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April 16[edit]

Please Explain Why This (Presumable) Crank Is Wrong...[edit]

Hi. Whilst randomly clicking through YouTube, I discovered this video. I didn't watch very much of it, but did watch enough to see the discussion between 0'25" and 3'00". In essence, the presenter (who bears all the hallmarks of being a crank) claims that energy is expended in maintaining an object at the same height in a gravitational field. For example, he claims that if a person holds up a weight at a constant height, so as to maintain a constant potential energy, the person is expending chemical energy through his muscles to keep it held at that height. Similarly, he observes that a helicopter uses more fuel to maintain a constant height when it is carrying a load than when it is empty. That is to say, energy is required simply to maintain an object at a constant height in a gravitational field. I believe he's wrong, but can't work out what his mistake is. I assume that there's some simple misconception, but I'm not smart enough to work out what it is, so I'd appreciate an explanation from a bona fide physicist, if there are any here. (In the interests of my understanding, and with all due deference to other reference desk editors, I'd appreciate it if the "usual suspects" (you know who you are) resist giving their opinions and speculations on this unless they are actually physicists.) Thank you. RomanSpa (talk) 00:14, 16 April 2015 (UTC)[reply]

You do notice that while his arm is holding the cannon ball, there's nothing but air between the cannon ball and the ground. The stick, on the other hand, lies between the ball and the ground. If he stood still and balanced the cannon ball on his head, he'd have a similar situation. I make no statement about the rest of it, except to note that he's NOT set up identical situations; he's set up very different situations: One is a stick resting in mid-air over the ground (i.e. his arm = a stick) and the other is a stick resting directly on the ground (i.e. the little wooden pillar he takes out). I'm pretty sure that mistake renders the rest of his explanation null and void. and for the record, I'm a bona fide chemist, but I've had enough physics classes along the way to at least recognize that an arm with nothing between itself and the ground is not identical to a pillar resting right on the ground --Jayron 32 01:04, 16 April 2015 (UTC)[reply]

He makes one slightly interesting observation, that something must be compressing the stick - the energy to do so is exactly equal to the loss of PE of the canonball as it compresses. Otherwise he is committing a logical fallacy that in late latin would be 'the specific doesn't prove the general' Hasty_generalization. Just because his two main examples burn fuel to hold the cannonball up, he can't generalise it to say that it is a necessary condition. Greglocock (talk) 01:42, 16 April 2015 (UTC)[reply]

It doesn't matter whether there's air directly beneath the cannonball. If you replaced the guy with a plaster cast of himself in the same posture, it would suspend the cannonball indefinitely without energy expenditure. If he balanced the cannonball on his head, he would still have to expend energy standing up, probably more than if the cannonball weren't there. The difference between a statue and a human being is that our muscles don't have a locking mechanism. It has nothing to do with shape. -- BenRG (talk) 03:13, 16 April 2015 (UTC)[reply]

The confusion permeating the first 3-minutes of the presentation is not understanding the concepts of force, work (physics), and energy correctly. Consider the toy-helicopter holding up the ball example:

Due to the gravitational force acting on the ball, it would accelerate towards the Earth, if there were no counteracting force (Newton's second law of motion).
The helicopter produces this opposing force on the ball, and as long as this force exactly counteracts the force of gravity, the ball's height does not change
The way the helicopter produces this force is by "moving air around"™ (the exact details do not matter for our purpose).
In order to move air around it needs to burn fuel, since moving air around, by definition, increases the air's kinetic energy. So in essence the helicopter converts chemical energy to kinetic energy of the surrounding air.
However–and this is the important point– the helicopter does not do any work on the ball, which consequently neither gains, nor loses potential energy.

A stick produces the force that holds up the ball by a different mechanism, which does not involve moving air, and hence expending energy. So steps 3-4 in the above explanation are different, but steps 1, 2 and 5 remain essentially the same. Ditto for using your arm to hold up the ball at shoulder height; in that case chemical energy is expended to keep the appropriate muscles contracted and is converted to (mainly) heat, but again no work is done on the ball.

Btw, you can't go wrong reading Feynman's lectures on work and potential energy. I haven't checked to see if he addresses misconceptions like the one in the video; but if he does, just ignore my explanation. Abecedare (talk) 02:23, 16 April 2015 (UTC)[reply]

This is really an issue of human physiology. There's nothing to say about it from the perspective of physics, except that you can make any process less efficient by producing extra heat. Our muscles are inefficient: they emit waste heat that tables don't when doing the same job as a table. If you put the guy+cannonball in a calorimeter, you could verify that the heat he emits is equal to the food energy he consumes, leaving no excess energy to disappear into the cannonball. -- BenRG (talk) 03:13, 16 April 2015 (UTC)[reply]

There's no obligation to disprove a crank who makes arbitrary assertions: "In a sense, therefore, the arbitrary is even worse than the false. The false at least has a relation (albeit a negative one) to reality; it has reached the field of human cognition, although it represents an error—but in that sense it is closer to reality than the brazenly arbitrary." source Aristotle calls the same using words as sounds with no corresponding meaning. μηδείς (talk) 03:10, 17 April 2015 (UTC)[reply]

Sigh. I wasn't asking that the crank be disproven, I was asking to have his misconception explained to me by an actual physicist, because I didn't understand the subject sufficiently well to work out what the error was. I'm still not entirely sure that I understand it, though Abecedare's response at least made clear that I need to think in terms of force as well as energy. I'd hoped to restrict respondents to physicists only, because I know what the Reference Desk is like, but despite my specific request for this that appears to have been a forlorn hope.

Just out of curiosity, were any of the respondents physicists? Thanks. RomanSpa (talk) 21:54, 17 April 2015 (UTC)[reply]

BenRG is. --Jayron 32 21:56, 17 April 2015 (UTC)[reply]

Well, you don't really get to say who can post here, though of course you are free to make requests. A reall physicist might flub something occaisonally, and a non-physicist might get it right. This is a reference desk, not an "expert desk". BTW, I thought I might be able to explain the misconception to you, but I refrained, as you asked non-physicists not to reply ;) SemanticMantis (talk) 18:10, 18 April 2015 (UTC)[reply]

Sorry, I am not a physicist. BTW, "sigh"ing in public is just about as polite as announcing you have farted. μηδείς (talk) 20:25, 18 April 2015 (UTC)[reply]

Plastic noodles test[edit]

According to this video here, a man claims that some of the noodles in his bag of noodles are plastic. His test is to burn them and he notes that plastic doesn't burn, it melts while real noodles catch on fire and burn. Is this a good test to see if his noodles are made of plastic or not? ScienceApe (talk) 01:38, 16 April 2015 (UTC)[reply]

I buy Mad Magazine brand "No more than five percent plastic!" brand noodles all the time, and burn them regularly after cooking, so as to char the edible ones and melt the inedible ones. I've not yet eaten a plastic noodle! Why would this strike you as odd? μηδείς (talk) 02:15, 16 April 2015 (UTC)[reply]

All plastics burn. Thermoset plastics smoke and then burn, while thermoplastics melt and then burn. However, pasta should never melt when exposed to heat. So, if the noodles melt, then I'd tend to believe they were (thermo)plastic, yes. But, thermoset plastic noodles might behave much like pasta, when exposed to fire. StuRat (talk) 02:22, 16 April 2015 (UTC)[reply]

Wikipedia Science Reference Desk RULE #1: All so-called "science" videos on YouTube are rigged. YouTube science videos are anti-facts...if a YouTube science video says that something is true - you can be 99% certain that it's false. Honestly - It's so childishly easy to fake a video - and so many YouTube posters are more interested in people watching their videos than they are about truth. They aren't even worth the effort to debunk. SteveBaker (talk) 06:50, 16 April 2015 (UTC)[reply]

If [1] is to be believed, this was actually real. The company says it was a disgruntled employee, and that plastic would cost more than real noodles (I suspect this is mostly true, although it would likely depend on the source of the plastic, if it's something no one wants that may be a different story, but of course it may be difficult to find scrap plastic that you can make look anything like noodles). Nil Einne (talk) 13:24, 16 April 2015 (UTC)[reply]

My ex, when she buys discount cloth, burns a little sample. If it's natural fiber (or rayon), it turns to ash; if it's polyester or the like, it melts to a hard blob. —Tamfang (talk) 07:43, 17 April 2015 (UTC)[reply]

Soreness[edit]

Two days after giving a natural birth, where is a woman most likely to feel sore? Vagina or belly?World bymyself (talk) 20:27, 16 April 2015 (UTC)[reply]

I googled "post childbirth pain" and got plenty of hits. Here's one:[2] ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:41, 16 April 2015 (UTC)[reply]

The lower back. Kind of like the delayed muscle ache caused by Viagra. μηδείς (talk) 01:33, 17 April 2015 (UTC)[reply]

(I haven't given labor, I am going on my sister's rather quick labors, the last being under two hours, with no epidural or episiotomy. This quick labor ended up causing complications for the baby, since a certain amount of straining clears the child's lungs of amniotic fluid. μηδείς (talk) 17:28, 17 April 2015 (UTC))[reply]