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March 28

Physics collision/momentum problem

Resolved

– Evidently Mastering Physics can be picky at times. Ks0stm ^{(T•C•G•E)} 04:00, 28 March 2012 (UTC)[reply]

I am having issues with a physics homework problem. The problem is: "Olaf is standing on a sheet of ice that covers the football stadium parking lot in Buffalo, New York; there is negligible friction between his feet and the ice. A friend throws Olaf a ball of mass 0.400 kg that is traveling horizontally at 10.1 m/s. Olaf's mass is 74.4 kg. A) If Olaf catches the ball, with what speed do Olaf and the ball move afterward? B) If the ball hits Olaf and bounces off his chest horizontally at 7.30 m/s in the opposite direction, what is his speed after the collision?"
My work for problem A is:
$.4(10.1) + 74.4(0) = V(74.4 + .4)$
$4.04 = V(74.8)$
$0.05 = V$
My work for problem B is:
$.4(10.1) + 74.4(0) = .4(-7.3) + V(74.4)$
$4.04 = -2.92 + V(74.4)$
$6.96 = V(74.4)$
$0.09 = V$
It says that both of my answers are incorrect, but I can't figure out where I'm going wrong. Any suggestions? Ks0stm ^{(T•C•G•E)} 23:44, 27 March 2012 (UTC)[reply]

Your method looks right to me (although I haven't checked your arithmetic). Do you have the "correct" solutions? Perhaps we can work backwards from those and figure out what is going on. --Tango (talk) 00:40, 28 March 2012 (UTC)[reply]

Maybe you have too few significant figures in your answers. Plasmic Physics (talk) 00:41, 28 March 2012 (UTC)[reply]

That would be my guess, the problem carefully states all the numbers with three sig-figs, but KsOstm's answers only have one. APL (talk) 02:06, 28 March 2012 (UTC)[reply]

Agreed. After the final division steps, more decimal places should be retained. StuRat (talk) 03:36, 28 March 2012 (UTC)[reply]

I would say that's the problem except that Mastering Physics (which I'm doing the homework problems on) tells you if your answer is wrong because of significant figures (it gives you a message saying that your answer is close but perhaps included a rounding or significant figure error). Ks0stm ^{(T•C•G•E)} 03:38, 28 March 2012 (UTC)[reply]

In that case, do you need to include the units in your answer ? StuRat (talk) 03:41, 28 March 2012 (UTC)[reply]

It gives you the units to answer in...in this case both problems say "Express your answer numerically in meters per second" and have "m/s" after the text box where you put your answer. Ks0stm ^{(T•C•G•E)} 03:43, 28 March 2012 (UTC)[reply]

Well, we're back to significant digits, then. It may say it checks for that, but maybe they turned that feature off. If that's not it, we're down to it wanting/not wanting a leading zero, wanting it in scientific/engineering notation, or maybe you're typing the letter O instead of a zero ? Or, if it's set in European standards, perhaps it wants a comma instead of a decimal point. We're grasping at straws here, but you might as well try all the possibilities.

Also, did they specify no air resistance and that no energy is dissipated as heat from the collision ? I'd assume so, unless they gave some figure for how much to figure in. Then there's the possibility of them wanting a negative sign, but, since they specified speed instead of velocity, that should always be positive. StuRat (talk) 03:49, 28 March 2012 (UTC)[reply]

Well oddly enough it was a combination of significant figures and scientific notation, apparently. I typed in .0540 and .0935 and it changed them to scientific notation and said they were correct. Evidently something about the fact that it wanted it in scientific notation makes it pickier on significant figures; normally it doesn't care how many significant figures you give so long as your answer is within a certain range of the correct answer, even on problems that specify to answer to a certain number of significant figures. All in all it accepted my answers finally, so we're all good. Thanks for the suggestion. Ks0stm ^{(T•C•G•E)} 04:00, 28 March 2012 (UTC)[reply]

Glad we could help ! StuRat (talk) 04:13, 28 March 2012 (UTC)[reply]

Resolved

Breathing hydrogen

I strongly urge that no one attempt this, since a spark could produce a lethal explosion, and even breathing even a nonexplosive gas could cause death from oxygen deprivation, but I understand that breathing helium or a helium and oxygen mixture causes the human voice to be distorted and to sound higher pitched. I wonder if breathing hydrogen or an (extremely explosive) hydrogen and oxygen mixture would similarly cause the human voice to sound high pitched? (If this question seems inappropriate for the science desk I do not object to its removal to the talk page for determination of its suitability.) Edison (talk) 05:48, 28 March 2012 (UTC)[reply]

I would say yes, it should raise the pitch of your voice, probably even more than helium. StuRat (talk) 05:53, 28 March 2012 (UTC)[reply]

See Breathing gas#Hydrogen and Hydrox (breathing gas). SkyMachine ⁽⁺⁺⁾ 05:57, 28 March 2012 (UTC)[reply]

Yes. As a spherical cow approximation, a human vocal tract is roughly a cylinder. The resonant frequency of a cylinder is proportional to the speed of sound, and at standard temperature and pressure the speed of sound in hydrogen is about 35% faster than the speed of sound in helium. So ignoring the realistic complications with the physiology of the vocal folds and what-not, I would expect the frequencies involved when breathing pure hydrogen to be roughly 35% higher than the frequencies when breathing pure helium. Red Act (talk) 06:42, 28 March 2012 (UTC)[reply]

Well, keep in mind that what the gas affects is the properties of the resonating chamber, not the driving frequency. The driving frequency is mainly determined by the larynx. So it's not so much that the frequencies per se are higher; it's that the higher harmonics are stronger. Timbre rather than pitch. --Trovatore (talk) 06:52, 28 March 2012 (UTC)[reply]

Thanks for the quick and informative responses. I've tried putting speech through an audio filter, and it can be made to sound tinny, but I never noticed it sounding like helium speech. Helium speech sounds like the fundamental is higher, not just like the low frequency tones are missing, although I have certainly read this explanation in other sources. In a famous tape recording Mercury astronaut Scott Carpenter tried to call President Johnson from a sea lab while breathing a helium mixture, and the White House operator refused to put the planned call through, because she said the speech was unintelligible. If it was just a frequency balance issue, the 1960's NASA techs should have been able to compensate for the frequency imbalance and make the speech sound more normal. Active filters existed even then. The Scott Carpenter speech sounds almost like single sideband radio transmission picked up on a cheap shortwave which can't properly decode it, like 30 seconds into this recording of ham radio. Edison (talk) 23:01, 28 March 2012 (UTC)[reply]

Black body radiation

If we assume a possibility of heat trapping as in green house effect, what would be the maximum measured surface temperature before the body is forced to equilibrium again? I mean can we apply some thermodynamics laws in order to achieve a rough limit?--Almuhammedi (talk) 11:36, 28 March 2012 (UTC)[reply]

Assumming that we are talking about an abstract greenhouse, then there is no physical limit. Given an external heat source, you can keep increasing the thickness of the infrared trapping layers to get an arbitrarily large warming at the surface. In the limit of a thick greenhouse the response function must be logarithmic, so there is a practical issue of diminishing returns. In other words doubling the thickness will only get you a fixed temperature increment, e.g. if the initial concentration gets you 5 C of warming, then to get a 50 C warming you need 2^10 = 1024 times as much thermal insulation as the first layer. Dragons flight (talk) 17:27, 28 March 2012 (UTC)[reply]

Note that Venus#Atmosphere_and_climate gives us some idea of what the upper limits might be. StuRat (talk) 18:25, 28 March 2012 (UTC)[reply]

Expansion of the universe

Hello everyone. After the Big Bang, did the universe expand at the same speed in all directions, or did it expand faster in some directions than in others? Thanks. Leptictidium (mt) 11:58, 28 March 2012 (UTC)[reply]

In the earliest phases, the Big Bang was "homogeneous". At some points in the timeline of the Big Bang there were spontaneous symmetry breaking events which created distinct locations (particles) within the universe. At this point, there would have necessarily been differences in different directions, insofar as one could point at a location and say "there's a particle there" but could also say "there isn't one there". Most of this happened, IIRC, in the inflationary epoch. We believe, for example, that the early universe was homogeneous because the Cosmic microwave background radiation is homogeneous. That is, it isn't more concentrated in any direction. Since we have a universe today which is inhomogeneous (i.e., clumpy, there's stuff in some places, and no stuff in other places) there had to be some time when the universe transitioned from a homogeneous place (as it had been since it was a singularity) to a clumpy one. This is one of the great unsolved questions in cosmology/cosmogony; how and why the universe came to have such a structure as it has today. One thing to clear up, also, is that the Big Bang itself creates the concept of direction. There is no "center of the Universe" in the sense that we can't point to a location in the sky and say "That's where the Big Bang happened". This isn't a technological limitation (that is, it isn't just that such a center exists, but we can't find it), this is fundemental to the Big Bang: since it creates not only matter and energy but time and space itself, everywhere is the center. The heuristic model which kinda works to explain this is to imagine a balloon on which you draw dots all over, then inflate. If you put yourself on any one dot, it always looks like all other dots move away from you, but none of the dots is in a privileged position: every dot is moving away from every other dot equally. This is a model in three dimensions which works like the universe does in 4D spacetime: there is no privileged location in the universe which isn't in expansion away from the other locations, so there isn't any place which is the "center". Thus, there is no direction "away from" the Big Bang to decide if the universe moved "faster" in that direction. --Jayron32 12:23, 28 March 2012 (UTC)[reply]

If all dots are moving away from each other equally, wouldn't this necessarily imply that the Universe is spherical? Leptictidium (mt) 12:33, 28 March 2012 (UTC)[reply]

That rationale is applying too much emphasis to the non-rigorous use of the word "equally" within an analogy. If it helps, instead read the phrase as "each dot sees every other dot moving away from it", as evidenced by Edwin Hubble's observations of galactic redshift. — Lomn 13:15, 28 March 2012 (UTC)[reply]

Indeed, the point isn't that the universe is exactly spherical, which would imply a location at the center of the sphere which provides a privileged point. That location wouldn't be moving, which would provide a privileged location. You may find Shape of the Universe an interesting article to read. --Jayron32 14:55, 28 March 2012 (UTC)[reply]

The universe is expanding faster in all directions - see Accelerating universe. Blakk and ekka 16:39, 28 March 2012 (UTC)[reply]

We can infer that quantum fluctuations occurred just prior to the inflationary epoch which resulted in large scale structures of the universe and polarization of the cosmic microwave background radiation. Those features suggest that inflation was isotropic. However, most cosmologists will admit when pressed that the actual evidence for anything involving facts prior to mid-reionization (i.e., redshift z > 10) is extraordinarily flimsy and such inferences would be relegated to mere conjecture for the same quality of evidence in, e.g., biology or chemistry. 70.59.24.75 (talk) 20:53, 28 March 2012 (UTC)[reply]

Washing hands / skin contact with diesel and kerosene

Many people wash their greased hands with kerosene and/or diesel. Do these fuels have any carcinogenic or harmful properties?— Hamza ^{[ talk ]} 12:15, 28 March 2012 (UTC)[reply]

Both fuels may contain benzine and/or toluene, although I can't find a reliable reference to demonstrate whether they're present in sufficient concentrations, in modern fuel formulations, to be of concern. 87.114.248.222 (talk) 12:28, 28 March 2012 (UTC)[reply]

ref for diesel constituents 87.114.248.222 (talk) 12:30, 28 March 2012 (UTC)[reply]

Save for the fact that Rudolf didn't invent the compression ignition engine – they where already around in Europe. What he did was patent was the Diesel Cycle to get around existing patents by people such as Wright. In the event, the Diesel cycle, proved a complete failure in economics, reliability and poer to weight ratio, and thus never used, other than for a few years buy few marine engine companies. Something that Wikipedia has not bothered to correcting yet. Next thing I'll expect to be reading it that Mr. Steam invented the steam engine and Mr Gas the gasoline engine etc..--Aspro (talk) 15:15, 28 March 2012 (UTC)[reply]

Thank you Mr. 222. Found it. Material Safety Data Sheets for High Sulfur and Low Sulfur diesel fuels. — Hamza ^{[ talk ]} 12:57, 28 March 2012 (UTC)[reply]

The amount of benzene (and other aromatics) in gasoline (in the U.S.) is regulated to pretty low levels (0.62 volumne % for benzene) [1]. Buddy431 (talk) 17:03, 28 March 2012 (UTC)[reply]

Aside from the quantity present, the length of the exposure also matters. Since gasoline, diesel fuel, and kerosene all stink, I'd expect anyone using those as solvents to immediately wash those off afterwards. So, the time in contact with the skin is quite minimal. Is inhalation also a concern ? If so, then those crazy folks who add them to their washing machine to remove grease are also at risk from that, besides the obvious risk of fire. StuRat (talk) 18:19, 28 March 2012 (UTC)[reply]

Direct penetration through the skin is also a concern. [2] Wnt (talk) 20:24, 28 March 2012 (UTC)[reply]

infinite universe yet always finite rate of expansion

Inflation (cosmology) says that in the first fraction of a second after the Big Bang, the universe expanded by a factor of 10⁷⁸, and I assume that that is the fastest it has ever expanded. So the universe has expanded at finite rates for a finite length of time; so it seems to me that it must be finite in size. But Shape of the universe#Global geometry says Two strongly overlapping investigations within the study of global geometry are whether the universe is infinite in extent.... How can that be, with finite rates of expansion ever since the Big Bang? Duoduoduo (talk) 18:01, 28 March 2012 (UTC)[reply]

If expansion happens at faster than the speed of light, how do you find, reach, measure, get information about, or even posit the existance of the edge of the universe? From any point within the universe, the observable universe from than vantage point will never include the edge of said universe. If I'm in a place where no location within that place can ever observe an edge; well, that's a pretty good definition of a place with an infinite extent, n'est ce pas? --Jayron32 18:06, 28 March 2012 (UTC)[reply]

No, here you're confusing epistemology and ontology. The question is not whether we can ever observe an edge, but whether there are an actually infinite, for example, number of baryons, whether or not we can even in principle observe them. --Trovatore (talk) 18:12, 28 March 2012 (UTC)[reply]

Not necessarily. It all depends on what one is saying is infinite, and what the extent of infiniteness is. The question is one of the infinite, but closed universe or an infinite and open universe. As a sort of example, think of a shape like the Koch snowflake. It has a boundary of infinite length, but confines a space of finite area. In a much more complex way, one can envision geometries of the universe which are boundless in one way (such as volume) but bounded in others (such as number of baryons). Then there is the problem that some of this conjecture may be untestable at a fundemental level. There's a reason why cosmology (the very BIG) and quantum theory (the very small) attract the same sort of confusion and are studied by the same kinds of people. Philosophically, these subject matters themselves confuse ontology with epistimology. To borrow from the quantum end, ontologically quantum mechanics is very unsettling, because it doesn't say that the location of a particle is unknowable in the limit of understanding (that is, epistimologically speaking), that unknowability is built into being itself. Cosmology proposes similar problems with knowability and existance: from a scientific point of view, how do you prove the existance of something which is fundementally outside of the possibility of knowing? How do you understand the unprovable? Or, if something's ontological existance cannot be fundementally understood, not because of our own limitations, but because of its actual nature, where does that leave us epistimologically? In layman's terms: if something exists, but can never be known to exists, how is that fundementally different than not existing at all? If the universe has a geometry which the properties of the universe itself prevent anyone from knowing about, what good does it do us, as the knowers, to say that it does, or does not, have any particular geometry beyond what we can find evidence for? --Jayron32 18:31, 28 March 2012 (UTC)[reply]

You're getting into logical positivism, which is, not to put too fine a point on it, wrong. It is possible to give a consistent, philosophically realist description of an actually infinite universe, not merely logically consistent but (as far as I know) consistent with observations and known physics. --Trovatore (talk) 18:39, 28 March 2012 (UTC)[reply]

Not really, there would be an infinite number of logically consistent descriptions of a universe which is consistent with observations and known physics. All we can actually say is what the observable universe behaves like, how do you falsify the unobservable? From astronomy's own past, the deal with epicycles was that they worked and they matched observation, and matched observation fairly well. We laugh at the Ptolemaic system in hindsight, but we have the privilege of extra knowledge to allow us to do that. How would one decide between the Ptolemaic system and the Copernican system if one were physically (as opposed to technologically or socially) prevented from probing it with observation. There's nothing logically which decides between the two systems, they both match the observations and "work", so one needs better observations to decide between them. If data doesn't allow us to falsify a proposed geometry of the universe, and neither does the mathematics, and neither does logic, then what? That doesn't mean that particular geometry is real or correct, it just means the question is unanswerable. If so, why pretend to answer it authoritatively? People do, because one geometry or another matches ones own biases about how one believes the universe should work, but in the absense of any ability to probe the veracity of such a belief, on what does such authority rest? --Jayron32 19:10, 28 March 2012 (UTC)[reply]

You keep talking about how we would find out. I'm talking only about what might be, independent of our ability to find it out. A question being "unanswerable" in the sense that we can't justifiably claim to know an answer, is not the same as it not having an answer. --Trovatore (talk) 19:12, 28 March 2012 (UTC)[reply]

Why bother to say what might be, if your "might be" can't be shown to be any closer to reality than my "might be"? Why have an answer if the answer has no basis in reality? --Jayron32 19:18, 28 March 2012 (UTC)[reply]

No basis in reality, is that really what you mean? The heart of philosophical realism is that reality is there whether we can find out about it or not. --Trovatore (talk) 19:22, 28 March 2012 (UTC)[reply]

That's exactly what I mean. The idea that reality exists outside of our ability to observe it is 100% the central core of my entire thesis in this discussion. The problem isn't the existence of that reality, the problem is the insistence to claim our descriptions of unknowable (but very real) reality can be authoritative. The problem is not in reality really existing, which it does, the problem is in our answering questions with authoritative answers where such answers cannot be falsifiable, and therefore cannot be authoritative, at least in the realm of scientific understanding. --Jayron32 19:37, 28 March 2012 (UTC)[reply]

But I haven't said anything about it being authoritative. Just about it being true. --Trovatore (talk) 19:43, 28 March 2012 (UTC)[reply]

Is that like, a joke, because it seems like one, and I just want to confirm before laughing. If your statement is serious, I will still laugh, but for different reasons. --Jayron32 19:48, 28 March 2012 (UTC)[reply]

It's not a joke whatsoever. If you're laughing at it, it means you have a very fundamental philosophical error. --Trovatore (talk) 20:20, 28 March 2012 (UTC)[reply]

It is a fundemental philosophical error to ask that, if you make an assertion that a statement you make is true, that the statement of truth needs to be falsifiable in some way? Falsifiability is a core tenet of scientific thought. Perhaps I am misunderstaning you, but you seem to be asserting that we accept your statements as being true merely because you say it, and for no other reason. If that is not what you are trying to say, on what basis are you asserting that your statements are true? --Jayron32 20:37, 28 March 2012 (UTC)[reply]

Jayron, I have not made any assertion that the universe is infinite or finite. I am saying only that the well-specifiedness of the question does not depend in any way on my ability to make a justified assertion in either direction. It either is or is not infinite. I may not be able to say which, but that's just a limitation on me. --Trovatore (talk) 20:43, 28 March 2012 (UTC)[reply]

So if we're both clearly in agreement on that, why are we arguing? --Jayron32 01:08, 29 March 2012 (UTC)[reply]

I don't know. I think I expressed myself clearly all along; you were the one saying it wasn't so. --Trovatore (talk) 01:11, 29 March 2012 (UTC)[reply]

As to the more mathematical point, if the universe (or rather a spacelike slice of it) is a compact manifold without boundary, then it does pretty well follow that there are only finitely many stars. I understood the question to be about the case of infinitely many stars. --Trovatore (talk) 18:51, 28 March 2012 (UTC)[reply]

(edit conflict) It may already have been infinite in size at the Big Bang, we can't know for sure. Keep in mind that the expansion of the Big Bang didn't happen at a particular point, rather it happened everywhere. To use the balloon analogy, when you blow into the balloon, its entire surface is stretched. When the Big Bang occurred, all of space was stretched. If you can imagine marking two points in space at the Big Bang and watching them, then the distances between any two points would have grown immensely, but we don't have any way of saying how much space already existed before the earliest parts of the Big Bang that we can currently understand. The size of that space, beyond the immediately visible universe, is largely unknowable. It could be infinite, in which case it was presumably already infinite at the earliest moments of the Big Bang for which current physics makes sense. Dragons flight (talk) 18:12, 28 March 2012 (UTC)[reply]

Here's a really simplistic toy example. Imagine a one-dimensional infinite universe, with stars laid out at regular intervals. For convenience we can index them by the integers:

 ... s_{-3} s_{-2} s_{-1} s_0 s_1 s_2 s_3 ...

but there's no "preferred star"; the situation will look exactly the same from the vantage point of star s_1776 as from that of s_0.

Now, suppose that the distance between s_n and s_m, at time t, is always t|n-m|.

Then at any time t>0, the universe is infinite; there are infinitely many stars and they are unboundedly far apart (the distance between any particular pair of stars is always finite, but for any distance D, there are stars farther apart than D).

However, at t=0, the distance between any two stars is zero, so it's all lumped together in a single point.

The purpose of this exercise is not to suggest that the actual universe looks like a 3D version of this, but just to give a simple case to check your arguments against. Has this universe "expanded at finite rates for a finite amount of time"? Depends on what you mean by "finite rates". You may or may not have worked out exactly what you mean by that; here's a case where you can see what meanings of "finite rates" are consistent with an actually infinite universe. --Trovatore (talk) 18:37, 28 March 2012 (UTC)[reply]

Thanks, guys! Trovatore, if I understand your one-dimensional example, you're saying essentially that there was an infinite rate of expansion for an infinitesimal period of time (essentially a discontinuity), at the moment of the Big Bang, so the volume at t=0 was 0 but the volume at t=

\epsilon

for arbitrarily small

\epsilon

was infinity. Then we can still say that for all strictly positive t, the time derivative of the volume is finite (i.e., expansion "at finite rates for a finite amount of time"). Is that a valid interpretation of your example? Duoduoduo (talk) 19:21, 28 March 2012 (UTC)[reply]

Right, the volume would be discontinuous at t=0. But you know, really none of our physics makes much sense at t=0, whether for finite or infinite versions of the Big Bang. That might be what Dragons flight was getting at with the phrase the earliest moments of the Big Bang for which current physics makes sense.

Maybe it makes more sense to just omit the point t=0 from the description entirely. Perhaps there was no such point in time, but only times later than that by an arbitrarily small amount. If you look at the way cosmologists talk, they're always saying things like "10^-38 seconds after the Big Bang". --Trovatore (talk) 19:34, 28 March 2012 (UTC)[reply]

My impression is that the period from 10^-38 seconds to 10^-37 seconds was the same amount of "time", in the general sense of "stuff happening", interactions, interesting transitions in physics, objects crossing large portions of the known universe, as the period from 1.5 billion years after the Big Bang to 15 billion years after the Big Bang. Am I wrong? Wnt (talk) 20:27, 28 March 2012 (UTC)[reply]

I'm gonna hafta punt on that one. BenRG, are you around? --Trovatore (talk) 20:33, 28 March 2012 (UTC)[reply]

Wnt, are you suggesting that there would have been a relativistic time dilation due to the rate of inflation, or are you simply referring to the quantity of work done by the energy in the smaller volume of the universe at early inflation? 70.59.24.75 (talk) 21:04, 28 March 2012 (UTC)[reply]

I'm not that sure what I meant. ;) I wasn't thinking of time dilation, just subjectively that a lot of stuff seems to happen in that first second. I'm not sure what the quantity of work done was ... might be interesting. Wnt (talk) 01:29, 29 March 2012 (UTC)[reply]

Post-Angioplasty Stent

Hello. When is a stent after angioplasty necessary as opposed to angioplasty alone? I am asking out of curiosity. I respect the medical disclaimer. Links are appreciated. Thanks in advance. --Mayfare (talk) 21:29, 28 March 2012 (UTC)[reply]

There is not a simple answer to that. A blood vessel can close down again after angioplasty and stent keeps it open or a stent can be used to keep a vessel open (without), especially in the bifurcations neck regions. Can you focus you question down to something more specific?--Aspro (talk) 23:23, 28 March 2012 (UTC)[reply]

You're looking for the difference between angioplasty with and without stenting, so you could search for that, or for "coronary stent indications". One reference: [3]: "Controlled randomized trials have confirmed that stent deployment is superior to balloon angioplasty in certain lesion subsets or clinical scenarios. These include focal de novo native vessel lesions, lesions with late recoil after balloon angioplasty, acute closure after balloon angioplasty, and proximal left anterior descending coronary artery lesions. In addition, observational data is persuasive in focal coronary saphenous vein graft lesions and aorto-ostial lesions. " That article's a bit old (1996) so you may want a more recent one, unless your curiosity has been sated... - Nunh-huh 04:11, 29 March 2012 (UTC)[reply]