timo

You get the distance d from E=F*d and the initial conditions E=mv² for both cars. Again, m will cancel resulting in the same distance. It does indeed not sound logical (when you use the term "logical" in the sense of "intuitive" - in reality, logic is something different) but it´s the correct result of the calculation. I do not know to what extent a truck really stops as fast as a car, but in reality there are a few technical parameters (e.g. the tires) that need to be taken into account. They might make a difference, but I´m no car manufacturer, so I cannot judge that.

Newtons Law: a = F/m Frictional force: F = m*g*c, where c is the friction coefficient. Shouldn´t be too hard to find it out from these two equations.

Relativity allows for arbitrary coordinate systems to be used. The question whether you´re calling them equal just because you can use them all seems like a matter of taste - I´m not really sure what "equal" should or could mean in that context.

Colliding two electrons at high energies will, according to current theory, give different measurements in the detector. Whether this is because the initial conditions were not exactly the same or if completely equal intial conditions can really lead to different measurements is not known and, due to practical restrictions that you cannot prepare intial states arbitrarily exact, imho cannot be finally decided.

Since your problem is talking about forces (which you get from weight and friction coefficient) and distance, your best guess is to go via energy (force * distance) rather than momentum (force*time). It is possible to solve your question using momentum and momentum does -from the standpoint of modern physics- play the fundamental role in describing dynamics of a system. But in your case, you don´t really need to consider momentum (though you can if you want to).

The physics section might have been appropriate but the thread fits in here fine, too - I think. What you said seems basically correct but I´ll reiterate it and add a few points, anyways: The problem with fusion technology basically is that we don´t have it - at least not in the sense that we could obtain energy with fusion. There is also some amount of radiation involved in fusion but I cannot judge to what extend it is a relevant problem. For fission, there is 2-3 main problems: - Radiative waste: It´s not really that it is so much of it but rather that the stuff remains (measurably) radioactive for a very, very long time. Afaik, there is no real solution to the question what to do with the waste. Our currently-best idea is to dump it in special tunnels under earth which is a temporary solution for perhaps the next 1000 years. - Devasting effects of accidents: If a coal power plant burns down then you should possibly go on holliday for a few days if you´re living close by. If you get a serious accident in a nuclear power plant, then you can get something like Tschernobyl. - A less fundamental yet, for some people, the most convincing problem: Security and cost-efficiency. Nowadays, everyone is afraid of bearded people coming from the sky (except on christmas). Due to the severe effects of accidents on nuclear power plants, they need a high level of protection/security. This costs money which reduces the cost-effectiveness of fission-produced energy. I am not sure if that´s really a big issue as soon as you look at the numbers but it´s been mentioned in debates about whether to stop relying on fission power at all or not.

Dunno what a "free body diagram" is but three potentially interesting properties to look at are: - The position ("how far did he get already?") at a certain time t. - The velocity (=pretty much the same as speed) at a certain time t. - The acceleration (=pretty much the same as the force, for your case) at a certain time t. Once you know how these three properties behave, you should get some ideas what to talk about (ideally things that weren´t obvious to you bevore you investigated them).

Not sure what you mean. Both, +7 and -7 solve the equation: (+7)² = 49 and (-7)² = 49.

I have little knowledge about atomic and molecular physics or chemistry but I´ve seen simplified explanations of ionic binding stating that one electron is given to the other atom involved causing an attraction between the two unequally charged resulting ions. Translated to the picture I´ve used above this would mean that a part of one atom´s electron cloud is cut and added to the electron cloud of the other atom. I do not know to what extend it is sensible to describe the resulting molecular bond as still two distinct electron clouds of whether it makes more sense to see them as a combined electron cloud with just "more electrons" near one of the nuclei. So in short: Yes, that´s what I was saying. But I am not sure to what extend that´s correct.

I´d like to emphasize this point since it´s an understandable yet grave mistake to think that nuclear fusion (or "cold fusion") was about binding together atoms, such as in the reaction of hydrogen and oxygen to water. Atoms can be considered as consisting of two parts: The nucleus, which is very little in radius but rather heavy (for atomic scales, of course) and the electrons, which occupy an area with a much larger radius but have an almost negligible mass (compared to the nucleus). In chemical bondings, such as the 2H+O -> H2O, the different atoms' electron "clouds" around the nuclei can "melt" into a single cloud surrounding all of the involved nuclei (a better explanation by someone more familiar with chemistry than me might add a better explanation - I admit that it stretches imagination quite far to consider e.g. an ionic binding as the melting of two electron clouds). The typical energy scales are roughly a few eV (=electronvolt, a unit for energy). Nuclear physics and nuclear fission on the other hand deals with the other part of the atom, the nucleus. There, it´s not the electron clouds that form a binding, but it´s the nuclei of two atoms (for technical reasons it´s not really correct to speak of atoms here, you effectively have only the nuclei - but that´s a different story) that "melt" together forming a new single nucleus. The energy scales involved in nuclear physics are usually much higher than the energy scales of processes of the electron clouds - roughly in the range of a few million eV. In other words, the fusion of two nuclei brings you roughly (very roughly) a million times more energy than if the electron clouds of the atoms would fuse. However, while the forming of chemical bonds is absolutely common on earth, nuclear fusion practically doesn´t happen at all. That´s because the nuclei need a relatively high initial energy to be able to melt (they are both positively charged and repel each other, so they usually can´t get close to each other) which they only attain at extremely high temperatures. Attaining these high temperatures is the reason why fusion reactors are big and expensive machines that you don´t have in your home-lab. Getting a net energy gain out of the nuclear fusion is mainly a technical question. More precisely: It´s a yet-unsolved technical question (as someone already mentioned: The sun perfectly demonstrates that it´s possible in general). ITER is supposed to help solving this question (but afaik not thought to be the solution already).

I am always surprised by people claiming that our/their perception of the world was 3D. I am certainly aware of the time dimension; how would you plan a meeting with someone, otherwise? A somewhat nonsensical yet possibly funny question in this context is: What if we really couldn´t percieve time? Since we couldn´t distinguish between things we´re currently doing, things we did in the past and things we´re going to do, would we be fortune tellers (which admittedly wouldn´t help us, because we wouldn´t know what future is)?

You should go via momentum. Sadly, no one else bothered replying here and I will probably not have time to guide you to the equation within the next weeks. So I will just give you the formula I suppose you are looking for: [math] F = \frac{dp}{dt} \Rightarrow p = Ft, v = \frac{pc^2}{E} = \frac{pc^2}{\sqrt{m^2c^4 + p^2c^2}} \Rightarrow v = \frac{Ftc^2}{\sqrt{m^2c^4 + F^2 t^2 c^2}} [/math] The difference is in part 2) of the points I previously mentioned. In classical mechanics, the relation of velocity and momentum is p = mv. In relativistic mechanics, the relation is different (the one I used above).

I think you´re missing how a representation of vectors in (x,y) coordinates relates to a representation via the magnitude and an angle. If the x-axis is what you´ve called 0° and the y-axis is what you called 90°, then for any vector [math] \vec v [/math]: [math] \vec v = \left( \begin{array}{c} v_x \\ v_y \end{array} \right) = | \vec v | \left( \begin{array}{c} \cos \theta \\ \sin \theta \end{array}\right) [/math], where [math] \theta [/math] is the angle to the x-axis increasing towards the y-axis (same as in your diagram). Using this and the definitions of sine, cosine and tangens, you can transform from one representation to another and back. As for Rocket Man's post: Note that he completely omitted any units, which is usually a bad idea.

I know how to figure it out: 1) Find out what the momentum after time t is, when a constant force is applied 2) Find out how the velocity of an object depends on its momentum (and its mass/energy). 3) Plug it together.

The comparison has a slight flaw: The US are giving up the rights of non-US citizens. So the "excuse" should be "they must sacrifice rights for our security". This also slightly alters the second part of your statement.

I agree with Martin´s proposal to split the thread. I guess Martin is talking about practical units for calculations in physics. Turrican's reason to bother about units seems to be more an aesthetic/ideologic issue. So while there are common points at first glance, the motivation and aim of both approaches seem very different (both in aim and the kind of arguments relevant for the discussion) and should be seperated to avoid mixing up two distinct debates.

You only considered one H atom. But it´s H2... Just out of curiosity: Why did you post that under the relativistic physics section and not in the chemistry sections? EDIT: Seems that Baby Baggins did post it in the chemistry section, too: http://www.scienceforums.net/forum/showthread.php?t=24730 . I propose that further replies should go there.

I´d assume it´s this paper: http://arxiv.org/PS_cache/hep-ph/pdf/0604/0604255.pdf (hep-ph/0604255)

You don´t "just leave it out". It´s part of the (x-4)²: (x-4)² = (x-4)(x-4) = x² -4x -4x + 16 = x² -8x +16 If you read that equation from the right to the left (equations can always be read both ways round), then that´s exactly what you´ve done/used in the last step. I don´t fully understand the question. You´re asking why 16 (the square of one-half of the coefficient of the x-term) is added and substracted?

A sensible answer would be much easier to guess if you gave an estimate of how far away from earth it actually is detected. Assuming it is at least as far away as Pluto (it´s most likely we already had discovered a planet the size of earth if it was closer) or in a neighbouring solar system, then the next question is whether the shuttle shall be manned or not and whether it shall travel within the currently-known laws of physics or if you want to use some imagined exotic device. In case of the imagined exotic device: Anything is possible as long as you can imagine it, that´s what being a scientist is all about (couldn´t resist bringing this quote from Futurama, the alternate quote sais "magical faerie elf", btw). In case of currently-accepted science: If it´s unmanned you might get an almost unlimited acceleration that almost instantly brings the shuttle to the maximum possible velocity which is about 300000 km/s. For the nearest star, the time needed is ~4 years then, I think. Much less if you´re staying within the solar system. If it´s a manned probe then you´ll likely have the restiction that a human body will probably not stand an acceleration greater than 10 m/s² for long. This low acceleration will probably give a large increase to the time needed.

I am not sure if I see the problem. As I see it, you don´t think the stones have healing powers and hence don´t want to help selling them for a lot of money. I think it shouldn´t be too much of a problem if you tell your friends that you don´t want to help selling healing stones if you don´t believe in their healing powers yourself. At least most people I know would accept if I told them that I don´t want to rip-off other people - that´s what it would be if you´re not believing in the healing potential yourself. Convincing them to stop the whole thing is a different matter, but that´s not what you´re asking for, or is it?

Nice sketch. Since you´ve not asked a question I am not really sure what kind of replies you´re expecting. Apart from issues with the level of detail you can achive, the most striking problem that comes to my mind is the amount of light you need. If it´s supposed to work in daylight, then I am not sure that a normal flashlight will give you enough light for the picture to be well visible. Beamers tend to have specially-bright lamps in them that also need special handling (e.g. you cannot simply plug off a beamer after use but instead need to let the cooling work for some more time after use). Beamers also are portable, at least to my understanding of the term (i.e. they are roughly the size of the laptop I plug onto them). If you simply take a flashlight and shine through some transparency/dia on a wall you´ll see the tranparency projected there. What do you want to change/improve/... ? Btw.: Welcome to these forums.

Afaik the twins really interesting for investigations of genetic impact are not "normal twins" but twins that grew up in different enviroments (families). "Normal twins" both being raised by the same family are not so good for distinguishing genetic/biological influence and enviromental/educational influence because they will likely have the same enviroment, too. That makes a distinction between biological and enviromental impact complicated, to say the least. In the case of those "unusual twins", however, I would see a significantly stronger correlation (stronger than for persons from similar backgrounds which are not twins) between IQ scores as a sign of genetic/biological influence. Mostly because I wouldn´t know other factors that might explain the stronger correlation.

Photons can interact with electric charges, e.g. a photon can kick an electron or another charged particle out of its position. I can´t picture a "high voltage medium" so I don´t know about the 2nd part.

It´s rather a way to (mathematically) describe the exitation/state of a field than only a metaphor. You can describe the state of the ocean surface by giving a water-height to each point or you can start with a flat surface and add waves (areas where the height is raised) to it. The former method clearly describes the state but the latter might be more practically interesting for certain applications (e.g. a surfer). While describing a field in QFT is slightly different technically, the idea to get a practical (i.e. useful) description of the field by using waves/particles is the same.