Wikipedia:Reference desk/Archives/Science/2010 April 13

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April 13

Problem about capacitors in series

I don't seem to understand how capacitors can be connected in series. My question requires picturing a normal circuit diagram with two capacitors connceted in series with a cell. Now, I am rather puzzled with the fact that when you just connect the capacitors, you can get an ampere reading of say 2A between the two caps...I mean the Ammeter connected in series with the two caps except that you place it between them. Like II---A---II where the II is the cap symbol and A is the ammeter. I read in a book that capacitors arent supposed to pass any current between them. That's supposedly the point. But then, how come the Ampere reading. Upon searching in Google, I cam up with something about charges, but couldnt get the picture. I need to know because I am dying with curiosity.

If you are using alternate current than there will be a current going through the capacitor. There might also be some transient current even if the current is direct. Dauto (talk) 04:56, 13 April 2010 (UTC)

It is not true that capacitors can't pass current. It isn't possible for charged particles to move across the barrier, but when the amount of charge on one face of the capacitor changes, it produces an electric field that causes movement of charges on the other side, thereby inducing a current. Looie496 (talk) 05:07, 13 April 2010 (UTC)

If electricity in a wire is like water in a pipe, then a capacitor is like an impermeable elastic hshsghrt5hrseghsegtaehg

membrhgbdgdhgdtrhthutfr6htrhbrsane blocking the pipe. If you push the water in one direction with a constant pressure, the membrane will stretch until the restoring force balances the pressure, at which point the water flow will stop. The current on the far side of the membrane is always the same as the current on the near side, though—there's just a limit to how far the water can flow in total, for a given pressure. When the water is just sloshing back and forth (alternating current), there's no such limit because the membrane never stretches too far in either direction. -- BenRG (talk) 07:51, 13 April 2010 (UTC)

I like that analogy! One could extend it to explain the rules for capacitors in series and parallel, and to a warning about very high pressures (voltages) breaking the membrane. Dbfirs 08:29, 13 April 2010 (UTC)

Then you will love the Wikipedia article Hydraulic analogy. Cuddlyable3 (talk) 12:58, 13 April 2010 (UTC)

I did! It made very interesting reading. I've always thought of electricity intuitively in this way, but I've never thought of extending the analogy that far! Dbfirs 20:18, 14 April 2010 (UTC)

Wasp question (not about the insect)

Could anyone suggest some plausible reasons for a Wasp to suffer catastrophic failure? By "catastrophic" I mean severe enough to cause the engine to fail suddenly and completely during flight (as opposed to developing reduced power or gradually overheating) AND to require custom-made spares and/or a fully-equipped machine shop to repair. Any ideas, anyone? 24.23.197.43 (talk) 05:14, 13 April 2010 (UTC)

Bird strike (could it take out a piston engine?). Or Enemy Fire. Buddy431 (talk) 05:46, 13 April 2010 (UTC)

That didn't redirect to where I thought it would. I guess shooting is the closest I can find. That's really a pathetic article for an important topic. Buddy431 (talk) 05:48, 13 April 2010 (UTC)

The capitalization of "Fire" makes it a title (song title, in this case). However, Enemy fire is a redlink, I will redirect it to shooting. StuRat (talk) 09:45, 13 April 2010 (UTC)

Done. StuRat (talk) 09:45, 13 April 2010 (UTC)

Are you writing a novel or something? Avoid the word Catastrophic failure as in my book that means reduced to scape. Mechanical failure of some sort would be a good candidate. Components used in engines of that era where prone to fail unexpectedly. What you want is a failure that does not wreck the engine. A dropped vale in the cylinder head would cause the pilot to shut down the engine ASAP.

Here are a list of WASP engines photos on Wikimedia Commons.[1] There are lots of valves. Badly broken cylinder rings could be another - would cause a lot of dramatic smoke to trail from the aircraft etc., etc. A dropped valve would require the head to be removed and the old valve sleeve to be removed a new one pressed in, you need a workshop for that. From the photos I can’t tell if the valve would drop through into the cylinder but if it can, then that would damage the piston head a be a major repair job. Broken rings could score the cylinder walls and require regrinding as well as needing the piston to be removed. Again, you can’t do that sitting on the side of the tarmac. If its for a novel, then don’t get technical. Finally: I don’t know what year air worthiness certificates were introduced but the only need I have come across for a ‘custom made spare’ is when a script writer wants to get the message across to the audience that this is just another silly American B movie that lacks any credibility what's-so-ever; but if you must, here are some possible things that such an aircraft might be used for.--Aspro (talk) 08:00, 13 April 2010 (UTC)

I don't see anything wrong with giving details. Even if the audience doesn't understand them, they still "lend authenticity". As for a "custom spare", perhaps a component of the engine needs to be replaced, and no spare is readily available, so might be produced by a nearby machine shop ? This scenario might come up particularly during war, when getting the plane flying again may be critical and the normal delivery of spare parts may be interrupted by enemy action. StuRat (talk) 10:02, 13 April 2010 (UTC)

A brief comment as to why the component failed (say metal fatigue) is OK but there is that danger, that a technical explanation may lead the audience to think that this in depth information is going to turn out to be important, later on in the story. Indulge too often, and the audience can get lost with information overload. It is different for such shows as CSI: Crime Scene Investigation because there is always someone on hand, and at the right time, to ask the soppy questions, that give the investigators the opportunity to spell out for the audience, exactly how each factoid links together. However, if the OP was writing that sort of thing, he would already have the background knowledge to be able to do his own research in this area. Second point. For reasons of weight and performance, aircraft engines are built to very tight safety margins. Even in war time, critical engine parts, that might be held responsible for such a failure would be have been manufactured to such high quality standards, that it is unrealistic, that a workshop would have those engineering documents available to inform them how to do it. An airfield workshop would unlikely have suitable case hardening nor heat treatment ovens, and on so on, nor stocks of the right metal in that grade. If they need to case harden or heat treat a chisel etc., they would just use a forge. The engineer whipping off his leather belt and punching out of it a new washer for the lube system is a possibility, but that is the sort of thing that can be fixed easily and would not lead to a sudden catastrophic failure. The two conditions the OP requests thus exclude each outer, even for this era. --Aspro (talk) 11:52, 13 April 2010 (UTC)

Wrong fuel or sand in the fuel. Cuddlyable3 (talk) 13:05, 13 April 2010 (UTC)

Thank you all very much (especially Aspro). Yes, I'm planning to write a novel about a long-distance flight, and the particular scene with the engine failure is supposed to happen somewhere in Indonesia, and to lead into a comic scene where my heroine goes with a local trucker to get spares but the trucker drives so recklessly that they run off the road several times and end up having to make repairs to the truck as well. I think a dropped valve would be the best kind of engine failure in this case -- serious enough to make the engine fail at once and completely, but not so serious as to wreck it altogether. Plus, the engine would make a lot of dramatic-sounding noises at the moment of failure. (And yes, if the cylinder was one of the upward-pointing ones, then the valve would drop into the cylinder.) As far as "custom-made spares", my understanding is that they don't make Wasp engines or spares for them anymore, so any spare part that is specific to that engine and not just a generic engine spare would have to be made to order. Once again, thanks a lot to everyone, and clear skies to you! 24.23.197.43 (talk) 02:14, 14 April 2010 (UTC)

You could probably specify a broken valve spring - that would avoid the "dropping into the cylinder" thing - and it's likely that you could find a sufficiently similar spring (eg from a truck engine or something) to at least allow you to limp home. My problem is that this engine has a lot of cylinders and the failure of one valve wouldn't cause a total failure - just a loss of power. Just have the fuel line break...much more abrupt - and very certain to force a landing. Also more credible that you could find some replacement tubing. SteveBaker (talk) 18:49, 14 April 2010 (UTC)

generalising the pI = pH formula for situations where the opposing weak acid and base are of different concentrations

It dawned on me that a major shortcut for amine + carboxylic acid solutions of equal molarity was to treat them like an amino acid and therefore pH = pI = (pKa1 + pKa2) / 2. But what if they are of different concentrations? How would I modify the formula? Is there a way to "weight" the pKas? I'm guessing they wouldn't be weighted linearly? John Riemann Soong (talk) 06:54, 13 April 2010 (UTC)

Voltage Divider and a Wheatstone Bridge

The article on the wheatstone bridge uses the idea of a voltage divider to calculate the voltage across the bridge. But it seems to me that there's a problem with this approach. The equation for the voltage divider assumes that the current running through the two impedances is equal for both. However, if we consider, say, R₁ and R₂ to make a votage divider, then because there's a current across the bridge, the current across R₁ will not be the same as the current across R₂. So we can't use the voltage divider equation in this case. How, then, is the method describes in the wheatstone bridge article valid? 173.179.59.66 (talk) 08:07, 13 April 2010 (UTC)

The key point is that the position is adjusted so that the current through V_G is zero, then the current must be the same through R₁ as it is through R₂, because there is nowhere else for it to go. Think of current through (not across). Dbfirs 08:19, 13 April 2010 (UTC)

Except, from the article:

"If all four resistor values and the supply voltage ($V_{S}$) are known, the voltage across the bridge ($V_{G}$) can be found by working out the voltage from each potential divider and subtracting one from the other. The equation for this is:

$V_{G}={{R_{x}} \over {R_{3}+R_{x}}}V_{s}-{{R_{2}} \over {R_{1}+R_{2}}}V_{s}$

This can be simplified to:

$V_{G}=\left({{R_{x}} \over {R_{3}+R_{x}}}-{{R_{2}} \over {R_{1}+R_{2}}}\right)V_{s}$"

So it seems that there is a voltage across V_g, and hence a current. 173.179.59.66 (talk) 20:30, 13 April 2010 (UTC)

Er, not when the bridge is balanced (ie when the contents of tha braces equals zero) there aint.--BandUser (talk) 21:18, 13 April 2010 (UTC)

Try putting $V_{G}=0$ in your equation (the braces), and thus deriving the equation to find ${R_{x}}$: $R_{x}=(R_{2}/R_{1})\cdot R_{3}$ . Perhaps this needs to be spelled out in the article if it is being misunderstood. Dbfirs 21:48, 13 April 2010 (UTC)

From the article: "...if R1, R2, and R3 are known, but R2 is not adjustable, the voltage difference [ie V_g] across or current flow through the meter can be used to calculate the value of Rx...". So clearly, the formula presented above is used to calculate what the voltage across the bridge would be, whether or not it's balanced. Hence V_g isn't necessarily zero. 173.179.59.66 (talk) 23:08, 13 April 2010 (UTC)

This was an alternative for when the normal balancing procedure is not possible. The point of the normal balancing procedure is that it can give a very accurate result. I would question the accuracy of the "alternative" that you mention, but I haven't used it. (If you are not going to balance at all, then you might as well just use a multimeter!) Perhaps the article needs more explanation. Dbfirs 02:13, 14 April 2010 (UTC)

It's apparently a quicker method, because it's faster to read a voltmeter than to adjust a resistor. Regardless, my question still remains unaddressed: seeing as how V_g need not be zero (as the article, and hence the above formula, both assume), how is it that the approach used, that is, via voltage dividers, is valid? 173.179.59.66 (talk) 04:40, 14 April 2010 (UTC)

V_g, as a galvanometer, draws an extremely small current, so I assume that we are making a fairly accurate approximation. I have only ever used a Wheatstone Bridge in accurate mode, but I can see that for some applications, it might be more convenient to use the alternative, with good accuracy provided that the current through the galvanometer is extremely small. Our article doesn't explain how to apply the correction, but I'll leave this to an expert. Dbfirs 08:20, 14 April 2010 (UTC)

So if I understood correctly, the galvanometer will have a very high resistance to minimize the current lost, right? Originally I had thought that this must be true, but in an assigment I recently had, we were given the same layout as a Wheatstone bridge, except V_g was replaced by a regular resistor. We were then asked to find the current through this resistor. The problem is that, in the solutions, my teacher used the same method as above, by using voltage dividers. Is he wrong? 173.179.59.66 (talk) 08:32, 14 April 2010 (UTC)

No. Galvanometers are low resistance, very sensitive ammeters. For bridges with arms in hundreds of ohms region, you could take the galvo resistance to be effectively zero. I have never heard of using a bridge unbalanced for measurement of one of the arms. But if you wanted to do this I suggest using a high impedance voltmeter to measure the difference in voltage. This will draw effectively zero current and then you could work out the unknown resistor.

When you were asked to work out the current through the resistor, was the bridge balanced? If so, the answer would of course be zero. Hope this clarifies.--BandUser (talk) 17:21, 14 April 2010 (UTC)

(I think the usual arrangement is to use a galvanometer with a high series resistance to protect it, which makes it into a high impedance voltmeter if the resistance is known.) With the resistor in place of the galvanometer, I think you were just expected to use Kirchoff's circuit laws without the special Wheatstone Bridge equation that is only valid when the current through the galvanometer is zero. You might also like to read Network analysis (electrical circuits) for a general treatment. Dbfirs 19:47, 14 April 2010 (UTC)

No, the bridge was very specifically said not to be balanced. And yet, he used the voltage divider method above to get the voltage across the bridge, and divided that by the resistor value of the bridge (which was comparable to the value of the other resistors) to get the current. But from what I understand, this is a mistake, right? 173.179.59.66 (talk) 20:35, 14 April 2010 (UTC)

You could prove it wrong by applying Kirchoff's law to the currents (unless by some very strange coincidence the values chosen happen to give a correct answer from an incorrect method, but I can't find any values that would achieve this). The incorrect method gives a good approximation when the bridge resistance is large compared with the other four. Dbfirs 15:11, 15 April 2010 (UTC)

Plants - pinching new growth? pinching? really?

I'm new to this whole horticulture thing. I'm starting easy, with an avocado seed. It's got roots and I expect a shoot soon. I've read that if you don't want a 10 foot tall tree with 2 leaves at the top you should "pinch off" new growth every once in a while to encourage it to expand outwards rather than upwards. My question concerns the exact action - am I really just... cutting off the top of my plant? This seems rather barbaric. Is there a special way to cut it? 59.46.38.107 (talk) 09:14, 13 April 2010 (UTC)

Pinching is just that, taking your thumb and fingernail and nipping out the shoot. Wait until it gets to a manageable height and then take out the growing tip. If you're growing tomatoes, this process is called "stopping" and will result in more fruit. I've never grown an avocado so I can't vouch for it in this circumstance. This article [2] has more detail on the process and its results. --TammyMoet (talk) 09:49, 13 April 2010 (UTC)

Take note, the reason these jungle plants have a big seeds is so that they have the energy reserve to shoot up above the deep shade, given by the thick ground level foliage of the forest floor, until they reach sufficient light to sustain them further. It will bolt skywards given half a reason to. An indoor room give the same light level reading on a camera as the jungle floor. Pinch the top bud out as suggested. You could leave it outside during the day. As your new to this, set an alarm clock to ring in the early evening, so that you remember to bring it back inside, so as to avoid any frost damage. Avocados are cheap, so experiment. --Aspro (talk) 10:27, 13 April 2010 (UTC)

You don't need to lop off the entire top of the plant, all you need to do is eliminate the terminal buds (buds at the top/end of the seeding) by "pinching" them off. Let's say your avocado seedling is now a meter tall and growing straight up without branching. If you cut off the group of buds at the top of the seedling (at the end of the growing season) it will stop growing straight up next year . Instead the seeding will start sending branches off to the sides from a set of buds lower down the seeding (usually next to the leaves). These side branches will grow diagonally upwards the next season. When you think these side branches have grown high enough (2-3 meters?), you can then pinch or cut off the terminal buds of these branches. This process goes on and on and on until you have the tree with the high and form you want. The article "Pruning" may help. Sjschen (talk) 18:45, 13 April 2010 (UTC)

marine animals that have evolved from land animals.

Sir,Madam. I was thinking about the structure and movement of various marine animals, and got to wondering, is the upwards and downwards movement of the tail section of certain species whales and dolphins for instance, indicative of species that have returned from the land to the sea. bearing in mind the propensity of fish to move their tail sections from left to right. Yours Truly John Walshe.

I don't think that's a hard-and-fast rule, as there are marine animals not evolved from land animals which use the up-and-down motion, too, such as rays. There are also land animals, like snakes, which use a side-to-side motion. However, the up-and-down motion does facilitate sticking ones head (or just blow-hole) out of the water to breath, and only marine mammals need to do that, so there is some link. StuRat (talk) 11:06, 13 April 2010 (UTC)

I suppose the up-and-down motion of walking on land translates into up-and-down swimming in sea; the reason marine snakes, lizards and newts swim side-to-side like eels would then be that they crawl partly side-to-side on land as well. What about the ichthyosaurs, does anyone know how they swam? --91.148.159.4 (talk) 15:22, 13 April 2010 (UTC)

Considering that their tail fins were vertically oriented, I'd say it's much more likely that they used the fish style of side-to-side motion. Matt Deres (talk) 16:50, 13 April 2010 (UTC)

True. Well, I suppose this can be connected to the fact that they evolved not from dinosaurs (dinosaurs had developed the capacity to walk up-and-down pretty much like modern land mammals do), but from more archaic reptiles that presumably still crawled with partial side-to-side movement like lizards. Thus, one may say that while the implication "terrestrial origin -> walking -> up-and-down swimming" does exist, it only holds in those lineages where "proper" up-and-down walking has already been developed in the first place, and this took quite some time to develop in terrestrial animals. Penguins are another telling example - unlike ichthyosaurs, they do not originate from crawling reptiles but, ultimately, from walking ones (specifically from dinosaurs), so their swimming is primarily up-and-down oriented, as in marine mammals.--91.148.159.4 (talk) 18:42, 13 April 2010 (UTC)

Shrimp

The horizontally oriented tails of marine mammals developed from side-by-side hind legs, so there's a connection there. Fish on the other hand developed without any tie to what anatomy worked on land. Rckrone (talk) 17:46, 13 April 2010 (UTC)

Actually, looks like I'm wrong with regard to cetaceans and manatees whose tail fins are just tails. So scratch that. However, the Marine_mammal#Adaptations section touches on what 91.148.159.4 said about swimming and walking motions. Rckrone (talk) 17:58, 13 April 2010 (UTC)

Bear in mind that most "flat" fish evolved from bottom-feeding fish that habitually lay on their sides and whose eyes and various other body parts migrated to make that a permanent thing. Looked at like that, they are flapping their tails "from side to side" - but they are swimming on their sides so it looks like they are flapping up and down. SteveBaker (talk) 18:05, 13 April 2010 (UTC)

Dawkins addressed this very point in "Greatest Show on Earth" and he agrees with the premise suggested by the OP. it is the orientation of the mammalian spine that gives the up and down "galloping" motion in aquatic mammals. I think rays do not flex their spine up and down to swim, they use their laterals. And steve above is also correct. Also land snakes and lizards didn't evolve a mammalian spine so I don't see why the fact they walk side to side has anything to do with aquatic mammals. Another extremely interesting thing about land/sea migration is that it looks like some turtles/tortoises migrated from the sea to the land not just once but twice! Sea to land, back to sea and to land again! Including the ancestral fish to amphibian migration, that means they've been from sea to land three times! Vespine (talk) 22:29, 13 April 2010 (UTC)

Well the spine and the way of walking change together, one is a means for the other. But I don't think it's only a mammalian thing; as I said, I think that there are animals other than mammals that have adapted to swim, and that swim not laterally like fish, but mostly up-and-down. Penguins evolved from birds, which walk on land, and (swimming) shrimp evolved, perhaps, from bottom-walking crustaceans - although admittedly both seem to use their limbs more than fish and cetaceans usually do.--91.148.159.4 (talk) 22:57, 13 April 2010 (UTC)

Well that's exactly the point, penguins evolved to walk on land too but do not use their spine for swimming so they are a bit of an "inbetween" and shrimp don't have spines so they don't really count. I do agree with you that a spine which bends vertically is not THE definitive indicator of mammalian origins in an aquatic animal, but it is one of the very strong indicators. Along with things like lungs and bone structure. Vespine (talk) 00:20, 14 April 2010 (UTC)

Sure, no doubt about mammalian origins. The OP was about a connection between tail movement orientation and terrestrial origins in general, not only mammalian origins, so I was trying to say that the connection between walking and up-and-down swimming in a broad sense may hold not only in mammals. I'm conjecturing that dinosaurs' and birds' spines have also become sufficiently re-oriented to make vertical oscillation easier than lateral one. Of course, I could be wrong - I don't understand the underlying anatomy well enough, and I don't really have examples: it's true that penguins don't seem to rotate their bodies for swimming very much anyway, so it may have been too far-fetched of me to draw them into the picture. On the other hand, there may be some examples with swimmers that have a background as sea-bottom walkers: for example lobsters swim by flipping their tails up and down more or less like marine mammals (source in hidden text). Shrimp and crayfish use limb-like structures, but they also flip their tails like lobsters ((source in hidden text)). The flat ends of their tails are also horizontally flat, as in marine mammals. --91.148.159.4 (talk) 14:25, 14 April 2010 (UTC)

Primaeval woodland in the UK

Is there are woodland or forest left in the UK that is truely wild and has never ever been planted or cultivated? Even ancient woodlands such as the New Forest were planted centuries ago. Thanks 78.147.232.11 (talk) 12:25, 13 April 2010 (UTC)

According to episode three Man Made Britain of Britain From Above - which I've just (20 minutes ago) watched - the answer is pretty much no. Mitch Ames (talk) 13:51, 13 April 2010 (UTC)

Depends what you mean by "never ever been planted or cultivated". The wood at the bottom of my garden (Brandon Wood, Warwickshire) is an ancient woodland, mentioned as such in the Doomsday Book. But no tree in there seems to be more than about 150 years old. This could be because the trees have been harvested over the ages, and that might mean the woodland has been "cultivated". Also a woodland which contains pollards - that has been "cultivated", but it could date from "time immemorial". I beg to differ with Mitch, however, in that the Caledonian forest seems to be a remnant of the post-glacial coniferous forest which covered the British Isles after the last Ice Age. --TammyMoet (talk) 15:22, 13 April 2010 (UTC)

Caledonian Forest 89.240.34.241 (talk) 22:34, 13 April 2010 (UTC)

In a case like this, I wonder how great a role something like the Little Ice Age can play in skewing the results. I'm not a climatologist, but it seems to me that you'd have a hard time finding a section of forest that had existed continually for, say, a thousand years when you have temperature changes that great, lasting for so long. Matt Deres (talk) 16:44, 13 April 2010 (UTC)

300 years is a blink of an eye when talking about great trees such as oaks! --TammyMoet (talk) 19:07, 13 April 2010 (UTC)

Not if it gets so cold that it causes the tree to die or even if the climate shift is enough to curtail the growth of saplings in favour of more cold-hardy species. More philosophically, there's the possibility of a variation of the "Ship of Theseus" - is it still the same forest if the oaks survive, but the beeches and poplars are replaced with, say, birches and evergreens? Matt Deres (talk) 20:20, 13 April 2010 (UTC)

I've seen numerous oaks younger than that develop rot to the center and basically die of old age. Edison (talk) 16:14, 14 April 2010 (UTC)

What I should have said was, is there any land in the UK that has never been cultivated, managed, or planted? Unless its bare rock, its probably going to be a forest, climax community and all that. I understand that Britain would naturally be covered in wild forest but at various times all of it (or nearly all?) has been cultivated or cleared by man. 89.240.34.241 (talk) 21:32, 13 April 2010 (UTC)

Places such as Rothiemurchus, Abernethy and Glen Affric are remnants of the Caledonian Forest of which according to our article only 1% remains (but that 1% should fit your requirement).213.160.108.26 (talk) 22:25, 14 April 2010 (UTC)

The previous answers are correct, there is even much debate as to whether or not the UK was ever even entirely forested. Before we arrived on the scene there would have been plenty of herbivores (like elephants) that would have knocked down trees and probably maintained grassland/savannah. The most ancient woodland left is probably Hatfield Forest (go there now and there are no planes flying overhead into Stansted Airport!), Westonbirt Arboretum apparently contains a ring of genetically identical trees that are thousands of years old and may be a relic of the primaeval woodland you refer to. 131.111.30.21 (talk) 10:36, 17 April 2010 (UTC)

I had imagined that the elephants and hippos and so on would have been before the various ice ages, when the glaciers covered most or all of Britain. Presumably after the last ice age, Britain was covered in forest, and people eventually migrated here. I find it surprising that all of Britain has been cleared by humans - there were not many humans in past ages, and they must have worked really hard. I presume they used slash-and-burn farming techniques that I think are still used in the Amazon area. 78.146.229.142 (talk) 12:02, 17 April 2010 (UTC)

Gravity, friction and mass.

Would you please explain what is conflict in my logic based on my understanding of the laws of physics between statement number 1 and statement number 2. I suspect my knowledge of physics is flawed and I would welcome guidance.

Statement 1.

Two bodies(say a sphere) of identical size and shape are dropped at the same time from the same height. One sphere is filled with lead and the other sphere is filled with feathers. Both spheres will hit the ground at the same time.

Statement 2.

The same two spheres are each placed on identical skateboards and set off at the same time down an identical slope. The sphere filled with lead will pass the sphere filled with feathers.

If gravity is a constant accelerating force what is the explanation for the outcome of statement number 2 ?

Thank you.

86.12.198.101 (talk) 14:44, 13 April 2010 (UTC)

That's so easy. The coefficient of friction is not the same for every force applied. This is obvious if you think about what happens when you try to push with your pinkie a balloon versus a lead ballast of the same shape. In an "idealized" environment (where the physicist assumes zero friction) you can push each of them just as easily. Or maybe not, since they have inertia. What do I know, I'm just a useless hack. 84.153.214.140 (talk) 14:58, 13 April 2010 (UTC)

Statement 1 is true in an idealized setting without air resistance or other friction. Statement 2 is false in this setting (with no friction, the skateboard will behave like a sled - but see below).

Statement 2 is true in most realistic settings with friction.

However, there is an idealized setting in which both are true: No air resistance and rolling resistance, but perfect friction between skateboard wheels and slope. In this case, the potential energy of the system (proportional to the mass of the skateboard and the sphere) is converted into kinetic energy (corresponding to forward motion and proportional to the mass of the system) and rotational energy of the skateboard wheels (which is independent of the weight of the system). In the heavier system you need a smaller proportion of energy to make the wheels spin, so a larger part is kinetic energy, hence it goes faster. --Stephan Schulz (talk) 15:05, 13 April 2010 (UTC)

(edit conflict) Statement 1 is only true in vacuum. Otherwise air resistance will have an effect. For substances with such different densities as lead and feathers, air resistance will be a significant factor. In Statement 2 air resistance will have an even bigger effect, as the effective force acting on each skateboard and sphere parallel to the slope is less than its weight; it is weight x sin(θ) where θ is the angle between the slope and the horizontal. Gandalf61 (talk) 15:08, 13 April 2010 (UTC)

The problem here is that the two parts of the question make different kinds of assumptions. In effect, we have a spherical cow and a goat-shaped goat. In the first case, the questioner has seen fit to ignore the effects of air resistance - which would otherwise result in the lead ball hitting the ground before the feather-filled ball. But in the second case, friction with the track and between the axles and the wheels has not been ignored, because if it had then both skateboards would arrive at the end of the track at the same time. So this is really an issue of missing/unstated assumptions. If we were told (for example) that these two experiments were being carried out on the moon (no air resistance - but plenty of friction) then we might reasonably understand what the questioner had in mind...but without that context, we don't know what we're supposed to ignore and what we required to take into account.

The question is greatly confused by the fact that we're specifically told that the feathers are packed into a ball the exact size and shape of the lead ball. If we're supposed to be ignoring air resistance then why bother saying that? The usual assumption in this kind of question is that if such details are stated, then they are relevant. SteveBaker (talk) 17:48, 13 April 2010 (UTC)

Not quite, see my comment above. There is a reasonable abstraction (no friction, no air-resistance, but perfect no-slip wheels of finite moment of inertia) where you can have this result. --Stephan Schulz (talk) 18:13, 13 April 2010 (UTC)

Its like rolling two cylinders down an inclined plane, only this time it's the skateboard wheels that are absorbing the energy. The same thing would happen on the moon, so air resistance isn’t the prime factor.--Aspro (talk) 18:22, 13 April 2010 (UTC)

... and if you take the spheres off the skateboards and just roll them down the slope, then the lead sphere will still win (assuming the spherical shell is more dense than the feathers). There was a similar question on an Oxford Entrance examination in the 1960s. If you want the feathers to win, then you have to put less lead inside the other sphere and suspend it at the centre with light spokes. Dbfirs 21:12, 13 April 2010 (UTC)

Oxford Entrance examination of the 1960s??? Are these practical experiments preformed any more? I have read, just this month, that the modern school system only trains pupils to pass exams, instead of providing an education. There are many questions asked 'here' which seem to confirm that education (and the insight that it can provide) is a thing of the past. Can we get these school fees reimbursed? They have not delivered the goods.--Aspro (talk) 21:59, 13 April 2010 (UTC)

BTW: Just to be fully correct, the lead ball will hit the ground (slightly) before the feather one. It is not true that everything falls at the same speed, but rather it is approximately true. On earth the difference is not measurable, but it is there. To the OP: do not let this confuse you - most of the time the approximation is correct and should be used. Ariel. (talk) 02:40, 14 April 2010 (UTC)

I think I know why you think think that, and if yes, I had a major discussion with my high-school teacher about it. Both bodies are attracted towards the common center of mass, and the heaver ball makes the Earth move faster...right? Or wrong? --Stephan Schulz (talk) 07:40, 14 April 2010 (UTC)

Well, let's be a little careful about that. He who lives by the nit-pick must also die by the nit-pick! We know that $F=G{\frac {m_{1}m_{2}}{r^{2}}}$ - the force exerted on each body (both the ball and the earth) is proportional to the product of the two masses. However, the acceleration due to that force is given by A = F / m ...and the acceleration is what we care about here. So the acceleration of the ball depends only on the mass of the earth because its own mass cancels out...and the same is true of the acceleration of the planet...it depends only on the mass of the ball. So yes, the earth does move by the most microscopic amount imaginable in the direction of the ball due to the balls' own (insanely puny) gravitational field. If you drop the lead ball, it'll hit the ground a vanishingly small amount of time sooner than if you had dropped the feathery ball. But (and this is the point where your nit-pick fails!) if you drop both balls at the same time (as this experiment stipulates) - then even at this microscopic level of nit-picking, they still land together since the earth is being accelerated simultaneously by both balls. (Unless you drop the balls on opposite sides of the planet or something - but then our uber-nit-picker would invoke relativity and we'd have to ask what "simultaneous release" means for widely separated objects moving in different directions.) The balls themselves both experience the exact same accelerations. But in any case, the numbers involved are vastly too small to matter and I strongly urge our OP to studiously ignore this part of the thread because it's very, very, silly! SteveBaker (talk) 18:42, 14 April 2010 (UTC)

Homosexuality and Pedophilia

Hello, I don't want to do politics, so please don't do either, I was just wondering if there are any studies inverstigating links or relations between homosexuality and pedophilia. Again, no polemics, no political (in)correctness, just studies, please. Thank you very much. --Lorem ipsum dolor sit amet (talk) 18:00, 13 April 2010 (UTC)

Yes. This review article from 2007 should contain some relevant references. --NorwegianBlue ^talk 18:48, 13 April 2010 (UTC)

To clarify, this link is about childhood victims of homosexual paedophilia becoming homosexuals as adults, not about homosexuals being likely to commit paedophiliac acts (I'm clarifying, because there were some recent questions about the latter in connection to a statement by some papal spokesman).--91.148.159.4 (talk) 19:01, 13 April 2010 (UTC)

Not a study but I shall contribute a logical argument: how could homosexuality be any more related to pedophilia than various fetishes, such as shoe fetish, or even heterosexuality? Pedophilia itself is a sexual orientation, just like verious fetishes.--92.251.147.169 (talk) 18:51, 13 April 2010 (UTC)

See yesterday's thread at Wikipedia:Reference Desk/Humanities#Homosexuality = Paedophilia? Studies?. Comet Tuttle (talk) 20:23, 13 April 2010 (UTC)

Is pedophilia normal?--79.76.163.157 (talk) 23:18, 13 April 2010 (UTC)

What do you think? Look up the definition of "normal" and try to fit paedophilia into any of those definitions. I'll be surprised if you are left with any ambiguity after this exercise. Vespine (talk) 00:23, 14 April 2010 (UTC)

I detect that are implying that the answer is clearly abnormal, however it depends on the definition you take. Here are two, from WordWeb v5.0:

1. (psychology) being approximately average or within certain limits in e.g. intelligence and development - this make both homosexuality and pedophilia abnormal

2. In accordance with scientific laws - this makes both homosexuality and pedophilia normal

Abnormality says nothing about the potential harm to individuals nor what remedies (e.g. convince society that homosexuality is acceptable vs aversion therapy for homosexuals) are appropriate. An IQ of 150 is abnormal. It says nothing useful. 78.148.114.247 (talk) 00:56, 14 April 2010 (UTC)

Well, no, you're arguing semantics, I specifically said to look up the various definitions. Normal can also mean "(of a solution) containing one equivalent weight of the constituent in question in one litre of solution." but it's hardly relevant, just like "in accordance to scientific laws" is not relevant in this context either. dictionary.com normal has that definition at 4(b) of natural occurrence. google normal does not even have that definition. If the question was "is a society with paedophiles normal?" I'd say you might have an argument, but is being a paedophile normal? I think is clear cut. Vespine (talk) 01:23, 14 April 2010 (UTC)

Well if you use the definition "of natural occurrence" (is being a paedophile a/of natural occurence) then perhaps although it depends what you mean by 'natural occurrence'. If you mean innate then I think the answer is may be. As we've discussed many times before on the RD, the word natural is very fussy and it would seem difficult to argue conclusively any behaviour that occurs isn't natural. Well unless you believe in God and/or the devil in which case you could certain behaviours only come from God and/or the devil and these aren't natural but that isn't a particularly scientific answer. Nil Einne (talk) 01:58, 14 April 2010 (UTC)

Is being a homosexual normal? 78.148.114.247 (talk) 01:53, 14 April 2010 (UTC)

This is the science reference desk. We can talk about probabilities and perhaps evolutionary causes and maybe even biochemical and/or genetic causes. But words like "natural" and "normal" are not things that science can help you with. They are terms without solid meanings. We can't perform some kind of scientific test for "natural-ness" or "normality". This is a matter for society to decide. This question belongs on the "humanities" reference desk - and it's already been asked there. So unless someone can answer the actual question we've actually been asked ("Are there any studies relating these two facets of human behavior?") let's end this thread and direct readers and contributors to the correct place to discuss it. SteveBaker (talk) 02:07, 14 April 2010 (UTC)

Actually, true pedophiles (those whose primary sexual interest is prepubescent children) often don't prefer one gender over the other. It's the fact that a child IS A CHILD that attracts them. They are attracted to the way children look (i.e., the lack of adult secondary sexual characteristics is what turns them on) and think. Most pedophiles find some children more attractive than others, but what makes a particular child special may have as much to do with age and physical characteristics as with gender. Like people of more acceptable sexual orientations, a pedophile may have a "type." Gentlemen prefer blondes, women like guys who are tall/dark/handsome, and so forth - that doesn't mean that "preferring blond women" is a sexual orientation in its own right or anything other than an individual preference. An adult who chooses to act on an attraction to a person whose body is sexually mature but who is below the age of consent is another matter. Male homosexual culture, probably even more than our culture already does, idolize the beauty of youth, and as homosexuals are already existing for the most part outside of mainstream society, it may be easier for them to ignore the taboo on sexual involvement with a minor. So there may be cultural link or relation between homosexuality and sexual involvement with post-pubescent minors, but actual sexual involvement is a behavior. Attraction is involuntary, and although culture has an influence, biology is a huge factor in attraction. Are gay men more turned on by teenage boys than straight men are by teenage girls (whether either of them admit to or act on that attraction or not)? I doubt it. 71.104.119.240 (talk) 06:31, 14 April 2010 (UTC)

• Vatican Statement Regarding Comments by Card. Bertone
  • Interview with Scicluna --88.66.204.235 (talk) 17:12, 14 April 2010 (UTC)

Of course by definition, no perversions are normal. Maybe thats how we define perversions. Some perversions are legal (eg homosexuality), some are not (eg pedophilia). Other countries have different rules. Lawsin different countries are somewhat arbitrary. Morals are something defined by the religion you follow.--BandUser (talk) 00:09, 15 April 2010 (UTC)

It looks like your definition of 'perversion' is arbitrary too. And I think you'll find that morals are not solely defined by religion. AlexTiefling (talk) 14:18, 16 April 2010 (UTC)

Making my own tubular steel shelving in needed dimensions

I like this shelf but its dimensions render it inapplicable to my situation. I could buy some tubular steel or aluminium, but then what - how would I attach the ends to a plate in a way that didn't look messy? I'd like something that looks similar to the one in that picture (all-metal). I have a soldering iron and a weird little handheld butane thing I bought years ago and lost the instructions for - I remember it said something about brazing. Also, there are some putty things available where you mix two putties together and they react and harden after a period to a metal sort of bond (for fixing pipes etc). Anyone got any suggestions? --78.148.114.247 (talk) 22:18, 13 April 2010 (UTC)

You didn't link to a picture - so it's hard to guess what you have in mind. Soldering or brazing steel or aluminium tubing is a difficult and skilled process. You might want to consider using screw-fit plumbing fittings - then spray-paint it matte black or something. Take a trip to the plumbing section of your local DIY store and you'll be sure to find ready-made parts that can basically be screwed or even epoxied together to make a stiff framework - you'll even find parts that would be useful for feet and other bits and pieces. The "putty" stuff you are thinking of is called "Epoxy putty" - and that might well be all you need. If you are really itching to use the butane torch, you'd want the kind of copper plumbing fittings that have solder already applied inside the joints. That would be a much easier way to connect copper tubing - but it's not suitable (or even available) for steel or aluminium tubing. SteveBaker (talk) 23:37, 13 April 2010 (UTC)

Oh, right, thanks here is the image. I'd like it in this matt aluminium/steel finish. One possibility might be to cut off one of the sections with a multitool and then use a grinding wheel to neaten it? Most of all, I like this particularly cheap one but I can fathom no way that this would look neat after I had attacked it with a multitool. 78.148.114.247 (talk) 23:51, 13 April 2010 (UTC)

Ah - I see. Well, in the case of the one on eBay, instead of cutting off the tube that runs along the front edge of the shelf (leaving ugly, raw cut edges on the front ends of the support tubes). I would cut off the last tube at the BACK of the pipe and relocate the brackets forward one row...but everything depends on how those brackets are fixed. If they are just screwed into place - then it's easy - but if they are welded or molded on there - that wouldn't work. SteveBaker (talk) 02:22, 14 April 2010 (UTC)

Two methods:

• Buy the tubes, and flat metal for the sides, then connect them by buying long threaded dowels. Drill holes in the flat metal, put the dowels through the tubes and metal, then attach a nut on each side and tighten. Then use your multitool to trim the dowel right down to the nut. For extra reliability buy loctite threadlocker, use blue if you plan to take it apart or red if you want it permanent.

• Get short wooden dowels exactly the size of the inside of the pipe. If necessary make them using a hole saw. About 2 inches long. Glue them to the side metal, then glue the pipes onto them. For extra strength drill a hole in the metal and put a wood screw (sheet metal actually - you want a flat bottom on the head, not a cone like a typical wood screw) through the metal and into the wood.

Ariel. (talk) 02:31, 14 April 2010 (UTC)