timo

Yes, you are pretty confused. Thinking it´s 3E is a mistake I can understand up to some point. But why did you change your mind from (originally) 3B to 3A now? Perhaps you should slowly go through 1 to 5 in order and give your anwer with an explanation why you think this is the answer.

Some people sell you crystals that catch these positive energy waves created of the solar system´s eigenfrequency and therefore increase your overall health and life expectancy (and in this case also your mental capabilities since they match the frequency of the brain). EDIT: On topic: I have never head of it. I´d also not know what the frequency of the solar system should be. It sounds like a pretty much made-up statement to me. Did you notice that when you count the sun as a planet, the number of planets equals the number of your fingers? Too bad, I won´t be able to sell crystals with this wisdom.

I started to write a rather long text explaining what´s going on in detail but in the end I think it´s best to keep this as simple as possible and let you make your own picture of the physical background of it. The first reason I why I assume the force in 3) is upwards was already menationed in the previous version of this post: If you´re at point x with velocity 0 and acceleration 0, then due to x(t) = x(t=0) +v*t + a/2 * t², you won´t get away from there (note: This statement is incorrect but it´ll hopefully help you understand why a must be upwards). Since you move upwards from 3) to 4) and have v=0 at 3), your only option left is an upwards acceleration. Another reasoning would go like this: If you place the cart on the sponge it will squish a bit until the setback force of the sponge compensates gravity. But if you come in with some additional kinetic energy, the sponge must squish more to also compensate for this. Now assuming that more squishing leads to a bigger setback force, it should be clear that at 3), the setback force (which is upwards) exceeds the gravitational one ... because you squished bob beyond the point where it´s equal. I hope either of those two reasonings are a help for you to understand what´s going on. A very similar physical standard problem is the ideal spring oscillator (with the solution x(t) = sin(t)), in case you´re interested in further readings.

4F At the very point where the cart hits the sponge the answer would be A. At the point where the sponge is completely squished, the answer is B (with any scaling). At any sponge squish between 3 and 4, the anwer lies between A and B depending on how much the sponge is squished. Since this info isn´t given, you can´t know the force. EDIT: @5614: A zero velocity does not at all mean zero accelleration. Accelleration is the (time-) derivative of velocity. A function can very well have points with f(t)=0 and df/dx (x) != 0. The easiest example would be f(x)=x for x=0.

I fail to see any similarities between symmetry operations and random walks. Space and time as well as the symmetries assumed in gauge theory are continous, not discrete. Also, quamtum mechanics is -with the exception of the measuring process- strictly deterministic and in no way random. Given an initial state you know -in theory- exactly how it develops over time. That´s what the Hamiltonian is for. I think you are what you called "completely off base". But maybe you could elaborate a bit on what you actually meant. I don´t really understand it.

- What´s a "relativist"? Neither my online dictionary nor wikipedia knew that term. - If you have questions considering something go ahead and ask. - If you don´t care for something I see little point in mentioning it - mentioning something is not really the smartest way not to talk about it.

@Ecoli: I was exaggerating a bit, of course. My point is: This is a scientific forum. Is it really a good idea to be biased when reading someone´s posts? I don´t feel so. Of course you could claim that you are not biased towards someone´s post only because he has a certain personal rating but -besides that saying so you´re probably lying to yourself- then, your argument I refered to imho would become invalid. Much more helpful than a number telling me how much other people like the poster imho is the info people (usually don´t) give in their personal profile. A good share of the time it takes me for formulating a post is guessing the appropriate "level" the reply should be. @"only positive ratings": This eliminates the problem of people voting you down because they don´t like your viewpoint. But it still leaves the problem that people subjectively vote you high because you are on "their side". It also leaves what my main horror of a rating system is: People posting what they think the others want to hear and not what their own opinion is.

Apart from possible technical difficulities and potential abuse I think an interesting advantage would be that people get a feedback on the quality of their posts. But then, the question whether people who need their value being expressed in numbers really can read those numbers correctly. - How many people being assigned low ratings would think beyond "the others are all too dumb to understand me and can´t think out of the box" and consider that he/she is simply posting crap. - How many people being assigned good ratings would think beyond "yay, I´m the greatest/smartest with the longest" and consider that he ("she" would be a bit curious here) perhaps is simply good at ass kissing. Apart from the "does these rating really give you a usefull feedback"-consideration there´s another potential problem I see. I can imagine that some people would be tempted to post what they think the others want to hear. In the worst case this leads to a forum full of zombies which throw the commonly agreed upon standard phases towards every newcomer who accidently enters these forums. I definitely prefer a forum with 50% crap posts over one with 95% zombie posts. [x] No. I do like the idea but I wouldn´t want to have it realized in these forums (I guess that´s how to understand the poll). I think the negative effect of a possible fear of confrontation outweights the positive effect of getting (questionable - just think about the topics "religion" and "politics") feedback for your posts. As a sidenote: I think there already is the option to rate a whole thread; it´s just that noone uses it. That´s a possible way of giving people feedback. Another one is simply writing "thank you" of someone was extremely helpful of "well, that didn´t help me much" if the respective person´s posts were completely useless. It´s a much more powerful system of giving feedback than any number-based system could be. EDIT: One thing I didn´t consider in above but wanted to comment on, too: - People with a good ability to make posts get good posts to show as return. - What good would the indicator do? "oh, that member has such a low number, I don´t have to read the post"? "Oh, such a high reputation, so it´s probably my fault that I don´t understand anything"?

Temperature is a measure for energy distribution. For classical ideal gases that do not have any particle interaction I can very well imagine that equal temperature directly translates to equal energy distributions and therefore equal average kinetic energies. I do not think that "equal temperature = equal average KE" still holds true for more complex systems with particle interactions and bound states. A rather (although quantum mechanical and therefore probably leading beyond the homework scope) radical example of two systems with equal temperature and nonequal kinetic energy would be the free bosonic gas and the free fermionic gas in the limit of temperature -> 0.

@cpwmatthews: JonM was probably talking about the very basics of physics, not about how humans experience time. When you look at a piece of paper, it seems naturally that you can halve it´s size as often as you want if you only had sufficiently small scissors. Also, I could put my cup of coffee (or something way smaller) on any position on the table in front of me if I had enough fine-tuning. For the piece of paper, a big scientific revolution at the beginning of the last century was that this isn´t true. There is a smallest possible quantity of matter (atoms) and matter can only come in the form as a natural multiple of that smallest unit. Now you could ask yourself whether your observation that you can displace the cup of coffee by any distance you want is also wrong. Perhaps you can only displace it by a natural multiple of a smallest quantity of distance. A similar analogy (yes, I know they are flawed at several points but one should get the spirit of the idea) could probably made up with time instead of space but I can´t think of a good one at the moment.

I skipped the part of my post that complained about your nonspecific generalized statements Dudde (it was within the context of forum comunications - I should have let it in, I think). Whom exactly did I insult? Are you seriously telling me "do not insult exercises"? Would you be very surprised if I found your recent post a bit absurd? I was actually expecting an apology from you for not reading my post properly before you blamed me for insulting others. However, the kind of attitude/behaviour you show in your last post does not really give much hope to come to that level. I can live with that. If you feel the urgent need to discuss with me about this matter any further you are welcome to write me a private message.

It was certainly not meant as a "Kandi, what a stupid question you are asking!". But I am pretty convinced that this should be obvious if one read my whole post which I´d expect from Kandi (since it´s "his thread" so he should be interested in the replies) and especially from Dudde (since he felt competent enough to comment on my post). I probably should have had used a formulation like "stupid exercise" to avoid an ambiguity but such ambiguities easily happen in a forum form of conversation. Learning to reread a statement and rethink if it could mean something else than you first thought -especially if you haven´t read many posts of the person in question before- imho is a social skill that you can learn very well in internet forums.

Standard physics does not assume dicrete units of time or space. But afaik there is also no valid argument that time and space must be continual. The introductionary chapters of Bjoerken, Drell: "Relativistic Quantum Field Theory" talk a little about the possible implications that a quantized spacetime might have. I don´t have the book here at home (I decided not do do/have any physics at home so all except my most basic books are in my office) but I think I recall that the ultimate reason for not assuming discrete time and space is that we simply don´t have a working mathematical model for such a scenario. The book is roughly 50 years old, though, so it possibly not the most recent information you can get.

What name calling exactly? "Stupid question"? I do think the question is stupid and I gave an explanation why I think so. Do you think the question will be offended?

EDIT: Forget it, I didn´t see that you don´t count up if you found a common factor.

It´s totally unreadable. Let alone that in the very first line of section 3 the author starts with "Newton’s equation for free-fall velocity in a uniform gravitational field ..." combined with the fact that I fail to find the point at which he realizes that he´s not talking about a uniform gravitational field (and no: I did not spent hours trying to find it) took away all the fun of trying to find an error in it. The next possible one would be eq [3] which seems rather unjustified at first glance. But I lost my interest in the lines before that, already.

Yes, that´s the paper I meant. I am not really convinced that reading the paper will be much of a help to me since my background knowledge about LQG is exactly none. But the answer that the paper I mentioned only handles the graviton as a pertubation to a flat minkowsky space seems as a reasonable shorthand explanation to me so thanks for now.

And it probably should also read "What is the role of ethics in science discovery", didn´t it? There is an edit-button you can use to edit your posts. Well, a few random thoughts: - It is not always possible to see the full impact of a discovery from the beginning on. - Sometimes, new inventions can be used for either the good or the bad. The easiest example is probably a knife although it´s not really a recent invention. - There are so-called "ethical commissions". Some experiments (especially those involving experimenting on humans) need the approval of such a commission before they may be carried out. - There´s always psychos who want to turn even the most humane discoveries into something unethical. We had a thread here lately in which some guy obviously read something about cancer treatment by destroying the tumor with radiation. He wanted to know how this could be turned into a weapon which doesn´t leave any visible marks on the victim.... - Ususally, new technologies that can be used for unethical reasons at least need a little bit of additional research to be turned into it. You usually don´t build an hydrogen bomb by accident. - Some research like "how could a terrorist attack on the US electrical supply net deal the maximum of damage" should obviously not be made public. Yet, researching this -for example if you´re financed by the US goverment- can be quite interesting and even ethically good.

Sounds like a stupid question to me. If the plane is flying 130m/h on a course S 20 E (whatever this direction means) then it´s most certainly flying 130m/h on a course S 20 E. What you are probably supposed to do is to rewrite 130m/h on a course S 20 E as a vector, add up the windspeed and rewrite it as {magnitude, direction}. But strictly speaking that´s not what is asked for. If I were you I´d answer "130 mph, on a course S 20 E" because that shows that you can think beyond dumb "we just learned using vectors so I misinterpret the question to become a vector addition". But then, I was never known to be the kind of pupil a math or physics teacher wanted to have in his/her class.

How can you guys blame him for hacked satelites falling on his house?

I didn´t have the time to read much of your posts Martin but still I have two questions (one not-so-serious one and one which is a bit of personal interest for me): 1) Why did you put this under "Quantum Mechanics" and not under "Modern/Theoretical Physics"? Just because it´s not about classical electromagnetism? 2) I printed out the paper about the graviton propagator but I didn´t read it, yet (I´ll most probably have the time to do so this weekend, though). Nevertheless, I´m rather surprised that the graviton propagator should be a big deal. I´ve seen that in diploma theses (hep-ph/0306182, paragraph 2.5) already. Sure, the paper I posted is about Kaluza Klein gravitons but the general idea of constructing a spin-2 particle out of spin-1 particles via the Clebsch Gordan coefficients should work quite straightforwardly for any graviton. So what am I missing here?

What is "2, 3, 7, or 8 in the units"? Do you mean as a digit? 121. Do you mean as a prime factor? 4. Something else? Dunno.

You understand what "a+b+c=1 for a,b,c in R³" means? What?

Same for me (in my advanced physics labs), except that I ran into little ... well, problems ... using it. The manual which said "put in the silicon for 5 minutes" was outdated and refered to another concentration. It should have been only one minute. After about 1-2 minutes the whole stuff started boiling, after about 2-3 minutes my container started melting. Luckily, I was coward enough to break off the experiment at this point.

... unless momentum is transferred, too. Yep Correct Most probably not. I´d have never heard of people adjusting lasers because of earth´s magnetic field. What else? To my knowledge, photons do not interact with electromagnetic fields which arises the following question: @Severian: What is photon-photon scattering? Is it some indirect scattering like [math] (\gamma \rightarrow q \bar q) \, \gamma \rightarrow (q \bar q \rightarrow \gamma) \, \gamma [/math] scattering, some other SM mechanism or some exotic BSM physics?