What keeps electrons spinning around their nucleus?
in [Relativity]
|
From: Gregory L. Hansen on 28 Mar 2005 20:31 In article <d2a0ma0td8(a)drn.newsguy.com>, Daryl McCullough <stevendaryl3016(a)yahoo.com> wrote: >mmeron(a)cars3.uchicago.edu says... >>I don't see anything in the formulation of newton's laws saying that >>forces must be traceable to fundamental interactions. What should be >>taught is that there are "physical forces" which are, indeed, a >>measure of interactions between objects, and there may be additional >>"inertial forces" which are an artifact of the choice of reference >>frame. > >To me, the part that is misleading is the treatment of *acceleration*. >The use of "inertial forces" amounts to defining acceleration to be >the quantity > > A = e_i d/dt V^i > >instead of defining it to be > > A = d/dt (e_i V^i) > >If you use the first definition of "acceleration", then you >have to introduce fictitious forces in order to preserve F = mA. >If you use the second definition, then no fictitious forces are >needed. I think that for more advanced physics, it is a mistake >to equate a vector with its components, which is what is happening >in defining acceleration component-wise as A^i = d/dt V^i. I'm not sure I get your point. The second is how the inertial forces are calculated when you transform from an inertial to an accelerated frame, it can't be ignored. But if you're using an accelerated frame, the basis vectors don't change, so the first is appropriate. Suppose you're on a carousel and define a coordinate system with the origin at the center and theta=0 on the line extending from the center to the black horse. Then what *should* the d/dt e_i be? There is no time dependence unless you simply declare by fiat that mechanics should never be done from an accelerated frame, and insist the coordinate system should be converted to one with theta'=0 extending from the center to the snack bar. -- "In any case, don't stress too much--cortisol inhibits muscular hypertrophy. " -- Eric Dodd
From: Gregory L. Hansen on 28 Mar 2005 20:44 In article <Rj12e.16030$C7.2138(a)news-server.bigpond.net.au>, Bill Hobba <bhobba(a)rubbish.net.au> wrote: > ><mmeron(a)cars3.uchicago.edu> wrote in message >news:yr02e.13$45.3299(a)news.uchicago.edu... >> >Which is just another reason force should be considered as a secondary >> >concept - the PLA is the primary concept. End of rant. >> > >> Oh, the PLA most certainly is ***the*** primary concept. So primary, >> in fact, that it serves as foundation not only for Newtonian mechanics >> but (with appropriate generalizations) for most of physics. No >> argument about it. >> >> This said, the fact remains that, for applying the PLA, a level of >> mathematical knowledge and sofistication is required which is way >> beyond the capabilities of a beginning (high school) physics student >> and, in fact, way beyond this that most people ever reach. So, we >> maintain forces as a crutch to be used till more is learned. That's >> really their remaining role. > >Most certainly. But if we are to use forces then I think we need a >discussion of what they really mean along the lines of what Feynman did in >the lectures. In fact I consider that to be compulsory reading even for >grade 8 students (at least the chapters they have the mathematics to >understand - other chapters can be added as their mathematical knowledge >grows). Having understood what Feynman wrote I think a lot of confusion can The Feynman lectures? For 8th graders? Aren't the Feynman lectures the ones based on the college courses he taught that the professors attended and the freshmen dropped out of? >be avoided. In my case I always wondered why a definition could be a law. >It confused me for many years until I read a rather nifty old book on >classical mechanics. It clearly explained the real import of Newton's Laws >was in his third law. That was the start of actually understanding what was >happening. The full resolution came with Landua - Mechanics. I think it >would be great if students did not need to go through this process and were >taught what was happening right form the start - at least as much of it as >they can initially handle. I think a lot of this will be resolved for the person who can accept that there can be several completely valid ways to approach a subject. The importance of the PLA (Principle of Least Action, right?) doesn't make Newton's approach bad, just different. One objection I would have in trying too hard to teach it "the right" way, besides confusing the students, is that too often it denies the things they know. Everyone knows what centrifugal forces are, nobody is confused on the fact that you feel it in something that's spinning and you don't feel it in something that's moving uniformly. And then these eggheads come along and say centrifugal forces don't actually exist, although the layman knows those nonexistent forces seem to work pretty well when the laundry machine hits the spin cycle. And the layman isn't wrong. Insufficient centripetal force to retain the water and sufficient centrifugal force to expel the water are separated by a simple transformation. The layman doesn't think of it in that way, but he knows that things happen when you spin. Does the egghead actually accomplish anything by trying to excise the word "centrifugal" from the language? Should we skip the chapter on Newtonian gravitation in favor of the equivalence principle? -- "'No user-serviceable parts inside.' I'll be the judge of that!"
From: bz on 28 Mar 2005 20:25 mmeron(a)cars3.uchicago.edu wrote in news:8B12e.18$45.3391(a)news.uchicago.edu: > In article <Mo12e.16031$C7.902(a)news-server.bigpond.net.au>, "Bill Hobba" > <bhobba(a)rubbish.net.au> writes: >> >><mmeron(a)cars3.uchicago.edu> wrote in message >>news:XQ02e.15$45.3352(a)news.uchicago.edu... >>> >coordinates, but I think what is misleading is to call the correction >>> >terms "forces". >>> >>> Only if you attach more meaning to the term "force" than it deserves. >> >>Ahhhhhh. Yes. As Feynman says it is half a law. It gains its full >>meaning when combined with other laws and/or concepts such as Coulombs >>law or the introduction of non inertial reference frames. >> > Yes, it is a rather complex issue. I wrote some stuff about it in the > past, here, but I never kept a copy. But it certtainly needs some > sort of broad framework, to make sense. Especially when someone keeps insisting that force is always the result of acceleration and that without acceleration (as for example when a gyro precesses at a constant rate, or when a mass moves at a constant velocity because it is overcoming drag or friction) there is no force and no work. When I push my car, I exert a force. If the brake is off, the ground is level and I push hard enough, the car begins to move. I am doing work as long as it is moving, even if it has ceased to accelerate, as long as I must exert force to keep it moving, I am doing work. I am having the hardest time getting that point across to someone in another thread. Of course, maybe I am the one that is totally wrong. If so PLEASE tell me. -- bz please pardon my infinite ignorance, the set-of-things-I-do-not-know is an infinite set. bz+sp(a)ch100-5.chem.lsu.edu remove ch100-5 to avoid spam trap
From: Gregory L. Hansen on 28 Mar 2005 20:49 In article <slrnd4h9dg.6h5.dubious(a)radioactivex.lebesque-al.net>, Bilge <cranks(a)fghfgigtu.com> wrote: > mmeron(a)cars3.uchicago.edu: > > >I don't see anything in the formulation of newton's laws saying that > >forces must be traceable to fundamental interactions. What should be > >taught is that there are "physical forces" which are, indeed, a > >measure of interactions between objects, and there may be additional > >"inertial forces" which are an artifact of the choice of reference > >frame. But being an artifact, does not mean that they've no > > ``Inertial force'' is an oxymoron. If ``inertial forces'' are >to be considered forces, then the word ``force'' is nothing but >a placeholder for a noun, since anything is then a force. A force makes something accelerate. If you're at rest in an accelerate frame you might see a ball accelerate spontaneously. If it accelerated, a force must have acted on it. "Inertial force" might not be the best name for that because they don't appear in an inertial reference frame. -- "Very well, he replied, I allow you cow's dung in place of human excrement; bake your bread on that." -- Ezekiel 4:15
From: TomGee on 28 Mar 2005 20:55
PD wrote: > TomGee wrote: > > PD, > > > > didja ever learn anything in school? > > > > 3rd grade Physics: Centrifugal force. > > > > Don't ask me to explain that to you. Go to class and ask your > teacher. > > > > TomGee > > Tom, you can either keep babbling nonsense or you can start asking > honest questions. > > A few things: > 1. The force *inward* in central motion is called centripetal force, > not centrifugal force. > > So you made it through 2nd grade, so what? No one has said otherwise, yet you keep on repeating that. > > > The electrostatic attraction between electron > and proton is an example of a centripetal force. > > So what? Did anyone ask you to give an example of it? And the basis of that centripedal force is??: Em attraction twix the electron and the protons, as I said at first. > > > 2. There is no such thing as centrifugal force, strictly speaking. It > is a misleading term for a "false force" that has no true agent to > provide it. > > There, you see? You just learned that and you failed to understand what is false and what is real! How many times will I have to tell you that your measurements are false but not the forces which pull in and push out. Call them eggs if you wish to be contrary, who cares? They exist and no one has yet agreed with you that they do not exist. > > > If this comes as a shock to you, perhaps it's because > you're relying on the physics you learned in the 3rd grade. > 3. In circular motion, you'll note that the velocity is tangent to the > circular trajectory, perpendicular to the radius of the circle. Both > the true, centripetal force and the false, centrifugal "force" are > along the radius of the circle, perpendicular to the velocity. > > Velocity is a vector but you are using it to mean "direction". Speed cannot be tangent nor perpendicular to anything, so you're babbling now. > > > There is > no way that a force that is perpendicular to the velocity can change > the magnitude of the velocity, nor does it help in any way to maintain > the velocity. > > Mygawd Im talking to a child! There is no magnitude in velocity! It is a measure of the rate of change of position of something wrt time, speed, and direction. > 4. Newton's 1st law should also have been taught to you in 3rd grade > physics, and you should be reminded that, even in the absence of > forces, objects in motion tend to stay in motion. > > I can't believe you are so ignorant as to claim that the 2nd law allows objects to remain in orbit if all forces are removed. I let that foolishness go by the first time you said it, to keep from making you look sillier, but it is you who brings that onto yourself, not me, with your absurdities. > > > Now, if you find any of this to be intuitively wrong, then your problem > is not with special relativity, it's with 3rd grade physics. If you > would like corroboration that any of the above is true or false, then > simply itemize the thing you think is wrong and ask the newsgroup. > > All but a few have dared to agree with me or disagree with you, but, as I said in my first post in this thread, I find it incredible that the first 8 posters could not answer a simple question properly. Either they all think you're the greatest thing since Einstein or they will agree to anything so long as they don't have to agree with me. TomGee |