel lies, and on what information you base your assumption. Dolphin  ( t ) 00:06, 9 June 2017 (UTC)
 * AFAIK the model with the long-range tanks in the fuselage was the 10-E and not the 10-B (which was probably why Amelia chose that particular model) -- as far as the CG of the extra fuel, that's what I was asking about in the first place! But if I read your comment correctly, you're saying that unless the tanks were a retrofit by some third-party contractors who had no idea what they were doing (which was certainly not the case, since Amelia specifically wrote in Last Flight that all modifications were done in-house by Lockheed, except for the avionics which were installed by their respective manufacturers), then the effect on CG would be minimal -- is that correct?  (BTW, Pellegreno did in fact mention porpoising as well -- I just didn't mention that she did, because I found it much more shocking that she actually needed to push forward on the stick to prevent uncontrollable pitch-up.) 2601:646:8E01:7E0B:4F8:7AED:9CA5:90AA (talk) 00:49, 9 June 2017 (UTC)

Svalbard Global Seed Vault and Permafrost melting
At Talk:Svalbard Global Seed Vault I have been in an interesting discussion about recent sources that reported flooding of the seed vault. At issue is whether the flooding was caused by rain and snowmelt, or whether the permafrost itself actually melted, with what looks like reasonable sources for both claims. I could use some help sorting this one out. --Guy Macon (talk) 03:40, 4 June 2017 (UTC)
 * I think the major reason of that flooding is the existence of the vault simply physically as a path for the water to go where no natural path would usually be. As already pointed out in the discussion you linked temperatures above the freezing point can occur during summer even further north, even up to at the pole. Of course only at the surface - unless someone dug a hole there :p --Kharon (talk) 01:05, 6 June 2017 (UTC)

== Feynman Lectures. Exercises PDF. Exercise 6-3 JPG== . .

Here is my solution https://s.sender.mobi/u/image/2017/6/4/ucf3TrNFe/-.PNG but I'm not sure about the text highlighted in red. How can we express a probability as a cross-section area? By integrating it can be shown that probability of a hit between angles φ1 and φ2 (0 < φ1 < φ2 <0.5π) is 0.5[Cos(2φ1)-Cos(2φ2)].

And what does Leighton mean by "zero deflection"? Is it something like a check of formula 6.19 $$\int_{-\infty}^{+\infty}p(x)\,dx=1$$ ?

Username160611000000 (talk) 10:10, 4 June 2017 (UTC)


 * Zero deflection means that the BBs miss the sphere. Therefore they are not deflected.--Phil Holmes (talk) 12:02, 4 June 2017 (UTC)
 * Yes. But this is more complicated than it looks. First, we do not consider B-B's that don't touch the target sphere (do we?). Second, for B-B's that just touch it at an angle φ = 0.4999π the probability is almost zero, and not π(a+b)2. Besides  the probability is usually a dimensionless number.Username160611000000 (talk) 12:21, 4 June 2017 (UTC)


 * In case it is unclear, π(a+b)2 is the cross-sectional area generated by all BB trajectories that will collide with the sphere. Dragons flight (talk) 14:12, 4 June 2017 (UTC)
 * I know. I have written it down here. Is my solution correct? It is unclear how to represent the probability as the area. Can you show the procedure for general problem. We just multiply ordinary probability by target area, don't we? Username160611000000 (talk) 15:14, 4 June 2017 (UTC)


 * My guess is that you're supposed to figure out the area of the sphere that deflects particles by an angle of at least 2*theta as a cross-section (area). So as theta approaches zero, the full area pi (a+b)^2 is the value.  Of course, truly zero deflection can be achieved over an infinite area, so I assume he means the limit from above, not the actual value.
 * Now to deflect a bearing by 2*theta, that means that when it hits, the line from its center to the sphere diverges from the perpendicular plane by theta. For convenience' sake, let's call the closest point on the sphere the North Pole.  Then for a 60 degree deflection the bearing hits at latitude 30 degrees N, for example; hitting 90 degrees N, the North pole, sends the bearing straight back at 180 degrees.
 * Well then: we know that the area of the part of the 2D disk around the North Pole that they hit will be proportional to the cosine of this latitude theta (or the sine of 90 - theta).  For a 180 degree deflection theta is 90 and this disk has radius 0; for a 0 degree deflection theta is 0 so the cosine is 1 and we use the full distance.  The full distance, note, is made up of the sphere part and the bearing part; this could get complicated since the sphere at that latitude has area pi a^2 cos theta, but then part of the bearing overlaps part of that area, and the extent of overlap depends on the angle...  But the line from center of sphere to center of bearing goes straight through the point of impact, so they both get the same multiplier.  Therefore I think it's pi cos theta (a+b)^2 for the final answer. Wnt (talk) 19:45, 6 June 2017 (UTC)

Suicidal ideation
I was reading suicidal ideation. I always thought people suicide due to severe life problems that can't be resolved, e.g. finance/spouse/kids, or severe grief like loss of relatives. But that page suggests some people think about suicide for no good reason? I dont really understand, do some people get depressed without any external life problems? I thought depression was always due to some big external influence. Money is tight (talk) 10:34, 4 June 2017 (UTC)


 * In short, yes, some people get depression without any identifiable external cause. For example, the depression that occurs in patients with bipolar disorder is essentially the direct consequence of a brain disorder, and doesn't need external events to cause or influence it.  Dragons flight (talk) 11:12, 4 June 2017 (UTC)


 * For many people the suicidal ideation is a result of OCD (obsessive compulsive disorder) rather than depression. Alcoholics commit suicide at a much higher rate. Men are 4 times as often to complete suicide as the women for whom suicide attempts could be a way to influence others and not intended to go through. --AboutFace 22 (talk) 14:46, 4 June 2017 (UTC)


 * Also, some people who manage to commit suicide weren't actually trying to, but just looking for a thrill by "Taking it to the limit". See autoerotic asphyxiation, for example. StuRat (talk) 21:47, 4 June 2017 (UTC)


 * Actually, the sine qua non of psychological depression is to be "depressed without any external life problems". That's what depression disorders are.  People get sad when loved one's die, that's normal.  People are stressed when their job isn't going well, that's normal.  However, where depression disorder and anxiety disorders and other related mental health issues come in is where people have these moods for reasons unconnected to their external life situation.  Some recent examples include Robin Williams and Chris Cornell, neither was in a particularly difficult life situation at the time of death.  Robin Williams in particular had a type of early-onset dementia called Dementia with Lewy bodies, while Cornell's brain chemistry has not been studied in detail, he had dealt depression disorder for years: .  The article Mood disorder gives a good overview as well of these sorts of things.  -- Jayron 32 05:25, 5 June 2017 (UTC)


 * Nailed it perfectly Jayron. Richard Avery (talk) 07:23, 5 June 2017 (UTC)

How do you solve part A?
69.22.242.15 (talk) 13:50, 4 June 2017 (UTC)
 * --Jasper Deng (talk) 18:58, 4 June 2017 (UTC)


 * This is from the textbook. You can see the answer. It's not the homework. 69.22.242.15 (talk) 20:16, 4 June 2017 (UTC)


 * The questioner isn't asking for an answer to a homework question; he's asking *how* to solve a problem for which he already has an answer, presumably so he can learn how to solve similar problems. It's a perfectly valid question. - Nunh-huh 19:39, 4 June 2017 (UTC)


 * Our article on the topic is LC circuit. Conservation of energy applies, so energy moves from the capacitor to the magnetic field in the inductor, and is not lost. In "capacitor" you can see the charge to voltage formula C=Q/V and hidden there is also the energy formula E=CV2. You should be able to use these formulae to work out the solution. Inductor will tell you about that component. Graeme Bartlett (talk) 22:33, 4 June 2017 (UTC)


 * I did that and got a voltage of 5V, which makes sense because the loop rule should give a difference of 0V. 69.22.242.15 (talk) 03:38, 5 June 2017 (UTC)
 * →69.22.242.15 You would be correct, and the 4.25 would be wrong. Is the 150μJ right? (partb) (clue 160-10) Graeme Bartlett (talk) 13:19, 5 June 2017 (UTC)


 * Lc circuit.svg


 * As we're discussing this brief question, I'm going to Fair Use the whole text for future reference after the imgurl goes away: "A capacitor in an LC oscillator has a maximum potential difference of 20V and a maximum energy of 160 &micro;J. When the capacitor has a potential difference of 5 V and an energy of 10 &micro;J, what are (a) the emf across the inductor and (b) the energy stored in the magnetic field?  ANSWER: (a) 4.25 V  (b) 150 &micro;J"
 * The article doesn't give a picture of the LC oscillator, so I've pasted one from Commons at right that presumably represents the situation. This is obviously a simple theoretical concept rather than a real device.  We know that 150 uJ has to be somewhere, presumably the magnetic field, which is why the OP didn't ask about it.  The capacitor is indeed acting in accordance with what Graeme Bartlett says above - at 1/4 the voltage it has 1/16 the energy.  And electromotive force is simply voltage (coming from something, typically); at least as we show it I don't see how it can be anything other than the same 5V on the capacitor in parallel with the inductor. Wnt (talk) 20:03, 6 June 2017 (UTC)

STOP! The question is about a precharged capactior. Now put is into the circuit. If the picture fits zu the question, the capacitor will discharge over the coil, but the current will produce a magnetic field. The magnetic energy is to be calculated. Some more analytics after the homework: Later the magnetic field will induce a negative voltage and feed the capacitor with it. So this sine wave will fade out like the sound of a bell. An oscillator recharges the capacitor in the right time like riding a bike, the time, you step on the pedal. Step the wrong time, it will not accelerate the bike. -- Hans Haase (有问题吗) 19:35, 7 June 2017 (UTC)

Do animals know that mating leads to procreation
When animals mate,do they know that it will lead to procreation?Uncle dan is home (talk) 14:58, 4 June 2017 (UTC)


 * There's been some very clever studies done on all sorts of animal behaviors, but I just can't imagine what a person would do to determine this. How would a person know if their neighbors knew this other than asking them? Because they used condoms? On that basis a lot of people through history didn't know it. Dmcq (talk) 15:13, 4 June 2017 (UTC)


 * "Know" is probably a big ask, but it is interesting that some birds and other animals instinctively build nests / burrows / etc. before finding a mate. Now it is entirely possible that building a nest is simply an engrained instinct without an internal conception of why that is important; however, such events do show a complex ability to prepare for the consequences of mating whether or not the animal is consciously aware of why they need to do that.  Dragons flight (talk) 17:28, 4 June 2017 (UTC)


 * Human tribes have existed who didn't know this. Count Iblis (talk) 19:40, 4 June 2017 (UTC)


 * Impossible. How could have some human tribes not known this?Uncle dan is home (talk) 21:18, 4 June 2017 (UTC)


 * Well, if everyone mates from an early age, then females would just start to get pregnant some time after puberty, with no correspondence noted between mating and pregnancy. Of course, some people are infertile, so not all females get pregnant, further countering the idea that mating (after puberty) always leads to pregnancy.  There could just be the concept that females automatically get pregnant some time after puberty, with no further explanation as to why, or perhaps a supernatural explanation. StuRat (talk) 21:26, 4 June 2017 (UTC)


 * So they assumed that all the lesbians who never mated with males were infertile? Sagittarian Milky Way (talk) 21:34, 4 June 2017 (UTC)


 * Perhaps. Or they might get cause and effect mixed up and assume that not having children made them become lesbians.  Also, in a society where everyone is expected to take a mate of the opposite sex, the rate of exclusively lesbian relationships might be quite low and hidden, making it too difficult to see a pattern, if anyone was even looking for one. StuRat (talk) 21:41, 4 June 2017 (UTC)


 * Stu, and Iblis, you really need a source for this. I have read extensively on tribes from Siberia to Africa and the Amazon, and only ever heard this claim from a layman.  I have even heard atheists make the claim that the Jews didn't know about paternity at the time of the birth of Jesus!, so CN, please. μηδείς (talk) 00:17, 5 June 2017 (UTC)


 * Indeed. This article on Slate suggests that all groups of humans have appreciated a link between sex and babies for at least 50,000 years. Adrian J. Hunter(talk•contribs) 04:34, 5 June 2017 (UTC)


 * Here is the answer I gave the last time I saw this question come up. Matt Deres (talk) 13:50, 5 June 2017 (UTC)


 * I was merely commenting on the Q of how it could be possible, as opposed to claiming that any extant or extinct tribes didn't know the link. StuRat (talk) 15:02, 5 June 2017 (UTC)


 * Here is Cecil Adams' answer, in which he states that the connection between sex and pregnancy was only discovered about 10,000 years ago. Unfortunately he doesn't provide references for this answer.  As to how it's possible, it seems to me more valid to ask how on earth the connection was ever discovered.  It's not at all easy to connect a baby being born to an event that happened nine months earlier, especially since the woman may have had sex at various times before and during her pregnancy.  CodeTalker (talk) 17:09, 5 June 2017 (UTC)


 * Mating with men who look significantly different than the rest of the community, thus producing a baby that resembles the "outsider", would be one way. However, in ancient times people rarely had access to other people who looked significantly different from themselves, with exceptions such as the Watusis and Pygmy.  Those who traveled long distances, or interacted with others who did, would be more likely to notice this.  In ancient times, this would probably mean travel by sea, or perhaps multi-generational travel by land.  However, if you go back far enough, mating with Neanderthals and Denisovans was possible.  StuRat (talk) 20:34, 7 June 2017 (UTC)

FIV+ cats
How can FIV not infect humans? Given that HIV has non-human primate origins, can FIV turn into HIV? 50.4.236.254 (talk) 18:03, 4 June 2017 (UTC)


 * A cat is not a primate; cats are much further from humans phylogenetically than are animals susceptible to, say, SIV. All viruses have specific tropisms, an affinity for certain species. In part this is based on the surface proteins expressed on the cells of the susceptible animal.  HIV enters cells that express a CD-4 marker and an additional chemokine co-receptor; FIV utilizes CXCR4.   FIV also codes for a protease that is not contained in the HIV genome. HIV also has a far greater genetic variability, while FIV is more limited to about 5 subtypes, which is the reason there is a vaccine for FIV but not HIV. - Nunh-huh 19:34, 4 June 2017 (UTC)


 * However, cats and humans can share some diseases. For example, see cat scratch fever and toxoplasmosis. StuRat (talk) 21:30, 4 June 2017 (UTC)


 * Not really a "however" as much as examples of a host range broader than FIV or HIV. Cat scratch fever is a good example of species tropism. It's more of a problem for humans; cats can coexist with the organism as part of its flora without actually being symptomatic (i.e. colonized rather than infected). Toxoplasmosis is a good example of a parasite that has a complex life cycle involving both definitive hosts (cats) and intermediate hosts (humans and others). - Nunh-huh 22:02, 4 June 2017 (UTC)
 * Of course neither are viruses either. Nil Einne (talk) 03:25, 5 June 2017 (UTC)


 * A lot of relevant papers can be found with https://www.ncbi.nlm.nih.gov/pubmed/?term=FIV+tropism - of these, a particularly useful and freely available one is here; it points the finger at the association between the FIV envelope protein and the cysteine rich domain of CD134. It says "a fully functional receptor for the PPR strain of FIV could be reconstituted by exchanging residues H45S, R59G, S60D, N62D, V64K in human CD134".  Even so, note that HIV actually uses a different receptor (CD4) and coreceptor (CXCR4) despite having similar effects!   So these viruses, ever-mutating, can over evolutionary time check out various ways in to the cells they want, and use what works.  The catch is that these ways aren't precisely the same for different species. Wnt (talk) 15:33, 5 June 2017 (UTC)

Are free-moving pets able to recognize the litter box the same way humans can recognize a toilet?
Human children are toilet-trained. Somehow, they recognize that the bowl of water with a flush is where feces should go. Can dogs and cats recognize that a litter box is where to defecate regardless of the actual location of the litter box? If there is no litter box, then will they defecate outside or on the owner's furniture or indoor floor? 50.4.236.254 (talk) 18:18, 4 June 2017 (UTC)


 * Cats already know where to defecate, they'll do it in the soil and then bury it. The litter box in the house works by simulating this environment inside the house, since there is no other place in the house where the cat can go about defecating in the normal way, the cat is going to use the litter box. But outside the house the cat can defecate in an even more natural way, so it's not going to come back to the house to defecate. Without a litter box and when locked up in a house, a cat will just defecate on the floor. Count Iblis (talk) 19:38, 4 June 2017 (UTC)


 * So, it's really the human's fault for not providing a litter box that is simulating an outdoor environment or not allowing the cat to go outside. That makes sense. But what about dogs? Can dogs be trained to use the litter box? 50.4.236.254 (talk) 19:48, 4 June 2017 (UTC)


 * Note that an accessible flowerpot can serve as a litter box for a cat, so it's wise to only have hanging plants inside. Also, placing the cat in the litter box when they first enter the home helps them identify it as the "toilet".  One problem I've noticed, or perhaps an evolutionary adaptation, is that cats tend to associate painful urination or defecation with the location, and then go elsewhere.  This can be annoying to the owners, but there is some possibility the location does carry microbes which caused a bladder infection, etc.


 * Dogs lack the natural instinct to go in the same place and bury it, so training them is harder, but still possible. StuRat (talk) 21:36, 4 June 2017 (UTC)


 * What about human children? Are they like dogs or cats? 50.4.236.254 (talk) 21:42, 4 June 2017 (UTC)


 * More like dogs, I would say. We lack instincts to defecate or urinate in any particular place, and instead learn that as part of our culture.  Note that different cultures and times have done this differently, from a toilet (possibly with a bidet), to a urinal, to a chamber pot, to an outhouse, to a hole in a pit, to an open field.  If we had an instinct to do it one particular way, this variety wouldn't be as wide (although cats can be trained to use a toilet). StuRat (talk) 21:51, 4 June 2017 (UTC)


 * Are you sure we don't have an instinct to use the same place? I mean, if you hang out in the woods a while, do you really pee wherever, or do you quickly decide on a spot?  Why?  (questions of instinct are hard, the definitions need to be considered; don't regard this as an assertion of the opposite) Wnt (talk) 12:27, 5 June 2017 (UTC)


 * I would argue that this is a culturally learned behavior. For example, nomads on a trek through uninhabited land don't have much need, for health reasons, to "organize" their urination or defecation.  Just leaving it behind wherever works just fine for them.  However, this same behavior, when utilized by city dwellers, can lead to a disastrous spread of diseases. StuRat (talk) 13:58, 5 June 2017 (UTC)


 * Dogs don't bury their scat, but they do learn "not to foul the nest" rather quickly, in nature and when given a little encouragement by humans. My dog would scratch at the back door when she needed to pee, poo, or barf (cats barf indiscriminately, having smaller brains) and would go to the basement and use newspaper laid out for her when she was sick and making messes at all hours.  Training her to do this took very little effort, basically leading her when she showed signs of being ready to make and praising her when she did it in the woods or on the paper, rather than on the grass or the carpet.


 * μηδείς (talk) 00:13, 5 June 2017 (UTC)


 * I wish my cat would barf indiscriminately. When I hear him making pre-barf noises and try to move him to a non-absorbent part of the floor, he consistently runs to the nearest carpet. —Tamfang (talk) 05:33, 8 June 2017 (UTC)


 * Personal observation: I had a pet hedgehog. I took it to visit another person who had a pet hedgehog. It was a bit of a drive. When I arrived, I let my hedgehog out of the cage and it went straight to the other hedgehog's litter box to relieve itself. So, it was clear that it understood what a litter box was, how to use it, and was capable of using a litter box that belonged to another hedgehog. I have no way to know if this observation expands to all hedgehogs or other animals in any way. 209.149.113.5 (talk) 12:53, 5 June 2017 (UTC)


 * There is an alternate explanation. Many animals show dominance by leaving their "scent", which often includes urine and feces.  Covering up another of their species' scents by leaving a more recent scent in the same place is particularly effective. StuRat (talk) 13:56, 5 June 2017 (UTC)