Wikipedia:Reference desk/Archives/Science/2013 October 20

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October 20

Measuring tension in a belt

I have a long, thin drive belt running between two pulleys that are 1.2 m apart. I'm trying to estimate the tension in the belt - but it's hard to do directly. What I did was to buy a machine that fishermen use to weigh their catches - a little gizmo with a hook on one end and a digital display on the other. If I hook the machine over the belt midway between the two pulleys and pull on it until the belt deflects by 5cm - it reads 2170 grams.

Is this enough information to calculate the approximate tension in the belt? What would be the equation?

SteveBaker (talk) 04:38, 20 October 2013 (UTC)

I don't think you can calculate the tension in the belt when you aren't pulling it sideways - depending on the elasticity, it could in theory be completely slack. AndyTheGrump (talk) 04:49, 20 October 2013 (UTC)

Oh - yeah, I appreciate that. What's the tension when it's deflected? In this case, I doubt the tension is much different because drive belts are fairly inelastic and the deflection I'm applying is tiny compared to the overall belt length of around 2.4 meters. SteveBaker (talk) 04:59, 20 October 2013 (UTC)

(Hmmm - if it helps, I can measure the force to deflect the belt by two or more different amounts.) SteveBaker (talk) 05:00, 20 October 2013 (UTC)

(ec) You can approximate the tension by assuming the belt is a string (i.e., assuming its tensile forces are symmetric and isotropic). Then convert the measurement from your spring scale - you want to convert from grams (as indicated) to force (in newtons, or pounds) by multiplying mass times g (9.81 m/s). (Actually, the spring-scale measures weight: force on the spring, not mass, so it ought not display in grams!)

Then, you can draw out the geometry of your deflection: a force of (2170 · 9.81 N) gives a 0.05m deflection, let's say at a perfect right-angle to the belt, which is 1.2 m long. Draw the triangles, and solve for the tension along the hypotenuse - just straightforward trigonometry! The forces on an ideal string, in x- and y-, are exactly proportional to the deflections in x- and y- so you can use "similar triangles", or compute the tangent, or use whatever other favorite trigonometry formula you like.

The catch is, belts are not perfectly isotropic and symmetric; it is not "string-like; tension out of the plane is not perfectly additive with tension in the plane of the material. Even worse, your measurement is highly sensitive to error because of the tiny deflection you can realistically create. A tiny measurement error will be multiplied by an immense scale factor. (If you look online for belt tension gauges, as used in automobile maintenance, those devices work around this problem by controlling the geometry - they hold two points of the belt stationary while a second armature performs the deflection. No matter how long the belt is, the measured vertical deflection is always about the same size as the measured horizontal span, which keeps your calculations in a less pathological part of the tangent function; or put more simply, you want to avoid measuring a deflection using a long and skinny triangle!) My number-crunching, using Steve's data, estimates about 100 pounds tension in the belt: not impossible, but higher than I'd expect.

Among our many contributors, one of us has an ASME handbook; we can probably dig out an appropriate "engineering fudge factor" to adjust the simple equations, if your belt is made of a standard material, size, and shape... Nimur (talk) 05:01, 20 October 2013 (UTC)

Ah - so I understand the sensitivity issue. What I need to do to get more precision then is to perhaps add two fixed pulleys - maybe 10 cm apart and pull on the belt midway between the two to get the 5cm deflection. The problem with that is that the belt would need a lot of force to deflect it by so much over such a short distance. I'll play around with it...maybe I can figure out something better.

Many thanks! Having more eyes on the problem always helps. SteveBaker (talk) 16:59, 20 October 2013 (UTC)

I think you should lay out which alterations are possible and which ones aren't - also whether you need to know the tension moment to moment or if you are just measuring it once to position the pulleys. For example, if you have the option to anchor one of the pulleys by a material with Hooke's Law elasticity such as a spring, you can calibrate how far the pulley moves with varying forces you apply yourself, and then measure the position of the pulley moment to moment to know the tension. If you have the option to modify the belt itself, you could have a region of the belt with a known elasticity, with a small insert that is inelastic anchored to one end of the region, and have it report how far the other end is displaced. Wnt (talk) 15:56, 20 October 2013 (UTC)

I can't change the geometry or material of the belt - both pulleys are fixed (and moving them would alter the tension - which kinda defeats the object of measuring it). SteveBaker (talk) 16:59, 20 October 2013 (UTC)

Can you do the following?

• make two marks on the belt and measure precisely the distance between them
• take the belt off the pulleys and put clamps on the belt at the two marks
• apply measured tension to the clamps to stretch the part of the belt between the clamps, to the extent that the two marks are the same distance apart as when the belt was on the pulleys
• use the applied tension as an estimate of the tension of the belt when it's on the pulleys

--173.49.18.190 (talk) 08:34, 21 October 2013 (UTC)

Could you remove the belt, get another 2 pulleys, fix one, attach a newtonmeter to the other, and move it to the correct position (ensuring it is held by only the newtonmeter and the belt? The distances won't be 100% accurate, but it's probably closer than you'll get by the other techniques. If you set up some kind of fixed sand 120cm+x (where x is the length of the newtonmeter, plus a fudge to account for the extension while in use) from the fixed pulley, then you don't even need to hold it. MChesterMC (talk) 08:46, 21 October 2013 (UTC)

If the belt is sagging measurably by it's own weight, and you know the weight per unit of length of the belt, you could calculate the tension without applying springs or other devices by using the formulas for a catenary. But that's unlikely, drive belts usually have high tension compared to weight... Measuring the same way you did for a few other deflections (2, 3 and 4 cm, best not larger, don't want to stretch it) and comparing it could give you some idea. You have to measure the deflection in respect to a straight line between the pulleys, not from the belts "resting position" (because of sagging). To give approximate value, use 120*M/D with M= measured grams and D is deflection in cm. Not completely exact, but for small deflections it doesn't matter in comparison with the measuring accuracy. Ssscienccce (talk) 13:36, 21 October 2013 (UTC)

Is reading in your DNA?

Is the skill or tendency to be good/easily learn to read in human's DNA? Throwaway62 (talk) 04:47, 20 October 2013 (UTC)

The capacity for language is practically universal in humans. It's the exceptions and the struggles which get the attention, such as dyslexia. That suggests that language capability is hard-wired, as sure as the ability to walk upright. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:17, 20 October 2013 (UTC)

It's certainly both, as being exposed to reading early on also affects one's ability and willingness to read. StuRat (talk) 13:16, 20 October 2013 (UTC)

You can say it in a sense, because of course there are genetic learning disabilities that can prevent people from learning the skill. However, despite some interesting phenomena such as dyslexia which involve a more visual-level confusion that people can circumvent with training, I'm not aware of any genetic syndrome that specifically prevents reading while allowing other activities to work normally. This should not be surprising since reading was developed too recently for evolution to affect specifically to any great degree. Wnt (talk) 15:46, 20 October 2013 (UTC)

That last sentence doesn't make much sense to me. Reading is based on symbol recognition, and that ability goes way back in human evolution. In fact, it may even predate humans, as other animals seem able to recognize some symbols, too. So, there should be genes for that, which will affect the ability to read. StuRat (talk) 02:17, 21 October 2013 (UTC)

Well, if it comes to recognizing whether a symbol is a "b" or a "d", dyslexia affects that, and that may be very vaguely comparable to an animal trying to decide if an insect is an edible or inedible species. But recognizing that three letters together in a certain sequence, without spaces, but with spaces around them, have a particular meaning... it's hard to picture an equivalent from other animals. (There's a whole article on reading (process) which I should refer to rather than philosophize about from scratch, I suppose) Wnt (talk) 05:14, 21 October 2013 (UTC)

Though on consideration, it's possible I'm pursuing a No true Scotsman fallacy here... Wnt (talk) 17:12, 21 October 2013 (UTC)

Yes, I think very similar logic is used to determine that "that object is a chair" as is used to determine "that object is the word 'chair' ". It's all just various forms of pattern recognition. Heck, in some languages the word chair might actually be a picture of a chair. StuRat (talk) 17:23, 21 October 2013 (UTC)

But then there's grammar, and also the tendency of, say, Egyptian hieroglyphs to pull away from meaning exactly the thing represented in the picture (how exact that can ever be anyway is a side-track) into the domain of metaphor and sounds. You can say that symbols for sounds are combined into words and the words recognised as patterns representing things, but some of those things are very context-dependent abstract concepts or helper words, some of the words may be new but still understood, and it's all cognitive in fancy ways that amount to something more than pattern recognition ... arguably.  Card Zero  (talk) 18:59, 21 October 2013 (UTC)

This 2009 paper on the genetics of reading disability by by Tracey Petryshen and David Pauls says: "although there may be some disagreement regarding the genetic contribution to specific components of [reading disability], all studies demonstrate that [reading disability] is influenced by genetic factors. However, the specific underlying genetic mechanisms are not known. " Gandalf61 (talk) 09:21, 21 October 2013 (UTC)

Just a note that while the ability to read may be coded in our DNA, the skill is not. Skill in reading falls squarely on the nurture side of nature versus nurture and is not heritable. q.v. Epigenetics.--Auric talk 22:58, 21 October 2013 (UTC)

Dropping a Slinky

I was watching an episode of QI yesterday and on it Stephen Fry claimed that we don't know why the bottom of a dangling Slinky remains stationary until the top compresses down on to it at which point the whole thing falls to the ground. You can see what I'm referring to here. The man in that video though seems to suggest that we do know why this happens. Granted, it's simplified for laymen. Oversimplified you might say. I'm curious though and want a bit more explanation. I'm not looking for math formulas and things though because that would be over my head. But could someone explain it a bit more in depth? I have an idea in my head but I don't really know how to best express it. Something about the tension in the up vector, mid-drop, being greater than the force of gravity.

Note: This is not a request for medical or legal advice. Please do not hat this question. Thanks, Dismas|^(talk) 05:50, 20 October 2013 (UTC)

Isn't this the same as Steve's question above, about belt tension? Or is that just the delusion of generality perpetuated by the addled brain of a physicist...? Yes, tension in the real world is more complicated than the idealizations we teach in elementary statics!

The answer is quite obvious, and the video you linked above explains it very well. The tension at the bottom of the spring exactly counters the force gravity. At time t=0, when the top of the spring is released, the tension at the bottom has not changed. So, there is no net force on that part of the spring material; and it remains at rest.

Why hasn't the tension changed yet? Information about the dropping of the top of the spring should travel fast - at the speed of sound. In spring steel, that's something crazy fast, like eight kilometers per second! Even at 300 frames per second, that would take less than a frame of video to reach the bottom of the spring.

But slinkies are crazy contraptions! Tension between the coils is a totally separate entity from tension along the coils, and if we wanted to, we could describe the slinky as a "meta-material" with multiple speeds of sound! When energy propagates as a compression wave among the coils, (rather than as a compression wave along the interior of the metal material), the speed of propagation is quite slow.

And finally, since we have to satisfy action and reaction, consider the top of the slinky. At time t=0, the exact same rules apply. The tension has not changed and is still pulling that part of the spring downward; this is added to the force of gravity; and the contact force where the man was holding the spring has suddenly vanished. There is a net force: gravity plus downward tension. The top of the spring falls downward, faster than a free-falling object.

If you had enough time - say, you dropped the slinky off a tall building - that compression wave might reach the bottom of the slinky a long time before the slinky hit the ground, and the bottom would move. The wave might reflect, and the slinky would expand and contract as it fell; and then you'd get a wobbling slinky. Chances are pretty good that the entire slinky would start tumbling, unless you were absolutely perfect in the way you dropped it, and air flow was perfect, and so on.

If you study advanced (i.e., continuum) mechanics, your teachers will force you to solve you will have the opportunity to solve the equations of motion as a continuum - rather than treating the spring as a point particle, like you would in engineering statics (or "high school physics"). You will have to express the tension, and in fact the mass, as a function of spatial coordinate, and then construct an equation of motion that satisfies all the constraints; forces must be resolved, and you may also require that the spring remains intact (as a single, solid but perhaps non-rigidly connected object). Usually, this requires learning how to live and work in Lagrangian coordinates. Once you learn that technique, most of the rest of physics - even the weird stuff like relativistic motion and atomic physics - all seems pretty mundane! Nimur (talk) 06:09, 20 October 2013 (UTC)

Even though the speed of the actual movement of the object might be slow wouldn't tension in a substance like that cause effects that may propagate at about the speed of light.175.38.168.70 (talk) 08:41, 20 October 2013 (UTC)

(edit conflict) Further to Nimur's comments above, a simple way of looking at the situation is that the centre of gravity of the slinky falls as expected, but at the same time the slinky contracts because it no longer has a stretching force. Thus the top falls at twice the "expected" speed (because it has both gravity and internal tension acting on it) and the bottom doesn't fall at all until the contraction ceases. Dbfirs 06:19, 20 October 2013 (UTC)

Thank you both. I think I understand better now. And I don't read the Science Desk on a regular basis, so I wasn't aware of Steve's question above. Dismas|^(talk) 06:59, 20 October 2013 (UTC)

Going back to Nimur's answer above, in the HD video here at about one minute in, the slinky is dropped from a tall building and eventually - at about 1.30 - the top of the coil appears to overtake the base, which is still stationary. Richerman (talk) 10:02, 20 October 2013 (UTC)

The mathematical explanation can be found here . Ssscienccce (talk) 13:53, 21 October 2013 (UTC)

It really isn't counterintuitive. With a slinky you can see the upward force, i.e. the tension that is implied by the spring being stretched out. Since the area right above the bottom has the stretched shape, you know full well the bottom can't drop because it's held up by that. What's harder to accept viscerally is that the center of gravity really has to follow Newtonian physics. I mean, why does the very top of the slinky have to start moving at the exact rate needed so that the mass it has will move at the speed it does so as to make up for all the mass in the center and the bottom? I suppose I see that its speed will be faster from the same force the smaller its mass, and the force will be greater the larger the mass it supports, so it has to work out, unless it's hindered by air resistance or the speed of light or something in which case I suppose the Newtonian model doesn't apply any more, but it's still an unexpected place to see math poking its nose into. :) Wnt (talk) 18:49, 21 October 2013 (UTC)

Power use in simple remote control

Help! Recently my girlfriend went to a battery shop and got them to change the battery in her remote garage door opener. The man who did it told her that the battery would last much longer if she only used the remote when she was really close to the door, rather than out on the street. It sounded like the usual rubbish "technicians" come up with to me, but then a recent question on the Ref. desk about radio communication got me thinking that maybe I was too quick to judge! I assumed that simple remotes like this just broadcast a coded signal at a certain strength. What is the real answer? 122.108.189.192 (talk) 07:19, 20 October 2013 (UTC)

I think your first instinct is right, and that's BS. One exception is if she starts trying to use it when out of range and ends up pressing the button more than once. StuRat (talk) 13:14, 20 October 2013 (UTC)

StuRat is probably correct, but there are hundreds of types, makes, and models of door openers; the electronics inside them are usually subcontracted recursively to various small electronics companies; so it's nearly impossible for us - or the retail technician, for that matter - to answer with great certainty. Even if we had a lot of time, money, and skilled scientists and engineers, reverse-engineering garage door opener radios is not as easy as it sounds!

Most garage door remotes are simple unidirectional broadcast transmitters, so they expend the same transmitter power whether the signal works or does not work. However, there are some garage door openers with more advanced radios - probably not as sophisticated as the mobile telephone radio protocols we discussed last week. But it's plausible that your remote might "retry" broadcasting the command until it receives an acknowledge response from the base station. That would mean that it would waste a lot of power retrying (until it succeeds, or times out), if the reception is poor or the distance to the base station is too far.

Why does this matter? Because power savings of a few microwatts might extend the life of the battery by several years!

And, there are other reasons to care about garage door openers. Some years ago, I learned a lot more than I ever expected to know about garage door radio protocols, because (as you can read in this 2008 JIEDDO summary report), door opener remotes were a very popular and very sinister IED trigger in Iraq and Afghanistan. They're cheap, they're easy to buy, and the radios inside them are all totally different. Retailed door opener products just get slapped together by contract manufacturing designers with any radio circuit that happens to be available that week! That made detecting and jamming the bombs almost impossible for American electronics warfare soldiers. "JIEDDO has spent more than $2.3 billion to develop, procure, field, and sustain electronic jamming technology and techniques to thwart simple, cheap triggering devices such as two-way radios or garage door openers." As it turns out, when the lives are on the line, it is actually very hard to know everything there is to know about a garage door opener radio protocol. Nimur (talk) 05:48, 21 October 2013 (UTC)

There is nothing in the JIEDDO report that indicates some remote garage door systems send back a confirmation signal from the receiver. Doing so would double the cost and increase the current drawn form the remote battery, even if the first ping worked. And it is totally unnecessary: If your garage door doesn't open, simply hit the button again, a bit closer. As in recent years all such devices operate in the high UHF band, the range is consistent from one day to the next. The owner will soon learn how close he/she needs to be. 60.230.213.251 (talk) 06:45, 21 October 2013 (UTC)

You're right; the report I linked only establishes that a multitude of garage door radio exist; it does not describe them in detail; but that was an overview report written for politicians. Technical details would bore the politicians, so those sorts of boring details are kept in confidential reports that aren't available on the internet at large. Nimur (talk) 15:51, 21 October 2013 (UTC)

I have a Chamberlain garage door opener which uses rolling codes for security (so somebody can't just scan the code you use then come back later and replay it to open your garage door). Logically, they must send a confirmation signal, or how else would you keep the rolling codes in synch between the sending and receiving units ? Of course, if the confirmation code isn't received, then sending the code again and again automatically from the sending unit would be unwise, in case the button is pressed when you are nowhere near the garage. StuRat (talk) 16:26, 21 October 2013 (UTC)

Assuming you have a KeeLoq encoder, see Rolling code and this datasheet linked from it, particularly pages 16 and 17. The transmitter just increments its code by one every time you press the button - the receiver keeps track of the last code sent successfully. If the received code is within 16 of the last code, it's accepted. If it's more than 16 but less than 32k away from the last code, you need to resynch the transmitter and receiver by pressing the button twice, so that the receiver gets two sequential codes. If it's more than 32k away, the transmitter is locked out and you'll have to reset it. But no information is transmitted from the receiver to the transmitter. Tevildo (talk) 19:21, 21 October 2013 (UTC)

That's too bad. That method means it will accept something like 32 codes, making it much easier to hack, than if it worked the way I hoped. StuRat (talk) 20:59, 21 October 2013 (UTC)

Adaptive signal strength is useful in cel phones because of the length of the transmission, but implementing it in a car door opener seems unlikely, the two-way transmission would use additional power for the required receiver, besides there are legal limits on the signal strength of these devices so they're limited to short distances anyway. Ssscienccce (talk) 14:12, 21 October 2013 (UTC)

(OP) Thankyou all very much, an interesting comment from Nimur particularly, Scary stuff!! 122.108.189.192 (talk) 06:44, 22 October 2013 (UTC)

Can airplane runways be sloped?

I think you can save energy and time by landing while going up the slope, and take off while going down the slope. What is the problem with this?--朝鲜的轮子 (talk) 07:40, 20 October 2013 (UTC)

A Google search on "sloping runway" gives lots of hits, and some dramatic images. HiLo48 (talk) 08:09, 20 October 2013 (UTC)

Yes. Here are some tips on taking off and landing at sloped fields. Dismas|^(talk) 08:14, 20 October 2013 (UTC)

While it's possible, it's avoided where possible, and when not possible may result in quite some limitations. Civilian air traffic is highly safety-conscious (which I as a semi-frequent traveller very much endorse ;-). Part of that safety is achieved by regulation, training, and routine - to the degree that not only all air traffic controllers and pilots speak English, but that they are even expected to use a particular set of words and phrases defined by the ICAO. Having a non-standard runway layout decreases the value of that training and routine, and hence increases the risk of operating such an airport. Similarly, you have to define specific procedures for just that runway, and you would have to do a careful safety analysis that cannot draw on past experience from the large number of normal airports. Finally, most runways can be operated in two directions. Runway 21 is not the 21st runway of an airport, it is its single runway when operating in the direction that is 210 degrees off north. The very same strip of concrete (or asphalt, or dirt ;-) is known as runway 153 when when it's operated in the other direction. When there is significant wind, runways are always operated in a way that aircraft take off and land into the wind. Strong crosswind is a hazard that can even lead to an airport being closed - this is why many airports have runways in two different directions. --Stephan Schulz (talk) 08:35, 20 October 2013 (UTC)

Correction -- the other end of Runway 21 is Runway 3, not Runway 15. 24.23.196.85 (talk) 01:48, 21 October 2013 (UTC)

Right! Thanks for the correction! --Stephan Schulz (talk) 11:57, 21 October 2013 (UTC)

See also Courchevel Airport, one of the most dangerous airports in the world. Rather you than me.--Shantavira|^{feed me} 08:37, 20 October 2013 (UTC)

Thanks for the explanation on runway numbering. I've wondered about that when I've flown, but never enough to look it up once I got back home. The configuration of Chicago Midway International Airport presumably takes advantage of different wind directions, and obviously they can't use both parts of the "cross" at the same time - and it's clear that a slope would be of no use at all. In fact, I would think airplanes are designed with the assumption that they're taking off from a flat surface. I've seen smaller airports where some taxiing runways have a slight slope due to the natural topography, but that's not a problem. As regards Courcheval, I infer from the article that it's not too bad IF you're flying a small plane and IF it's a clear day. And it's not especially comforting that it's only the seventh most dangerous airport in the world. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:59, 20 October 2013 (UTC)

You should have seen the sloping (and short, and rough, and high-elevation) runway at Matekane, Lesotho! 24.23.196.85 (talk) 01:48, 21 October 2013 (UTC)

Also note that over the length of a runway, building up even a modest slope would require moving massive amounts of Earth, especially if you don't want the ground to drop away right at the edges of the runway. So, the expense is yet another reason not to do this, in the case where the ground is naturally flat. StuRat (talk) 13:12, 20 October 2013 (UTC)

However, in a VERY few cases, a runway may be deliberately built on a slope, in order to allow heavily overloaded aircraft to take off downhill -- an example would be the airstrip at Scholkovo (east of Moscow, near the town of Zhukovsky) from which Chkalov took off on his flight across the Arctic Ocean. 24.23.196.85 (talk) 01:48, 21 October 2013 (UTC)

It would also mean that almost every single airport would need to double the number of runways if they were sloped. As an example if the airport has a 13/31 runway and the top of the slope is at the 13 end, then another parallel runway with the top of the slope at 31 is also going to be required or aircraft may be landing downhill and departing uphill. CambridgeBayWeather (talk) 06:51, 21 October 2013 (UTC)

The NAZI buzz bombs used sort of a sloped "runway". Since they didn't need to land, that eliminated the doubling of runways issue. I believe other unmanned aircraft use the same trick. Some are caught in nets, while others land on level ground. StuRat (talk) 16:51, 21 October 2013 (UTC)

A more recent example and also rather specialized is the Flight deck#Ski-jump ramp. Alansplodge (talk) 17:24, 21 October 2013 (UTC)

It's not obvious to me that you would save anything overall by taking off downhill anyway. You might reach take-off speed sooner, but at take-off you would be pointing in a less advantageous direction in terms of climbing to altitude, and it might take just as much fuel to correct that as was saved on the runway. 86.176.215.115 (talk) 19:49, 21 October 2013 (UTC)

Redirection would be a simple matter of making the end be like a ski jump hill, right? (but less extreme). I wonder what the G-force would be. Sagittarian Milky Way (talk) 11:03, 23 October 2013 (UTC)

That depends on the plane's speed and the angle of the ski jump; for a navalized Mig-29 taking off from the carrier Admiral Kuznetsov, it's on the order of 2.5-3 G. 24.23.196.85 (talk) 00:11, 25 October 2013 (UTC)

Galvanised nails as plant water sensors

I'm a serial plant killer. I would like to make an arduino-based project to protect my herbs from drying out. How long would galvanised nails function as water sensors? Would they need to be recalibrated as they age? I have six pots, all on the same window sill. — Preceding unsigned comment added by 2.97.26.56 (talk) 13:03, 20 October 2013 (UTC)

I assume the problem is you forget to water them ? If so, you might want to get some of those inverted bulbs which slowly deliver water. I haven't used them myself, though, as I don't have that issue. Also, are you using any type of plant food/fertilizer ? StuRat (talk) 13:07, 20 October 2013 (UTC)

I've not used any fertiliser but these herbs didn't really last long enough to use it. I'm not sure how well those globes will fit but I'm going to buy some and find out. Thanks for the suggestion! I may still decide to make an electronic system, just for fun, but it can now go on the backburner along with everything else! --2.97.26.56 (talk) 13:23, 20 October 2013 (UTC)

You're welcome. Also, you might want to use some simple reminder method. For example, if you use a paper calendar, you could write a W in the corner of each day, then cross it out after you water the plants that day. Or, if you make coffee each morning, maybe you could put a "water plants" note inside the coffee can. StuRat (talk) 13:33, 20 October 2013 (UTC)

Responding, we should note that arduino is a sort of open source single board microprocessor; at least the original premise is presumably to measure how dry the plants are rather than to assume it. However, there may be a problem there, if I'm correct in assuming the galvanized nails would be intended as electrodes, because pure water is not a very good conductor, but ions make it conductive. So fertilizer, accumulated salinity, etc. would throw off the measurement. You'd need some sort of actual hygrometer. It seems like it could be as simple as a human hair ...... but I suddenly realize that I have absolutely no conception of what the "relative humidity of dirt" is, how to measure or even define it, even though it's something that we take as a matter of common sense every day! Wnt (talk) 15:39, 20 October 2013 (UTC)

• Our relevant articles are available water capacity, Hydrology#Soil_moisture, Soil#Soil_water, and Permanent_wilting_point. We could probably use a whole article on soil water content, if anyone is interested in putting a stub together out of the links above :) SemanticMantis (talk) 17:54, 20 October 2013 (UTC)

One could also use some sort of scales for this: you add water, system notes the maximum weight and plays the fog horn once the plant+pot is 50 grams (or whatever) lighter. 88.148.249.186 (talk) 17:27, 20 October 2013 (UTC)

And then you go to make a big meal and cut a lot of herbs, only to get a fog horn sounding right next to you. :-) Katie R (talk) 19:02, 21 October 2013 (UTC)

Err. Let us brake things down to their components parts. (1) Galvanised nails: Are these truly galvanised nails or nails that have been quickly and cheaply zinc plated and sold as being ' galvanised'? Because the thickness of zinc will affect their working life. (2) Although zinc is important for plant growth... too much is toxic. One does not leave this type of hydrometer in a pot all day and every day – unless one is a serial plant killer.--Aspro (talk) 22:15, 20 October 2013 (UTC)

There are very cheap and very simple Arduino soil moisture sensors available on Amazon - they're basically nothing but traces on the PCB. I suspect that over time they will corrode and lose effectiveness, much like a simple nail-based solution would. Katie R (talk) 14:57, 21 October 2013 (UTC)

Googling arduino soil moisture gives many discussions and several types of hardware. Seems like a common "almost anything above entry-level" model is to alter the metal to avoid corrosion. DMacks (talk) 15:12, 21 October 2013 (UTC)

A plant moisture meter I've seen in a store consists of two different metals (such as aluminum and copper with an insulator between them, at the end of a probe which is stuck in the soil. At the top of the probe is a simple meter movement with a pointer and dial. You could insert a galvanized nail or strip of zinc at some small distance from a strip of copper or a copper nail and periodically measure the voltage between them, which should relate in some way to the soil moisture, though it would probably not be a linear relationship. If you constantly drew current from this electrochemical cell, it would consume the zinc electrode or the zinc coating on the iron nail. Iron and copper would have a different voltage output from zinc and copper. Some judgement would be needed as to what output voltage represented ideal moisture, too dry, or too wet. For a nail and a piece of copper wire, there would be an appreciable fraction of a volt output, but the available current would be extremely low, so design your measurement circuit accordingly. Edison (talk) 04:16, 23 October 2013 (UTC)

Effects of high speeds in outer space

Since there are no g-force and air in the outer space, what are possible effects of, say, putting an arm (of course, in a space suite and in a isolated compartment) out of a window of a spacecraft travelling at cosmic speeds (say, 500,000 kmh and over)? Or just traveling externally fastened to a spacecraft, going with those speeds (excluding possible injuries from meteors)? --93.174.25.12 (talk) 20:12, 20 October 2013 (UTC)

Space is a near vacuum, not a true vacuum. So, you would be hit by some objects. Individual atoms or molecules presumably won't each put a hole in the suit, but will wear away at it, like dripping water wears a hole in a bathtub. I'm not sure how long this would take, though. Also, this would cause heating of the suit. And there are also micrometeoroids that would put a hole in the suit. StuRat (talk) 20:19, 20 October 2013 (UTC)

(ec)There are g-forces in outer space, of course. But apart from that: If you are in a spacecraft in free fall, there is no problem with sticking your arm out, or flying alongside, unless you come to really high speeds (when even the very tenuous gas and dust in interstellar space will start to become significant). --Stephan Schulz (talk) 20:24, 20 October 2013 (UTC)

At constant speed in an absolute vacuum and all other things being equal, none I can think of. This is interestingly related (Space travel using constant acceleration) not sure about the case here. Ok I suppose going out side the craft tethered would be like absailing at whatever g the ship is pulling. — Preceding unsigned comment added by 122.111.243.217 (talk) 04:32, 21 October 2013 (UTC)

@StuRat, "like dripping water wears a hole in a bathtub", what? where? when? Never in my life ... Caesar's Daddy (talk) 06:58, 21 October 2013 (UTC)

It would take a lot of drips, and might be more likely to happen to an old-fashioned ceramic or iron tub than something made of plastic as they are now. ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:13, 21 October 2013 (UTC)

Not sure if plastic tubs are any better at resisting this. It might just be that they aren't old enough yet to show this effect. It takes decades. StuRat (talk) 16:18, 21 October 2013 (UTC)

Good point. Also, plastic doesn't rust. But a steady drip-drip-drip over many years ought to have some effect eventually. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:04, 21 October 2013 (UTC)

Yep, I'm old, and old-fashioned. I've seen that on old bathtubs. HiLo48 (talk) 09:39, 21 October 2013 (UTC)

Yes, so have I, though it's often combined with corrosion because of the constant damp. For pure erosion, the effect is more clearly seen in old stone sinks. For a more dramatic effect in nature, see Panhole (called swirlholes in the UK). Dbfirs 10:01, 21 October 2013 (UTC)

At that sort of speed, the atoms and ions will be like cosmic ray radiation and penetrate deep into that arm, causing radiation damage. Graeme Bartlett (talk) 10:49, 21 October 2013 (UTC)

500,000 km/h isn't that fast. Solar wind peaks at close to 3,000,000 km/h - prolonged exposure to peak solar wind outside the protection of Van Allen radiation belt will increase a person's lifetime cancer risk. And our solar system rotates around the galactic center at 700,000 km/h, while our galaxy is on a 500,000 km/h head on collision course with the Andromeda galaxy. Measured against the cosmic microwave background our galaxy zips along at 2,000,000 km/h[1]. It's all relative. 88.112.41.6 (talk) 15:17, 21 October 2013 (UTC)

Did the Nazis performed unethical psychological experiments too?

Think Monster Study, Stanford Prison Experiment without cutting it short, Learned Helplessness with humans, and so on. OsmanRF34 (talk) 21:01, 20 October 2013 (UTC)

I don't see anything listed in the Nazi human experimentation article. If anyone finds reliable info, please add it (with cites, obviously). DMacks (talk) 21:57, 20 October 2013 (UTC)

I don't believe so. Would they believe such psychology applied to them so what was the point for the war effort? They were mostly interested in how to treat various sort of trauma, transplants and diseases and viewed the prisoners as lab rats without even today's concerns. Dmcq (talk) 09:12, 21 October 2013 (UTC)