Wikipedia:Reference desk/Archives/Science/2009 January 10

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January 10

Force of rain dropping

Looking at our new page Orders of magnitude (force) I thought it might be nice to add something "down to earth". I was thinking of the force of rain hitting the ground. Unfortunately I sort of got stuck half way though. At a diameter of 1 mm we could assume a drop to be spherical and I think a drag coefficient of 0.1 for a smooth sphere could be applied (or do we need a rough sphere at .4?). That should give us the terminal velocity in free fall. I got 0.523598775 mm³ for a volume and seem to remember 1 liter of water weighs one kg. 1 L our page says is 1 cubic decimeter. Anyone willing to lend a hand?_Lisa4edit76.97.245.5 (talk) 00:05, 10 January 2009 (UTC)

you underestimate the diameter alot http://hypertextbook.com/facts/2001/IgorVolynets.shtml --Digrpat (talk) 00:44, 10 January 2009 (UTC)

To work out the force you would need to know how long it takes for the drop to slow down and stop, I'm not sure where you would get such data. --Tango (talk) 01:02, 10 January 2009 (UTC)

Isn't there a math desk? I did some math, and I got about 23 millinewtons with a 5 mm raindrop going 9 m/s and stopping in its diameter. Smaller than I expected, but ballpark, I think. I got the 5 and the 9 from our "Rain" article, and I let it stop within its diameter, although there would certainly be some splashing to mess up all our perfect mathematics. Wait and see what others get before you do anything. --Milkbreath (talk) 01:09, 10 January 2009 (UTC)

But we're talking about article-space here. You can't treat it like the WP:RD and calculate your own numbers. You have to cite reliable sources. Any calculations we'd do here would be irrelevent because they'd fall under OR or Synthesis. SteveBaker (talk) 02:16, 10 January 2009 (UTC)

Oh, right. Pesky rules. --Milkbreath (talk) 02:28, 10 January 2009 (UTC)

What do we have to do to get the ref desk designated a reliable source? It's far more reliable than most newspapers I've read... --Tango (talk) 03:15, 10 January 2009 (UTC)

I'm torn berween being happy that so many of our schools apparently do "Original Research" with our definition being that close to the bottom of the pit and being sad just how often we let principles get in the way of achieving something useful. There certainly are good reasons for our OR rules, but when rules start to be applied for the sake of following "the rules" they unfortunately tend to incur a high cost in diminished results. I'm with Tango. I've seen many examples of some "nutcase" with an impressive degree putting something in a book and all of a sudden it's "true and valid". One of my favorite quotes is "Nothing exists till a white man has written about it in a book." Sad, but only too true.

@dgrpat I had chosen 1mm for a diameter since bigger raindrops are not spherical as the link from rain - [1] says. Thks for your help everyone, anyway. —Preceding unsigned comment added by 76.97.245.5 (talk) 07:28, 10 January 2009 (UTC)

Yes - I think the OR rules are a bit over-enthusiastic in cases where math and well-understood, established physics is concerned. In a case like this, it's reasonable that we should cite references for the diameter of the rain drop and perhaps provide links to the physical principles we used to calculate the force - but the actual calculation itself ought not to fall foul of the rules. However, it does (notably the 'synthesis' part). But the "No Original Research" part is what saves us from the worst of the nut-jobs with crazy theories who'd like to promote them as fact on Wikipedia. NOR means that they've got to find some reliable source to publish their crazy theory as fact before they can come and annoy us with it...that's a good thing overall. SteveBaker (talk) 14:05, 10 January 2009 (UTC)

I agree with Tango. don't want to discourage Lisa4edit from adding useful examples, but I'm not convinced that the 'force' of a raindrop hitting the ground is a meaningful quantity. A falling raindrop imparts momentum to the ground and dissipates energy, both of which can be calculated fairly reliably, but there is no single number than you can call the 'force'. The force will increase from zero as the bottom of the drop hits the ground, up to some maximum or series of maxima that depend in an impossibly complicated way on the shape of the splat that the drop makes as it disintegrates, and then decay to zero again. Even if we allow OR, the result is not going to be useful. --Heron (talk) 13:11, 11 January 2009 (UTC)

Suggestion for (original) research: (Might make a science project for someone). I wonder if, experimentally, one could note the effect of one raindrop hitting a leaf (or some elaborate experimental raindrop target) and noting the deflection, by videography, then measuring the force needed to produce the same deflection. In physics lab we fired a 22 caliber bullet into a block of wood free to swing, and noted how far it swung with the embedded bullet, measuring the energy and from the mass, the velocity of the bullet, and compared it to a standard velocity meter which had 2 rotating paper discs a certain distance apart. Or would this deflection measurement due to a raindrop only give us energy? A leaf would be springlike, whereas the wood block was gravity restored to its original position. If there were a substance delicate enough that a falling raindrop could break it, that could be a force measurement. A delicate enough strain gauge should be able to directly measure the force. Perhaps a target attached to a phono stylus could measure the force. Edison (talk) 20:29, 11 January 2009 (UTC)

Rather than leaves observe raindrops hitting a puffball They are constructed so that spores are released during a good rainfall. Certainly a reliable source has reported on the metrics of this phenomenon. --Digrpat (talk) 15:02, 13 January 2009 (UTC)

Crude death rate map

In my wanderings around wikipedia, I came upon this map of the crude death rate for different countries of the world. When you see a map like that, you have an expectation of how it will look, you expect at least some correlation with affluence. You know, Europe, the US, Canada, Australia and a few others will be really pale, while Africa would stand out sharply, and South America and Asia somewhere in between (like this map, for instance). And while it does follow some of those conventions (Africa having a high rate, for instance), there are a number of things in there that are really strange. All of Europe, besides Ireland, are fairly high on the scale. Mexico is lower than the US and Canada. In fact, the countries with the lowest rate are Mexico, Saudi Arabia, Jordan, Syria, Algeria, Libya, Venezuela, Paraguay and Ecuador. It's not exactly the list of countries you would expect to stand out in a map like that. I mean, seriously, Syria is kicking Scandinavia's ass! So what's up with that? I mean, if your country really is really dirt poor, then it seems to have a high death rate (as evidenced by Africa), but once it passes a certain threshold, it looks essentially random. But it can't be completely random, because all of Europe is one color (which would be an extraordinary coincidence). There has to be some factors lowering the score for some countries, but I can't for the life of me think of what they might be. Any one got any ideas? Belisarius (talk) 02:04, 10 January 2009 (UTC)

I presume that's because there are two drivers of longevity - health-care is one driver - but the other is healty eating and good exercise. In the US, terrible diet and a tendency for people not to exercise appropriately somewhat cancel out. Other countries have better healthcare - others, whilst being more 'primitive' in the realms of healthcare have people who don't weigh too much and who get plenty of exercise. SteveBaker (talk) 02:12, 10 January 2009 (UTC)

The map you cite is from the CIA World Factbook. They have a "notes and definitions" page where it says under "death rate":

"This entry gives the average annual number of deaths during a year per 1,000 population at midyear; also known as crude death rate. The death rate, while only a rough indicator of the mortality situation in a country, accurately indicates the current mortality impact on population growth. This indicator is significantly affected by age distribution, and most countries will eventually show a rise in the overall death rate, in spite of continued decline in mortality at all ages, as declining fertility results in an aging population."

--Milkbreath (talk) 02:26, 10 January 2009 (UTC)

Since the death rate is deaths per 1000 population, it makes sense that countries in Europe would have higher death rates, since their population growth has plateaued. In most of Europe, the general population growth rate is flat or actually negative; thus deaths in Europe represent a greater proportion of the population in any given year than in countries with rising populations. The U.S. probably has similar longevity expectations as Europe, however since the population growth is higher, the number of deaths as a proportion of the population is driven downwards. In countries with appalling health conditions and lower longevity expectations, even high population growths are offset by MUCH higher death rates. Because death rate is intertwined with such data as birth rate, immigration, and emigration the number is probably less of an important indicator of overall living conditions than is perhaps life expectency, which is not dependant upon changing populations... --Jayron32.talk.contribs 03:16, 10 January 2009 (UTC)

I like the aging population thing, that sounds like it's the reason. Even though affluent countries have low mortality rates for individual age groups, because the population is, comparatively speaking, so old, the overall death rate is higher. That indeed makes sense. Belisarius (talk) 04:18, 10 January 2009 (UTC)

The infant mortality rate map also shows a skewed picture. Compare e.g. to this site [2] Among the industrialized nations the U.S. and Canada keep performing below par. Some of the reasons cited for the U.S. are a high rate of teenage pregnancies, unavailability of quality health care for low income and illegal immigrant families with higher than average birth rates in those same population segments, and at least one study cited that due to improved prenatal care and fertility treatments babies were being delivered that were pre-term and/or of poor health to begin with. _Lisa4edit 76.97.245.5 (talk) 07:05, 10 January 2009 (UTC)

Yes. Even though developed countries have lower death rates at each age compared to developing countries, they have higher overall death rates due to their relatively older population. You need to compare actuarial tables across countries to get a more accurate picture. Zain Ebrahim (talk) 07:30, 10 January 2009 (UTC)

op amp

why the op amp is given the dual power supply? what the meaning of dual power supply? how to know weather the capacitor is charging or discharging? —Preceding unsigned comment added by KARTHICK18ECE (talk • contribs) 11:49, 10 January 2009 (UTC)

First of all, have you read our Operational amplifier article? It's VERY good.

• Op-amps typically have power supply pins labelled something like V_s+ and V_s- but those are really just 'power' and 'ground'. They aren't labelled like that because sometimes you'd like to amplify signals that go from negative voltages to positive - in which case V_s- is just the negative power rail rather than ground. The OpAmp doesn't care about where true ground is so the pins are labelled as two power rails rather than as power and ground...but that's what they are (essentially).
• When you talk about "the capacitor" - do you mean the internal 30pF compensation capacitor? I'm not quite sure what you mean by that question - or why you'd care what was going on with it. It's slap in the middle of a ton of other circuitry inside the chip - so you certainly can't measure its charge directly.

SteveBaker (talk) 13:59, 10 January 2009 (UTC)

Since Karthick is talking about the power supply, perhaps he means the power rail decoupling capacitors. In the case of a dual-rail op-amp then there are two of them. A 'discharged' capacitor is one with zero volts across its terminals, and a 'charged' capacitor is one that has a non-zero (either positive or negative) voltage. When the circuit is switched on, both capacitors 'charge' until they reach the supply voltages. When the op-amp draws a surge of supply current, both capacitors briefly 'discharge' to supply the extra current. Then, after the surge ends, both capacitors 'charge' up again from the supply rails. --Heron (talk) 13:03, 11 January 2009 (UTC)

Ah - yes. That's a good thought. In which case, the capacitor is essentially just filtering out noise from the supply rails. Presuming that the capacitor is large enough and the noise level low enough, the capacitors should be essentially fully charged all the time...but if you went with capacitors with insufficient rating or a capacitance too low for very low frequency noise - then all bets are off. It's rather hard to think of that in terms of capacitor charge states rather than thinking in terms of low-pass filtering math. SteveBaker (talk) 16:59, 11 January 2009 (UTC)

Recuperation after a stroke

Why do some people recuperate their abilities after a stroke? After damage of neuronal tissue, it is gone forever, isn´t it?--Mr.K. (talk) 16:27, 10 January 2009 (UTC)

There are a large number of variables separating the prognosis of two patients. The location of the stroke, the kind of stroke, how quickly medical assistance was secured, the general health of the individual, genetic predisposition, etc. are all factors that come into play and there are probably several more. Any one of those could influence how much damage is done. While neurons generally can't re-grow, it could be the case that the stroke took place in an area where the deficiency was less obvious or where there was enough good tissue to compensate for the damage. In some cases, the recuperation is more like re-learning, as the patient slowly regains ability by having different areas of the brain take the load for the damaged bit. See stroke recovery for great deal of information. Matt Deres (talk) 16:42, 10 January 2009 (UTC)

Pretty much agreeing with Matt. It's not quite as black and white as you might believe. Stroke recovery is highly dependent upon many factors, such as those Matt listed above. Neurons often cannot repair themselves, but working neurones around them can take up the workload in some cases. Recovery generally depends upon how damaged the tissue is (+how widespread), and the time before medical teams stepped in. —Cyclonenim (talk · contribs · email) 17:22, 10 January 2009 (UTC)

For a little bit more of what Matt and Cyclo are talking about when they say other parts of the brain take the load, Neuroplasticity#Treatment_of_brain_damage. --Bennybp (talk) 20:14, 10 January 2009 (UTC)