mooeypoo

I have either mathematica or matlab. I'm stuck regardless If you have any ideas for a mathematica function, that works too! BTW, thanks for all the help so far, I'm going over them all and will reply soon individually if needed. This is driving me crazy.

Okay, Ironically, I don't even know if this is the right forum for it. Maybe Engineering would be better.. or computer science.. but it's mainly mathematical, so I was hoping the people who frequent this forum will be able to help me out. I want to model a system of objects, each with mass m, attached to each other by springs of constant k. But I have 20 such objects that move in time. My equation involves both an integer-counter x(s) where s is from 1 to 20 (counting the objects), but also x is a function of t (which is continuous). All I want to try and do is solve the relatively simple partial differential equation: d^2x/dt^2 - k/m x'' =0 Where x'' relates to differentiation in relation to s, so it's not continuous. I represent it by x'' = x(s+1) + x(s-1) - 2*x(s) And I want to solve for x, symbolically. I know that if I had everything in relation to a single variable (that is, x' and x'' differentials in respect to t) then it's a simple ODE. But.. it's.. not exactly that. I'm stuck, I don't even know how to start with it, really. I assume I'll be needing a "for" loop, but how do I solve each of those equations for the second derivative of x per t, and insert it into a double-index array (matrix, really). It's basically creating a matrix that has functions in it rather than just numbers, and then trying to model that. I'm sure it's possible, I just have no clue how to start it. Even giving me links for further reading would help. Anything.. help! Thanks a lot! ~mooey

That shouldn't be the case. We're all human, so we might fall into the trap of impatience ourselves. But we do try, and the staff also relies on the members to point out when things go awry. If you think someone is being impatient or condescending, you should report it -- it might be that the person doing the condescending doesn't notice it, or isn't aware of it; sometimes, a staff member can point things out and have the discussion revert to its helpful tones. That said, let's return to the subject at hand here, okay? dragonstar, if you want to debate the particular issue of the treatment to "noobs", feel free to do that in a new thread. ~mooey

This isn't helpful, though. Stop trying to educate him or play armchair moderator. Use the "Report" button so the moderators can catch these on time and decide on a course of action. We don't usually just "ban" people. Don't lower yourself to the level of those you report on, SMF You're better than that. Let the mods know, and we'll handle it.

That's what happens usually, but we all need constructive feedback. If the attempt to explain a complex law failed (IE, people still don't understand) you need to state that you still don't understand. This happens, and causes the explainer to try again, this time maybe with a different type of explanation. Sometimes, physical concepts ARE complicated to explain simply, even when you know them. It's not an attempt to be condescending, it's just trying to show that it's not so easy to explain these things. Cooperation is key; if you don't understand, say so.

SMF, stop feeding the troll and building a postcount over useless parental warnings. You're not his mother. ! Moderator Note Mystery_of_God, we have no problems answering questions about feces, but your questions seem to be pointless and repetitive. If you want to participate in this forum, you should take it more seriously. For that matter, I suggest you stop multi-posting the same question over and over again, for one. Here's a nice golden rule: If your science-related question could be asked without the use of feces as an example (such as in this case, since the question refers to molecules' half-life in general), use something else. Okay? Okay. Read our rules, etiquette, and the definition of "Trolling" please, before you post again.

That's quite rude, michel123456. If you don't understand, or if you think the statement was unclear, you can ask and participate in a civil debate. A backdoor ad hominem attack against someone who's trying to help is still a fallacy, and is still against etiquette. ! Moderator Note Argument from ignorance doesn't justify ad hominem.* *The above statement was meant to increase the effectiveness of this thread.

Hehe apparently I rushed too, since I didn't even notice its okay. One tip I can give you though if you know you are rushing, other than checking the answers (which is awesome) is to make a mental list of potential slipups. I rush too in exams, which makes it extremely annoying and frustrating to get a lower grade later. What I try to do is remember the things I slip on. For me, its mostly signs that I need to be careful of and translations regarding exponents. So I take extra seconds to verify these. Also, if I get stuck on one question over and over, instead of trying to figure out what I did wrong (which is tough sometimes, because you tend to stick to the same methodology and miss errors) I rip out the page, do another question to clear my mind off this one, and come back to doing the problematic one from scratch. I think of it as closing all the windows and resetting my brain. Maybe one day I will switch to mental linux Cheers and good luck! ~mooey

I have to run in a second, but when I did this with the previously given x^2-4x-12=0 I got 6 and -2 as my values for x. Since your "a" is positive and "b" and "c" are negative in this equation, the inside of the root in the quadratic formula should be positive.. +4ac, not -4ac.. that would prevent the complex number. Verify this... a complex number here seems odd. Okay ,I see what's going on.. you're rushing again. Which is ironic, since I'm the one in a rush this morning. Look here: Great so far, but then look how you write your log: No. In log form, the right hand side is log_9(0), like you wrote above in the first posts. meh. Sorry, morning and I'm rushing it too. To get log(something)=1, you need to have log_9(9), not log_9(0) like I wrote before which is undefined. Apologies. Still, don't rush it. The quadratic root should be positive. Don't rush..... Slow down, you're in the right track, your errors are just concentration and rushing it. Write this properly, and then solve the quadratic equation properly and pay attention to the signs. It has two solutions. ~mooey

This is the quadratic formula: [math]\frac{-b \pm \sqrt{b^2 - 4ac}}{2a}[/math] You wrote it slightly differently, which changed your answer (and made it wrong ). Try again.

Okay, so you have this: [math]x^2 - 4x = 12[/math] You don't need to go back to having logs or powers anymore, this is just remembering your albebra. Try putting this in a form where it might factor? If it can't be easily factored, can you think of ways to solve that type of equation? Perhaps some roots related formula? You already did this by rewriting the right side of the equation to log_9(0). I'm sorry, you just seemed to have skipped a few steps so we were trying to lead you to continue from where you stopped. From your second reply, it seems you already managed to get rid of the logs (which is good) and now you just need to continue and solve for x. You're in the right track, just keep going.

What are the rules of logs? Do you remember? What can you do with Log(whatever) + Log(whatever else) ?

Also, it seems Yomomma is content in just throwing away garbage threads and running from discussion. That doesn't really help convincing us, Yomomma. It's also against the rules of the forum. Open a blog or something.

Yeah, and religious preaching isn't really welcome in this forum either. Neither is thread hijacking. If you have a point to make (not that any point seems to be able to save this thread) make it in at least something RESEMBLING a scientific manner. Otherwise, spare us.

Double posting. Please see answer in your other thread. Don't post full homework sheets, we're not here to just hand you answers. If you want help, pick a question, tell us what you already tried to do and where you got stuck, and we will direct you to how to do them.

Asal, we're not here to answer questions for you. Pick a question, tell us what you already tried to do and we can direct you to the rigth track. Don't post the entire homework sheet. Did you try any of the questions? Where do you get stuck? What did you already try to do?

Which one do you want to discuss, Yomomma? We can't discuss all of these. Pick one.

If it's supposed to be in his belt, that's not a very high complement for Orion, the "big man hunter guy". In any case, if you're truly interested in knowing anything actual scientific about the possibility of having a sort of "accompanying" star to the sun, there are quite a number of real science physics explanations of why this is just completely proven false. Are you interested in the physics and science of these claims, Yomomma? ~moo

There are multiple contexts because it's a word, and you can use it differently. This, however, is a physical context - a very specific one. If you're mixing terms from different contexts, and instead of helping you understand the principle of your question, you just get more confused; Supersymmetry has nothing to do with this. For that matter, "power" has many contexts too. That doesn't mean that I should start treating the horse-POWER in my car as a super-POWER to calculate how fast my car can go up a hill, or that if I want to understand what electric power is, I should treat it as the power of a point. Be careful of mixing terminology from different contexts. It will just confuse you. Try not to mix linguistics and physics. If you add the wave functions instead of subtracting (which is a bit of simplification since you also switch terms, but okay) then in the case of electrons, you will need to have different spins to "break" the symmetry. Because that's how Fermions work. Take a look at the spin statistics theorem and at what Fermions are again. http://en.wikipedia.org/wiki/Spin-statistics_theorem ~moo

I think you might be misunderstanding what the Pauli Exclusion principle is. http://en.wikipedia....usion_principle You can read a bit about the derivation (a simplistic one, but generally helpful) in the "Connection to quantum state symmetry" section of that article. Symmetry, also, has a few "shapes" to it; try reading a bit about the difference between bosons and fermions, for instance. Boson particles obey Bose-Einstein statistics, while Fermions go by Fermi-Dirac statistics. They differ in behavior and in the type of symmetry that they can have. Fermions are restricted by the Pauli exclusion principle. Electrons are fermions, which are related to an "antisymmetric" wave function: no two fermions can occupy the same quantum state at the same time. Either they have the same orbital but different spins or they have the same spin but different orbitals. Here's another good article to start from: http://en.wikipedia.org/wiki/Fermion If you want to know why this is, you should look at where the Pauli Exclusion principle came from. I can't find an online source for the derivation, but many printed quantum physics books have the explanation of where this came from and why. I have Griffith's "Introduction to Quantum Mechanics" and there's a whole chapter about it with the mathematical principles as well. ~moo P.S, this is another good source to read about superposition and spin-states of fermions and bosons: http://en.wikipedia.org/wiki/Spin-statistics_theorem

Khaled, thanks a lot! I missed this reply for some reason, which is why I just now respond. I decided, actually, to switch to MatLab. Awkwardly enough, it's much more convinient to use.. Mathematica is just too complicated for its own good, it seems. I will try this in mathematica though (I now have both) - this seems like it might actually solve a huge chunk of my problem. Thanks! ~mooey

What, exactly, do you wish us to discuss about 2012 as opposed to, say, 2013 or 2011? As you mentioned, we're a science forum, so we go by substantiation and actual evidence. The myth discussions about 2012 being a Mayan magical date deal with magic, not science. Galactic alignment already happened in 1998 (with nothing happening, of course) and will not happen again in 2012, so that's out. Planet X/Nibiru doesn't really exist, so the chance it will hit us is absolutely nonexistent. Moreover, while we do have trouble spotting asteroids on potential collision course with earth until it's relatively "late" (a few months in advance) that is mainly because they're very very very very small. Very small. A planet is big. We'd have seen it. It doesn't exist. Solar maxima happen every 11 years or so, and it never destroyed the earth. The next maximum is going to happen around 2012-2014 (notice, this would mean "doomsday" average at 2013... run!) and is expected to be about average. It's true that we seem to be a bit more worried nowadays about solar flares than we used to, but this is because any slightly bigger solar flare that's directed at the Earth risks our satellites and other objects in space, including our astronauts. We didn't have astronauts or as many satellites 100 years ago. And yes, we were worried about solar flares and our satellites 11 years ago, too. Here are two sites with great summaries of the claims about 2012 and the explanations of actual science behind them: http://www.universetoday.com/14094/no-doomsday-in-2012/ http://astrobiology.nasa.gov/ask-an-astrobiologist/intro/nibiru-and-doomsday-2012-questions-and-answers So, yeah, I do consider the doomsday scenarios totally bogus. There's no reason for a doomsday at 2012 other than, perhaps, one that follows the US elections. ~mooey

Okay, I see it now, though there's no chance in hell I could've seen it earlier without really.. knowing this trick... But it helps knowing at least why it's like that, so thanks Dr Rocket! And timo, you're right, oops, I forgot the minus - but only online. It came out okay on paper. LaTeX is annoying. In any case, thanks a lot for helping.. I was very frustrating over the fact that it seems I got the answer but not quite the answer, and.. I had no clue why. I guess I should go over a bit of series expansions.... If you have any suggestions on what to start from, that would be great. Otherwise, I guess I'm going google hunting. Thanks! ~mooey

Thanks! Sorry to be really slow here, but I'm not sure I get how you got to this step: Shouldn't the sum in the right hand be s=1 to N now? I'm just trying to understand the steps. The 1+ came from n=0, right? (Sorry, this is obviously "low level" series manipulation, but sadly, I've never really taken any real math classes in those, so it's really a lot of hard work and guessing and tryingt o figure it out on my own here. Sorry if the questions sound stupid.) And thanks for your time! ~mooey

Hey guys, I'm practicing for a thermo test later this week. We have this question in the book: Zipper Problem: A zipper has N links; each links has a state in which it is closed with energy 0 or open with energy [math]\varepsilon[/math]. We require, however, that the zipper can only unzip from the left end, and that the link number s can only open if all links to the left (1,2,...s-1) are already open. (a) Show that the partition function can be summed in the form: [math]Z=\frac{1-\exp{(\frac{-(N+1)\varepsilon}{\tau})}}{1-exp{(\frac{\varepsilon}{\tau})}}[/math] Okay, so I approaced the problem by defining the possible energies as a sum of epsilon(n) where it goes from 0, 1, 2, 3 etc. Hence, my partition equation: [math]Z=\sum \exp{(\frac{-n\varepsilon}{\tau})} = \sum (\exp{(\frac{-\varepsilon}{\tau})} )^n[/math] I have been trying to manipulate this further forever. I know it should look like something starting with 1 + exp(..) since with n=0, the exponent will be =1, but I couldn't see how to transform it. Finally, I resorted to the answer sheet, and it says that the above is right, and therefore it's obviously [math]Z=\frac{1-\exp{(\frac{-(N+1)\varepsilon}{\tau})}}{1-exp{(\frac{\varepsilon}{\tau})}}[/math] Obviously? What am I missing? How did they get from the ^n to that? meh. Help! ~mooey p.s I see in another source that they rewrote the summation as: [math]\sum_{s=0}^{N} x^s = \frac{1-x^{N+1}}{1-x}[/math] where [math]x=\exp{(\frac{-\varepsilon}{\tau})}[/math] Great... that looks like an expansion of a power series, and I guess it makes sense if you have the instinct to translate it like the above. Assuming I got stuck with this for an hour, is there any other way to do this, or is this simple a "remember your power series expansion" problem... ?