What keeps electrons spinning around their nucleus?
in [Relativity]
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From: mmeron on 29 Mar 2005 17:55 In article <pPmdnT872IUW8tTfRVn-rw(a)rcn.net>, jmfbahciv(a)aol.com writes: >In article <7K52e.21$45.3808(a)news.uchicago.edu>, > mmeron(a)cars3.uchicago.edu wrote: >>In article <Xns9627C5AEB62D6WQAHBGMXSZHVspammote(a)130.39.198.139>, bz ><bz+sp(a)ch100-5.chem.lsu.edu> writes: >>>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 >> >>Cause, not result. >> >>> 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. >>> >>Where there is net force, there is acceleration. The F in Newton's >>law is the total (i.e.) net force acting. Since forces are vectors, >>it is perfectly possible to have different non-zero forces to sum up >>to a zero net force. > >This is why using algebra in first physics courses hurts learning >more than it helps. > I don't see why it should hurt. Of course, it is better if you can use calculus, not just algebra. Mati Meron | "When you argue with a fool, meron(a)cars.uchicago.edu | chances are he is doing just the same"
From: mmeron on 29 Mar 2005 17:56 In article <slrnd4j3jn.6h5.dubious(a)radioactivex.lebesque-al.net>, dubious(a)radioactivex.lebesque-al.net (Bilge) writes: > mmeron(a)cars3.uchicago.edu: > >In article <d2ac7c$65m$5(a)rainier.uits.indiana.edu>, > >glhansen(a)steel.ucs.indiana.edu (Gregory L. Hansen) writes: > >>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. > >>-- > >Got a point. Lets call them "non-inertial forces". > > That would be redundant. > There is no keepiong everybody happy, I see:-) Mati Meron | "When you argue with a fool, meron(a)cars.uchicago.edu | chances are he is doing just the same"
From: mmeron on 29 Mar 2005 18:07 In article <1112115765.897415.6600(a)z14g2000cwz.googlegroups.com>, "PD" <pdraper(a)yahoo.com> writes: > >mmeron(a)cars3.uchicago.edu wrote: >> In article <1112038415.659261.203550(a)l41g2000cwc.googlegroups.com>, >"PD" <pdraper(a)yahoo.com> writes: >> > >> >TomGee wrote: >> >> Wormy, Bilge, PD, and all you other lemmings, >> >> >> >> you cannot understand that it is the _measurement_ of the force >which >> >> is fictional and not the feeling of being pulled out as a carousel >> >> spins. You cannot make the force disappear just by invalidating >your >> >> own measurements. You should not think that websites are the >> >ultimate >> >> authority on anything, either, as subjective opinion runs rampart >all >> >> through it. You will learn that, Bilgy, when if ever you get to >the >> >> fifth grade. >> >> >> >> TomGee >> > >> >One more little experiment, Tom, on your carousel. Take your >> >pocketwatch on the carousel with you and hang it by its chain >between >> >your thumb and forefinger. If you don't have a pocketwatch, take >your >> >wristwatch and hang it by a string in the same way. Now have someone >> >spin you up on the carousel. >> > >> >You see the watch swings outward on the chain. Why, you ask? Is it >> >centrifugal force? No, your fourth grade teacher says, look again. >> >> Yes, that's probably what the teacher will say. physics teachers >know >> little physics. > >Gee, and I thought most university-based physicists were physics >teachers. > Oh, but are they good teachers?:-) >> >> There is ***nothing*** wrong with centrifugal force. The term >> "fictitious" is misleading. True, it doesn't stem from interaction, >> just from chice of reference frame. So? It is quite often >convenient >> to deal with a problem in an accelerated frame, rather than transform > >> to an inertial frame and back. And when you want to deal with a >> problem in the accelerated frame and still use Newton's laws, you >> include inertial forces (i much prefer the term "inertial" than the >> misleading "fictitious"). >> >> Try the following problem. Assume that you've, say, the space >shuttle >> in a circular orbit around Earth, with a small mass placed in the >> middle of the cargo hold, motionless relative to the shuttle. Now >> give it a slight nudge. Write the Lagrangian for the system, in the >> shuttle coordinates and observe the equations of motion. You'll find > >> a centrifugal term (and a Coriolis term, as well). Of course, if >> you'll use an inertial frame, insted, these terms will disappear. >But >> you'll find the inertial frame far less convenient. It is a matter >of >> pragmatism, not ideological soap boxes. > >Maybe it's just me, but computational pragmatism should follow physical >intuition IMHO. You are absolutely right that it's more convenient to >work sometimes in the accelerated frame and see things like effective >potential and a Coriolis term, and in such cases it's always a good >idea to mentally note "And that thing there is because we are working >in an accelerated frame." Sure, no argument with this. > >> >> Consider the equation >> >> ax^2 + bx + c = 0 >> >> I'm sure that you're aware that one can always eliminate the middl >> (linear) term by a proper shift of the origin of x. So? Do we call >> this middle term "fictitious" and warn students never to use it? >That >> would be nonsense. > >No, but by the same token, I wouldn't say, "This problem is much more >conveniently solved if we shift the origin." Indeed, I would probably >work it both ways (or have students do it) so that they could see both >the more intuitive but harder-to-solve method, as well as the clever >trick, so that they could look at what's going on in the clever trick. > Yes. And they would find the ability to see it both ways quite useful. The one thing you wouldn't do would be stating "only one of these ways is legitimate". >> >> Mind you, this has nothing to do with the value of TomGee's >gibberish. >> But then, since you chose to waste your time by responding to him ... > >Ah, we each have our chosen crosses, don't we? > Indeed, tell me about it:-) Well, good luck, then. Mati Meron | "When you argue with a fool, meron(a)cars.uchicago.edu | chances are he is doing just the same"
From: Bill Hobba on 29 Mar 2005 18:08 "PD" <pdraper(a)yahoo.com> wrote in message news:1112120245.503240.33900(a)g14g2000cwa.googlegroups.com... > > Bill Hobba wrote: > > <mmeron(a)cars3.uchicago.edu> wrote in message > > news:yr02e.13$45.3299(a)news.uchicago.edu... > > > In article <hT%1e.15978$C7.12491(a)news-server.bigpond.net.au>, "Bill > Hobba" > > <bhobba(a)rubbish.net.au> writes: > > > > > > > ><mmeron(a)cars3.uchicago.edu> wrote in message > > > >news:4q_1e.11$45.3008(a)news.uchicago.edu... > > > >> In article > <1112038910.784307.230090(a)o13g2000cwo.googlegroups.com>, > > "PD" > > > ><pdraper(a)yahoo.com> writes: > > > >> > > > > >> >Gregory L. Hansen wrote: > > > >> >> In article > <1112035436.549412.84900(a)z14g2000cwz.googlegroups.com>, > > > >> >> PD <pdraper(a)yahoo.com> wrote: > > > >> >> > > > > >> >> >TomGee wrote: > > > >> >> >> Wormy, Bilge, PD, and all you other lemmings, > > > >> >> >> > > > >> >> >> you cannot understand that it is the _measurement_ of the > force > > > >> >which > > > >> >> >> is fictional and not the feeling of being pulled out as a > > carousel > > > >> >> >> spins. > > > >> >> > > > > >> >> >Nope. You feel the force pulling you *in*, which is a force > you are > > > >> >> ... > > > >> >> >Let me ask you another question. As an elevator suddenly > starts to > > > >> >> >descend, you feel lighter. Are you in fact lighter? Why > should > > > >> >Earth's > > > >> >> ... > > > >> >> >Let me ask you another question. You are on a road making a > > circular > > > >> >> >bend left in your car. What force acting on the car enables > you to > > > >> >> > > > >> >> Reference frames, Paul. TomGee feels, TomGee is in, TomGee > turns... > > > >> > > > > >> >> Who's making the measurement? A hypothetical 2nd observer > > stationary > > > >> >with > > > >> >> respect to the Earth, or TomGee? > > > >> >> > > > >> >> In those three situations, TomGee is in an accelerated frame. > If > > > >> >TomGee > > > >> >> is pushed to the left against a wall, there's an inertial > force > > > >> >pushing > > > >> >> him to the left. A 2nd, inertial observer might say no, > TomGee is > > > >> >REALLY > > > >> >> being pushed to the right, but so what? He's not in the car. > He's > > > >> >just > > > >> >> asserting that there's something special about his reference > frame > > > >> >such > > > >> >> that he can make valid observations but TomGee can't, and > pretending > > > >> >> there's no valid way to transform between them. > > > >> >> > > > >> >> Centrifugal force is called a force because it acts like a > force. > > It > > > >> >will > > > >> >> cause something to accelerate relative to the stationary > observer > > > >> >who's > > > >> >> already pinned against the wall. The stationary observer is, > of > > > >> >course, > > > >> >> spinning madly about if viewed by the hypothetical 2nd > observer > > who's > > > >> > > > > >> >> at rest relative to the Earth, but so what? There's nothing > special > > > >> >about > > > >> >> the Earth frame, nothing wrong with the accelerated frame. > If > > TomGee > > > >> >is > > > >> >> pinned to the wall of a centrifuge, then TomGee is still at > rest > > with > > > >> > > > > >> >> respect to himself and he can define a reference frame from > his > > > >> >> perspective. > > > >> >> -- > > > >> > > > > >> >Yeah, but I'm opposed to this kind of presentation, especially > to the > > > >> >uneducated. I really believe that forces should be taught as > being > > > >> >characterized by interactions between two objects, and those > > > >> >interactions should be traceable to one of the four (or less) > > > >> >fundamental interactions, and that forces are the cause and > > > >> >accelerations are the effect. > > > >> > > > > >> 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 > > > >> observable effects on motion relative to said reference frame. > I see > > > >> no reason to treat it as some sort of a dangerous knowledge that > > > >> should be kept away from the uninitiated, else their mind may > > > >> explode:-) > > > > > > > >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 > > 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. > > > > Thanks > > Bill > > > > IMHO, the number-one problem students have in physics is making a > connection between physics and reality. You can appeal to them about > what they've all seen about the flight of a baseball, and then you ask > them to draw the trajectory of a fly ball in a physics problem and you > get straight lines, you get trajectories with corners, you get crazy > stuff. And I've *asked* them, "You've been to baseball games, right? > Does this look like what you see?" And they'll look at me blankly and > say, "But this doesn't have to look real. It's physics." > > The number-two problem is ill-formed and overlapping concepts, like > distinguishing displacement, acceleration, and velocity in the > catch-all "to go", or the distinction between force, momentum, kinetic > energy, and power. Teaching students to be careful and precise in their > definitions is asking a lot of them. > > The number-three problem is pre-existing misconceptions, like thinking > that a bullet falls to the ground because it is slowing in flight. > > None of these would be addressed by pulling out a Power Tool, making a > Tim-Allen guttural grunt, and saying, "Now THIS is REAL physics!" IMHO, > we do our students a service by teaching them to > * check against their intuition > * recast their intuitive ideas with carefully defined terms > * recognize places where the edges of their intuition are a little > fuzzy > * be rigorous in making explicit predictions they can check with > observation, and thereby incorporate those observations into an > expanded intuition. > To me, this is teaching them to think like a physicist. PD you have experience in teaching students physics - I do not (limited to teaching some math). So I must say your observations hold much greater weight than mine. But perhaps as I suggested in another post there may be some value in tailoring material to the requirements of each student? After all when a student does have questions like I did and they get no satisfactory answer are we not doing a disservice to that student? Of course the problem is how can we accommodate these students with limited teaching resources. Perhaps we need more teachers like Jaime Escalante http://www.govtech.net/magazine/visions/feb98vision/escalante.php He does not believe in separating students out - he believes in challenging them. But I am biased - along with Feynman he is one of my heroes. Thanks Bill > > PD >
From: Tom Capizzi on 29 Mar 2005 18:10
<puppet_sock(a)hotmail.com> wrote in message news:c7976c46.0503291409.5e027614(a)posting.google.com... > dubious(a)radioactivex.lebesque-al.net (Bilge) wrote in message > news:<slrnd4dc08.n0h.dubious(a)radioactivex.lebesque-al.net>... >> Nick: >> >What is the velocity of an electron in a shell? >> >> Velocity isn't a quantum mechanical observable. > > Of course it is. What ever do you think you get when you take > the time derivative of position? Electrons don't really have position. They have complex probability amplitudes. What do you get when you take the derivative of that? > Socks |