Wikipedia:Reference desk/Archives/Science/2018 July 15

= July 15 =

Photography - tracking fast moving subjects
In professional filmmaking, how are cameras stabilised when they are following a fast moving subject close up? For example of an actor is sprinting and the cameras is right next to them following, how is the camera stabilised? Assuming that it’s not possible to use a guide rail. Let’s say the actor is sprinting over many obstacles and turning a lot and the filming is done in a public area. — Preceding unsigned comment added by 90.206.71.55 (talk) 12:15, 15 July 2018 (UTC)
 * Wildlife photographer Roger N. Clark explains The key to fast autofocus and focus tracking is the use of phase detection focus. The link shows examples. DroneB (talk) 12:55, 15 July 2018 (UTC)


 * SteadiCam, and many similar products, provide smooth video of motion using mechanical stabilization and a trained operator. Nimur (talk) 14:43, 15 July 2018 (UTC)


 * Steadicam's introduction was apparently a really big deal. I came across this recently and there's a couple interesting historical pages from the current manufacturer.  The "art museum steps" scene from Rocky was inspired by a very similar shot in the original Steadicam demo reel.  The full sized ones (made for Hollywood movie cameras) are huge and expensive but they now have some small handheld ones for smartphone/gopro sized cameras that are pretty affordable I do not need another gadget....  Interestingly only a very recent (phone sized) model has a gyroscope: I had expected they all did, but apparently the traditional one is just an elastic mount on some gimbals, completely inertial. 173.228.123.166 (talk) 16:54, 15 July 2018 (UTC)
 * Yeah, SteadiCam is kind of an important invention; it is one of those inventions which is so simple in execution and so significant in addressing a very real problem that there's a tendency to try to genericize the trademark. This is one reason why the SteadiCam people are so aggressive in protecting their intellectual property - particularly when it comes to the very sensitive wording associated with motion picture credits!  Nimur (talk) 20:13, 17 July 2018 (UTC)
 * Also there is a Hollywood profession of focus puller, someone whose sole duty is to focus the lens while the shot is in progress. That is not so easy to do automatically.  The focus puller has to understand exactly how stuff will be moving in the shot and what parts are supposed to be kept in focus, so that the focus movements are pre-planned/choreographed but done in real time as the shot proceeds.  173.228.123.166 (talk) 16:58, 15 July 2018 (UTC)


 * A little web research shows bigger gyroscope stabilizers are a thing. ken-lab.com apparently originated them in the 1950s.  There are other brands and some DIY ones now (even one using old hard disk drives as gyros!) but it all seems cumbersome and low tech.  I tried some stabilized binoculars in a store once (they have little piezo actuators that wiggle the prisms in the binocular to compensate for wobble) and they worked amazingly well, and optical image stabilization in camera lenses is similar, so I guess this gyro stuff is for heavier gear, higher amplitude vibration etc.  173.228.123.166 (talk) 19:16, 15 July 2018 (UTC)

Differences between hydrogen and most elements
Why do isotopes of hydrogen make such a big difference?? If all carbon on earth were C14 instead of the standard C12, it wouldn't affect life very much. But if all hydrogen on earth were H2 instead of the more common H1, we would die. Why do isotopes of hydrogen affect the element's behavior more strongly than isotopes of other elements?? Georgia guy (talk) 18:29, 15 July 2018 (UTC)
 * What are the bases of your two premises? ←Baseball Bugs What's up, Doc? carrots→ 18:43, 15 July 2018 (UTC)
 * Bugs, this is kind of a well-known fact, except that the choice of C14 is unfortunate. C14 is very radioactive, with a half-life of only 6K years or so.  If all carbon on Earth were C14, we would die promptly from radiation poisoning.  But from a chemical point of view, I think GG is correct (the choice of C13 instead of C14 would probably have made the question correct).
 * I think the answer is that the extra neutron just makes a lot more difference when there's only one nucleon to start with. For example deuterium has double the mass of ordinary hydrogen.  That affects the energetics of chemical reactions.  Someone who knows more about quantum chemistry can probably elaborate, but I think this is the main point. --Trovatore (talk) 18:53, 15 July 2018 (UTC)
 * I guess this is also to do with energetics of reactions, just not chemical ones...but since the mass will effect the average velocity of the particles in solution, it would also have an effect on rates of diffusion. 61.247.39.121 (talk) 23:09, 15 July 2018 (UTC)
 * That's the basis for the original separation of uranium isotopes during the Manhattan Project. --Khajidha (talk) 14:20, 17 July 2018 (UTC)
 * The Heavy water says BTW it also mentions the, I agree well known, problems organisms have with heavy water. It's perhaps worth remembering that while it's sometimes worth challenging a statement made in a question where you have good reason to be uncertain if it's true, it's not necessary to challenge every single statement made especially ones you know nothing about. If you just wish to know more about something said, it would be better to phrase it as a question. Nil Einne (talk) 19:55, 15 July 2018 (UTC)
 * Which is what I did. And Trovatore immediately shot down the first of the two premises. And thank you for providing a source for the second premise. ←Baseball Bugs What's up, Doc? carrots→ 00:14, 16 July 2018 (UTC)
 * No you did neither of what I said. Your response wasn't a polite question for more information but questioning a basic well known fact that frankly anyone who hangs out at the Science desk should already some inkling of. If you had just wanted more info on this basic fact, you should have wrote a polite question asking for it, rather than unnecessarily questioning such a basic fact. BTW, it sounds like you remain confused. Trovatore clarified that while the OP's specific example was flawed, but both of their basic premises were in fact correct. As now supported by the article I linked to. (Albeit which didn't seem to be directly sourced.) Nil Einne (talk) 08:18, 16 July 2018 (UTC)
 * "If all carbon on earth were C14 instead of the standard C12, it wouldn't affect life very much." Is that a true statement? ←Baseball Bugs What's up, Doc? carrots→ 15:59, 16 July 2018 (UTC)
 * No idea why you're asking something we've already established was incorrect. If you're understanding of this discussion is so limited, I strongly suggest you just stay away from the RD for good or at least as a responder. This isn't even a case of a limited English ability where we can allow reasonable latitude. Your English ability is clear up to the task, your ability to understand very simple discussions appears not to be. Nil Einne (talk) 10:52, 17 July 2018 (UTC)
 * For the benefit of nitpickers, I should clarify that this depends on the precise wording and how it's interpreted and there's a chance we don't know for sure the answer to the question. Assuming that the statement is interpreted to mean it's always been that way, then there's a reasonable chance life would be different. It's possible it would not have managed to evolve although I expect more likely it would have evolved to cope with C14. It seems likely this would mean significant differences. Even more so with organisms that live very long (thousands years or more for example). Of course a wider point is that the short half-life means a world with C14 is unlikely to be plausible, I don't see how your world could ever produce enough C14 to keep it the primary form of carbon, well unless we're thinking about tiny amounts. If it's simply a case that all the C12 was magically replaced with C14, it would seem likely as Trovatore said that life would not cope with this and most would die out. Mind you, I'm not sure that there's anything in heavy water (or the other molecules of life with hydrogen) that means most of life couldn't also work with heavy water. So if life evolved in a world where most hydrogen was predominantly deuterium, I suspect life would also have managed to make a go with it. (But this is just a guess, I know too little to have any confidence.) This is of course moot to the question, since as I already mentioned anyone with basic scientific understanding could interpret what was meant. As someone had wanted to nitpick these and any aspects, that seems reasonable (especially if the question has been answered). But I still see no benefit to anyone questioning a very basic and basically correct premise namely that life which has evolved to cope with hydrogen-1 cannot cope with deuterium, rather than either politely asking for an explanation for something a person may reasonably be unaware of, or just leaving the question be. Nil Einne (talk) 11:28, 17 July 2018 (UTC)
 * You're starting to catch on. Life could have evolved to tolerate C14 and/or deuterium. But it didn't. Not on earth, anyway. ←Baseball Bugs What's up, Doc? carrots→ 17:22, 17 July 2018 (UTC)
 * All C14 would be a lot of radiation. Low-energy betas, but still.  I think there's a better chance life could have evolved to use deuterium than C14. --Trovatore (talk) 18:10, 17 July 2018 (UTC)

See kinetic isotope effect. Changing 12C to 13C only raises its mass by about 8.3%. Changing protium to deuterium raises its mass by about 100%. It is no surprise that the consequences are therefore much bigger. Double sharp (talk) 04:25, 18 July 2018 (UTC)

Rinsing out a vessel
You have a 1 litre vessel full of soapy water. You want to rinse it until 0% soap remains (or nearly 0%). Which is most efficient?


 * 1) Pour out the soapy water. Fill it 10% full of clean water. Swish it around. Pour it out. Repeat until 0% soap remains.
 * 2) Pour out the soapy water. Fill it 50% full of clean water. Swish it around. Pour it out. Repeat until 0% soap remains.
 * 3) Pour out the soapy water. Fill it 90% full of clean water. Swish it around. Pour it out. Repeat until 0% soap remains.

Anna Frodesiak (talk) 20:44, 15 July 2018 (UTC)
 * Strictly OR, but when camping (with limited potable water), I use something most similar to 1 for washing dishes, etc. However, I too, would be interested in a "scientific" answer. 2606:A000:1126:4CA:0:98F2:CFF6:1782 (talk) 21:17, 15 July 2018 (UTC)
 * I think the answer is going to vary a lot depending on what sort of "efficiency" Anna has in mind, and a little bit on what "0%" means. Of course you're never going to get to literally zero percent by any practically feasible method. But if you have a threshold in mind, you can optimize the procedure for getting to that threshold based on things like the amount of time spent, the amount of water used, how difficult it is to "swish". I don't think we can really come up with a definitive answer here. --Trovatore (talk) 21:18, 15 July 2018 (UTC)
 * I assumed "efficient" meant least amount of water used (and "nearly 0%" meant undetectable for ordinary use). 2606:A000:1126:4CA:0:98F2:CFF6:1782 (talk) 21:36, 15 July 2018 (UTC)

I guess by efficient I mean time and effort. Anna Frodesiak (talk) 21:41, 15 July 2018 (UTC)
 * In that case, I'd go with 3 (more intuitive than scientific answer). 2606:A000:1126:4CA:0:98F2:CFF6:1782 (talk) 21:45, 15 July 2018 (UTC)
 * (ec) Too complicated for a once-and-for-all a priori solution, I think. We'd need to have a model that incorporated the rate at which the soap diffuses into the clean water, the characteristics of the pot as regards "swishing", et cetera. Probably best to just try it a few different ways and see what works for you. --Trovatore (talk) 21:48, 15 July 2018 (UTC)


 * If the contaminant is just soap, a few quick rinses with small amounts of water is easiest and saves water. Say dumping out the vessel's contents leaves 1 ml of solution in it by surface tension (probably too high a guess).  If you started out with 1ml even of pure soap and treat the rinsing as pure mixing into a solution, 3 rinses of 100ml each would get to 1/1003 or 10-6 concentration, same as 2 rinses of 1000ml each, but probably quicker.  See homeopathic dilution if you want to get silly. 173.228.123.166 (talk) 23:43, 15 July 2018 (UTC)
 * That assumes that the soap mixes evenly with the water, in a reasonable time. It's not clear that that assumption is reasonable.  In my experience, it tends to stick to the pot, at least some sorts of pot. --Trovatore (talk) 03:43, 16 July 2018 (UTC)

Can this be changed to a more pure example to come up with a maths or physics type answer? For example, 1% dye. Add water. Allow for mixing by osmosis, etc. Anna Frodesiak (talk) 04:15, 16 July 2018 (UTC)

Trovatore, yeah, soap dissolves more readily in hot water if that helps. Anna, let's say it takes a seconds to draw a liter of water from your faucet, so x liters (say x=0.1) takes ax seconds. Then you spend b seconds sloshing it around and dumping it out (this is presumed independent of x), and there's c liters (like 0.001 liters = 1ml) of solution left in the pot after you dump it. You started with 1% dye (let's say you dump it immediately so only c liters of 1% dye are left in the pot before you start rinsing), so after n rinses of x liters each, the amount of dye left will be 0.01&sdot;(c/x)n and the time spent will be n&sdot;(ax+b) seconds. Let's say you want to rinse til the dye concentration is 10-6. If x=0.1 this takes 3 rinses and 0.3a+3b seconds, and if x=1 it takes 2 rinses and 2a+2b seconds. I guess it takes about 10 sec to get a liter of water from my kitchen sink, and maybe b=5 sec of sloshing. In that case, the 2x 1 liter rinse is 20+10=30 sec and the 3x 0.1 liter rinse is 3+15=18 sec. I guess if the number of rinses turned out to be much higher (like if you wanted 10-100 concentration, which is physically silly) or if you treat n as a continuous variable, you could frame this as an optimization problem where you solve for x to minimize the time spent. You'd draw a graph that would have some minimum, write out equations and find where the derivative is 0. But of course you probably would want more parameters in the model than just the water fill rate and slosh/dump time. Anyway this is the general approach to what I think you were asking. 173.228.123.166 (talk) 05:03, 16 July 2018 (UTC)


 * So, based on the above, is it better to use less water more times or more water fewer times? Anna Frodesiak (talk) 05:11, 16 July 2018 (UTC)
 * You still have an undefined goal of "better"/"efficient". Do you mean in terms of time? Number of actions (which may not be the same as "time" if you consider it takes longer to do something with a larger volume)? Total amount of water used? Is there no worry about evaporation or surface tension ("stickiness" of the soap to the surface)? Is the soap completely and quickly water-soluble (with no residue of relevance, and not difficult to rinse away)? If you are merely trying to get the lowest possible remainder, then there is no cost and a definite gain to using a larger amount of water in each rinse (though when a container becomes very full, it is harder to mix it evenly). And heck, may as well do it lots of times too (again, no cost). But if you are trying to conserve total water usage, then "many smaller batches" is generally more effective than "fewer larger batches" as long as each batch is large enough to mix well, not evaporate, and pour out easily (practical concerns of handling the material). DMacks (talk) 05:19, 16 July 2018 (UTC)
 * For clarity, the OP has already said "" which while maybe not completely clear, does seem to suggest water used isn't much of a concern in and of itself. BTW, I suspect there's also a reasonable chance it may depend on whether the container can be closed especially if we using an extreme like only talking about the amount of water used and not considering anything else. Nil Einne (talk) 08:22, 16 July 2018 (UTC)
 * Well, this is a standard math question in German schools (usually with only two fixed schemes given). Under the usual assumptions (spherical cows on a perfectly slippery surface in a vacuum, i.e. perfect mixture of rinsing water and soapy water, a fixed amount of liquid remaining in the vessel after each rinse, no evaporation or other funny effects, optimisation goal is minimal water usage) the solution in the limit is to do an infinite number of rinses with no water at all ;-). The less water you use for a given rinse, the higher the concentration of soap in the solution you pour out. So for a given amount of water you want it to go in in as small increments as possible. --Stephan Schulz (talk) 09:24, 16 July 2018 (UTC)
 * The reason why that works isn't mysterious, nor even mathematical. Suppose you have a vessel partly full of soapy water.  Do you wash it out better by (a) mixing fresh water into the soap-water, or (b) throwing out as much of the soap-water as you can first?  Clearly there can be no merit to contaminating the fresh water unnecessarily. Wnt (talk) 12:41, 16 July 2018 (UTC)


 * Anna re So, based on the above, is it better to use less water more times or more water fewer times?, if it's based purely on minimizing time, there will be an optimum. We had 30 seconds for 2 rinses with 1 liter, and 18 sec for 3 rinses with 0.1 liter.  Let's consider 0.01 liter so with the same parameters you need 6 rinses.  That would be 30.6 seconds, even worse than rinsing with 1 liter.  If the number of rinses doesn't have to be an integer, then in this case the optimum would be about 0.129 liters for 2.84 rinses taking 17.88 seconds.  That makes 0.1 liters very close to ideal. 173.228.123.166 (talk) 20:36, 16 July 2018 (UTC)


 * The OP posed the question with three techniques, each one starting with throwing out the soapy water. Thus each of the three starts with the pan in the same condition. She wants to end with zero soap in the pan, so each of the three ends with the pan in the same condition, whose degree of cleanliness is immaterial; it's the same state for each of the three techniques. She indicated that time and effort were her criteria. The three are fills of 10, 50, and 90 percent, swishing, and tipping out repetitively until zero soap is left. There are finite times involved: to fill to 90% will take nine times as long as for 10%. The effort of lifting will also be greater, but let us assume that this is not onerous and ignore it. The question then comes down to how much time is involved in filling versus how efficient the three different filling levels are, i.e. 10% will fill quicker from the tap but does it require more fills than 90% to achieve the same soap reduction? Kitchen experience tells me that 90% filling will dilute soap remnants much more than 10% will, but at the expense of taking longer to fill. Therefore I'd have to go with the mid-range figure of 50% as the best compromise of the OP's stated choices. There may be a more efficient filling of say, 25-30 percent, but the OP did not give that as an option. Akld guy (talk) 22:42, 16 July 2018 (UTC)

Thank you, everyone! I really, really appreciate the thoughtful answers. I am sorry I am so bad at asking questions. I guess that is part of being good at science: forming the experiment. Thank you again. I will try it all in the kitchen and see how it pans out, no pun intended. Anna Frodesiak (talk) 20:29, 17 July 2018 (UTC)