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Natural Science Forum / Physics / Relativity / May 2006Another Rotating Cylinder Problem - explain from moving frame view
Can anyone explain this rotating disk problem from the point of view of a moving observer? In the rest frame let there be two rotating disks of diameter D perpendicular to the x axis. Let the distance between the disks be L. Let there be a rotating cylinder of the same diameter and length connecting these two disk. Let the disks be massive and made out of steel and let the cylinder be made out of wax. Let the cylinder and disks rotate at one revolution per second. Let there be a frame moving along the x axis relative to this rest frame with some V. Let L and V be such that simultaneous events measured in the moving frame at each disk (separation L) are measured as a half- second time interval in the rest frame. At time t0 as measured in the moving frame a thin straight wire is simultaneously attached to the two disks at the top position of each disk and along the top of the wax cylinder. This is a straight line in the moving frame, but spirals around the cylinder making a half revolution as viewed in the rest frame. Now very slowly the tension of this wire is increased - the wire is stretched. This means the wire is very slowly approaching a straight line as viewed in the rest frame. As the tension is increased this wire cuts through the wax cylinder. Eventually the wire becomes a straight line and any further stretching of the wire does not change its shape. As viewed in the moving frame the wire is a straight wire on the surface of the cylinder rotating with the cylinder before we start stretching the wire. Now as the wire is stretched the center point of this wire eventually touches the center of the rotating cylinder (the x-axis) as the wire slices through the wax. Can anyone explain as viewed in the moving frame why the center of this straight wire cuts the wax all the way through to x-axis as the wire is stretched? If you post an explanation, does the same explanation work when the straight wire is simultaneously attached as measured in the rest frame and then the wire is slowly stretched? That is, the wire is stretched and stretched but it never cuts into the wax eventhough it spirals around the cylinder as viewed in the moving frame. This physics result of SR seems non-sensical to me. Thanks, David Seppala > Can anyone explain this rotating disk problem from the point of view > of a moving observer? snip << This physics result of SR seems non-sensical to me.>> Probably because you haven't read these: http://web.mit.edu/8.02t/www/802TEAL3D/teal_tour.htm "Retarded potential" http://farside.ph.utexas.edu/teaching/em/lectures/node50.html Sue... > Thanks, > David Seppala > Can anyone explain this rotating disk problem from the point of view > of a moving observer? Davis Seppala is one of the mysteries of this group. Unlike Spaceman, for example, he is smart enough to dream up endless SR puzzles, many of them involving accelerating reference frames, yet by his own admission he has practically no understanding of SR. Is he really an expert on the subject testing posters' understanding? Is he a bunch of psychology students performing some kind of experiment on us all? Is he just a troll who delights in stirring up discussion and argument? Any suggestions? Martin Hogbin >> Can anyone explain this rotating disk problem from the point of view >> of a moving observer? [quoted text clipped - 15 lines] > > Any suggestions? I suspect he simply wants to see if he can trip people up. It is not that he delights in thinking up endless thought experiments it is that he applies zero effort himself into actually solving them. As Sal has said - that is simply not nice - and I suggest those that respond do not tell him the answer but ask questions whose answer will lead him to the solution - that will trip him up nicely by forcing him to think rather than us doing it for him - which is probably what gives him his jollies. Thanks Bill > Martin Hogbin >>Can anyone explain this rotating disk problem from the point of view >>of a moving observer? [quoted text clipped - 15 lines] > > Any suggestions? Could be any of the above, or could be he's what he says (doesn't seem likely), but does it matter? After all, this is a Relativity group and his usually well-posed questions are on-topic and ought to be welcomed without regard to his motives. Most regulars here however, seem to be more interested in critiquing the incessant babble of the irrational know-it-alls that infest this place. 2 cent contibution attached. > Could be any of the above, or could be he's what he says (doesn't seem > likely), but does it matter? After all, this is a Relativity group and > his usually well-posed questions are on-topic and ought to be welcomed > without regard to his motives. Most regulars here however, seem to be > more interested in critiquing the incessant babble of the irrational > know-it-alls that infest this place. I think the "problem" is that David's questions are all of the extremely tedious, uninteresting variety. -- Jan Bielawski >> Can anyone explain this rotating disk problem from the point of view >> of a moving observer? [quoted text clipped - 15 lines] > >Any suggestions? He is none of the above. David's understanding of Einstein's notions so far is much like David's comprehension of E. M. Escher's drawings. He gets to points in problems where there seems to be contradictory results as in his posting on 4/10/2006 where a moving rigid rod is always parallel to the x-axis and loops about the x-axis in a circular pattern at a 10 meter diamter circle yet no forces are applied to the rod to make it continue in this circular pattern. Or in this posting where as tension on a straight wire increases the center of the wire moves away from a straight line. This is opposite to typical experiences - wires form straight lines when stretched from two points with nothing in between them to interfere with the straight line. This does not make sense to David. David is accustomed to more educated people pointing out the errors in his understanding. David really doesn't care whether others think he's a troll, a moron, a psychologist perform tests, fat, skinny, ugly or anything else for that matter. David simply finds relativity interesting but seeks out the help of others in resolving things that don't make sense to him. He finds it interesting that posters spend time speculating about his motives but he doesn't find it helpful to himself or others who may not understand the answers. If the problems are so uninteresting as one person has posted here why does he read David's postings? If the answers are so obvious why not simply post the correct interpretation and help everyone? Why do you care about David's motives? David >Martin Hogbin > >> Can anyone explain this rotating disk problem from the point of view > >> of a moving observer? [quoted text clipped - 27 lines] > with nothing in between them to interfere with the straight line. This > does not make sense to David. Then what David should do, as he has been told many times, is to make sure he fully understands basic SR _in inertial frames_ with only _inertial motion_ involved. If he could demonstrate a sound understanding of Einstein's postulates, what an inertial frame is, the train experiment, and the pole and barn paradox the others would be much more willing to help him. I should point out that is no good his saying, 'Yes, yes, yes, I know all that', he must clearly demonstrate that he fully understands basic SR before anyone will accept that his questions are serious. Will he make a concerted effort to understand the basics and show that he has done so or will he continue as before? We shall see. Martin Hogbin > Will he make a concerted effort to understand the basics > and show that he has done so or will he continue as > before? We shall see. No reply so far from David. My money is on his posting another question on a new thread before he addresses the issues I have raised. Martin Hogbin > > Will he make a concerted effort to understand the basics > > and show that he has done so or will he continue as [quoted text clipped - 3 lines] > posting another question on a new thread before > he addresses the issues I have raised. Yes, give him 4 weeks. It has been like that for 10 years. Maybe this helps though ;-) Dirk Vdm > > > Will he make a concerted effort to understand the basics > > > and show that he has done so or will he continue as [quoted text clipped - 6 lines] > Yes, give him 4 weeks. > It has been like that for 10 years. Yes, I know. I spent ages trying to explain things to him. Even his protestations of genuine interest are several years old. > Maybe this helps though ;-) That was the idea. Every time, he seems to convince a new person that he is genuine. Martin Hogbin | > > Will he make a concerted effort to understand the basics | > > and show that he has done so or will he continue as [quoted text clipped - 10 lines] | | Dirk Vdm Understanding the basics... Maybe this helps though ;-) http://www.androcles01.pwp.blueyonder.co.uk/Dork/PartialDerivative.htm http://www.androcles01.pwp.blueyonder.co.uk/Dork/real.htm http://www.androcles01.pwp.blueyonder.co.uk/Dork/shit.htm Androcles > | > > Will he make a concerted effort to understand the basics > | > > and show that he has done so or will he continue as [quoted text clipped - 17 lines] > http://www.androcles01.pwp.blueyonder.co.uk/Dork/shit.htm > Androcles It surely helps completing my little collection: http://users.telenet.be/vdmoortel/dirk/Physics/Fumbles/PartialDiff4.html Thanks :-) Dirk Vdm | > | > > Will he make a concerted effort to understand the basics | > | > > and show that he has done so or will he continue as [quoted text clipped - 21 lines] | http://users.telenet.be/vdmoortel/dirk/Physics/Fumbles/PartialDiff4.html | Thanks :-) You are welcome, impotent pee puppy. :-) Androcles. Understanding the basics... Maybe this helps though ;-) http://www.androcles01.pwp.blueyonder.co.uk/Dork/PartialDerivative.htm http://www.androcles01.pwp.blueyonder.co.uk/Dork/real.htm http://www.androcles01.pwp.blueyonder.co.uk/Dork/shit.htm Why do you not try and explain David's problem to him. Then we could all have a laugh. Martin Hogbin | Understanding the basics... Maybe this helps though ;-) | [quoted text clipped - 4 lines] | Why do you not try and explain David's problem to him. | Then we could all have a laugh. Nah... Yours is funny enough. Show me a relativist and I'll show you a "constant velocity" is there and back again with a reversal of direction. Androcles. "Martin Hogbin" <goatREMOVETHIS123@hogbin.org> wrote in message news:IoqdnXx8lZ7b89_ZRVnyiA@bt.com... | "Hexenmeister" <vanquish@broom.Mickey_b> wrote in message news:Vws0g.73789$8Q3.40565@fe1.news.blueyonder.co.uk... | [quoted text clipped - 6 lines] | Why do you not try and explain David's problem to him. | Then we could all have a laugh. Nah... Yours is funny enough. Show me a relativist and I'll show you a "constant velocity" is there and back again with a reversal of direction. You have never explained anything to anyone. You cannot even say how you would measure the speed of a train. Martin Hogbin | "Martin Hogbin" <goatREMOVETHIS123@hogbin.org> wrote in message news:IoqdnXx8lZ7b89_ZRVnyiA@bt.com... || [quoted text clipped - 15 lines] | You have never explained anything to anyone. You cannot even | say how you would measure the speed of a train. Sure I can. You start a clock (tA) as the train leaves the station (A) and wait for it to return to the station (A) and stop the clock (t'A). The round trip time gives the speed, 2AB/(t'A-tA) = v. Everyone knows this is the correct way to measure speed. Androcles. "Martin Hogbin" <goatREMOVETHIS123@hogbin.org> wrote in message news:u--dnXvMT5YpON_ZRVnyiA@bt.com... | You have never explained anything to anyone. You cannot even | say how you would measure the speed of a train. Sure I can. You start a clock (tA) as the train leaves the station (A) and wait for it to return to the station (A) and stop the clock (t'A). The round trip time gives the speed, 2AB/(t'A-tA) = v. Everyone knows this is the correct way to measure speed. Physics 1/10 Humour 1/10 Is that the best you can do? Martin Hogbin | "Martin Hogbin" <goatREMOVETHIS123@hogbin.org> wrote in message news:u--dnXvMT5YpON_ZRVnyiA@bt.com... || [quoted text clipped - 11 lines] | | Is that the best you can do? Physics 1/10? You'd only give Einstein that much? I thought he was your fuckin' tin god... In agreement with experience we further assume the quantity 2AB/(t'A-tA) = c, to be a universal constant--the velocity of light in empty space. -- Einstein. Oh.. I forgot... you are an illiterate c.nt who can't fuckin' read. Androcles. "Martin Hogbin" <goatREMOVETHIS123@hogbin.org> wrote in message news:b5OdnWLBpZFUw9jZnZ2dnUVZ8qCdnZ2d@bt.com... | "Hexenmeister" <vanquish@broom.Mickey_b> wrote in message news:p7V0g.279495$zk4.31895@fe3.news.blueyonder.co.uk... | [quoted text clipped - 13 lines] | | Is that the best you can do? Physics 1/10? You'd only give Einstein that much? I thought he was your fuckin' tin god... In agreement with experience we further assume the quantity 2AB/(t'A-tA) = c, to be a universal constant--the velocity of light in empty space. -- Einstein. [Hogbin] You really are running scared. You honestly cannot tell me how you would measure the speed of a train. | "Martin Hogbin" <goatREMOVETHIS123@hogbin.org> wrote in message news:b5OdnWLBpZFUw9jZnZ2dnUVZ8qCdnZ2d@bt.com... || [quoted text clipped - 28 lines] | You really are running scared. You honestly cannot tell me how | you would measure the speed of a train. You really are a stupid c.nt. You honestly cannot tell me how you would measure the speed of anything. Androcles "Martin Hogbin" <goatREMOVETHIS123@hogbin.org> wrote in message news:RIadnRyjGIiJctjZRVnyqQ@bt.com... | [Hogbin] | You really are running scared. You honestly cannot tell me how | you would measure the speed of a train. [Androcles] You really are a stupid c.nt. You honestly cannot tell me how you would measure the speed of anything. [Hogbin] No progress on the physics then? Martin Hogbin | [Androcles] | You really are a stupid c.nt. You honestly cannot tell me how [quoted text clipped - 3 lines] | | No progress on the physics then? 0/0 for humour, 0/0 for physics. You start a clock (tA) as the train leaves the station (A) and wait for it to return to the station (A) and stop the clock (t'A). The round trip time gives the speed, 2AB/(t'A-tA) = v. Every relativist c.nt who licks Einstein's dick knows this is the correct way to measure speed. You really are a stupid whore. You honestly cannot tell me how you would measure the speed of anything. Androcles. "Martin Hogbin" <goatREMOVETHIS123@hogbin.org> wrote in message news:JsidnXWDaLvcBNvZnZ2dnUVZ8tydnZ2d@bt.com... | [Androcles] | You really are a stupid c.nt. You honestly cannot tell me how [quoted text clipped - 3 lines] | | No progress on the physics then? 0/0 for humour, 0/0 for physics. You start a clock (tA) as the train leaves the station (A) and wait for it to return to the station (A) and stop the clock (t'A). The round trip time gives the speed, 2AB/(t'A-tA) = v. Every relativist c.nt who licks Einstein's dick knows this is the correct way to measure speed. You really are a stupid whore. You honestly cannot tell me how you would measure the speed of anything. Offensiveness 3/10 but still 1/10 for physics. It is not a trick question. I am simply asking how _you_ would measure the speed of a train. Martin Hogbin | "Martin Hogbin" <goatREMOVETHIS123@hogbin.org> wrote in message news:JsidnXWDaLvcBNvZnZ2dnUVZ8tydnZ2d@bt.com... || [quoted text clipped - 20 lines] | | It is not a trick question. Yes it is. 10/10 for lying. | I am simply asking | how _you_ would measure the speed of a train. All speeds are relative. If I'm riding the train I wouldn't bother, the answer is trivially zero. Otherwise v = dx/dt and the speed is |v|. What precision will you pay for? Androcles. >> >> Can anyone explain this rotating disk problem from the point of view >> >> of a moving observer? [quoted text clipped - 34 lines] >If he could demonstrate a sound understanding of Einstein's >postulates, I cannot demonstrate a clear "understanding" of Einstein's postulates. I know the two main hypotheses stated in relativity are that all physical laws are the same in any given inertial reference frame and that the speed of light is constant and independent of the motion of the emitting source. The translated text I read actually used the word "velocity" of light instead of speed. We all know the velocity of light (speed and direction) must vary with the motion of the light source but the speed can possibly be constant. Although stated as a definition and not as a hypothesis Einstein states that "time" at two points cannot be defined at all unless the "time" required to travel from A to B equals the "time" required to travel from B to A. I readily admit that this statement of time that Einstein characterizes as "true by definition" seems to me more like a hypothesis than something true by definition. >what an inertial frame is, An inertial frame is one in which no accelerations or higher order changes in position of objects occur. In in a real sense we do not find inertial frames except in rare situations since any kind of motion causes the frame to accelerate slightly. But these are negligible in a practical sense. > the train experiment, I'm not certain which train experiment you are referring to. To demonstrate that one inertial frame is identical to every other frame there is a real life experiment where the acceleration of a train is so small that passengers on either of two trains cannot easily tell which train is moving out of the station or not. This is not much of a physics experiment since it merely demonstrates lack of needed sensitivity in measuring devices, so I don't know if this is the experiment you are referring to or not. >and the pole and barn paradox the others would be much >more willing to help him. The pole and barn paradox. What am I supposed to do here? Plug in values in the time and length separation formulas to show in one inertial frame where the two door closings are simultaneous and the pole just fits gives values that in another inertial frame the two doors open and close at different times and this is compensated precisely by the legnth contraction? Maybe you want me to plug in the length contraction, and then add the product of the time difference & V (distance traveled) to see if I get the correct number. This only demonstrates an understanding of algebra. >I should point out that is no good his saying, 'Yes, yes, yes, >I know all that', he must clearly demonstrate that he >fully understands basic SR before anyone will accept that >his questions are serious. I suspect this posting still won't result in any physics answers to my two cylinder questions I posted - this one and the one 4/10/2006. But we'll see. David >Will he make a concerted effort to understand the basics >and show that he has done so or will he continue as >before? We shall see. > >Martin Hogbin > >If he could demonstrate a sound understanding of Einstein's > >postulates, [quoted text clipped - 6 lines] > of light (speed and direction) must vary with the motion of the light > source but the speed can possibly be constant. So, in what way do you not 'understand' Einstein's postulates? > Although stated as a > definition and not as a hypothesis Einstein states that "time" at two [quoted text clipped - 3 lines] > as "true by definition" seems to me more like a hypothesis than > something true by definition. I have not come across this statement before. Can you tell me where he says this. > >what an inertial frame is, > An inertial frame is one in which no accelerations or higher order > changes in position of objects occur. In in a real sense we do not > find inertial frames except in rare situations since any kind of > motion causes the frame to accelerate slightly. But these are > negligible in a practical sense. Indeed. Some frames, notably that of the Earth, can be considered inertial for some purposes (such as the MMX) and not for others (laser giro). It is important to make clear at the start whether a given frame can be considered inertial for the purposes of the proposed discussion. > > the train experiment, > I'm not certain which train experiment you are referring to. That surprises me somewhat. I suspect that you are trying to make some oblique point. > To > demonstrate that one inertial frame is identical to every other frame [quoted text clipped - 3 lines] > a physics experiment since it merely demonstrates lack of needed > sensitivity in measuring devices, This seems to be a perverse way of stating what you have said more clearly above. We have agreed that inertial frames are idealisations, just like massless, inextensible strings and point masses. As I have said above you must make clear whether a given frame is to be considered inertial at the start of a discussion. > so I don't know if this is the > experiment you are referring to or not. No, I meant Einstein's famous thought experiment that demonstrates that (if you accept his postulates) two (spacelike separated) events that are simultaneous in one inertial frame of reference may not be simultaneous in another inertial frame of reference. Is it coming back to you? > >and the pole and barn paradox the others would be much > >more willing to help him. > The pole and barn paradox. What am I supposed to do here? Plug in > values in the time and length separation formulas to show in one [quoted text clipped - 5 lines] > V (distance traveled) to see if I get the correct number. This only > demonstrates an understanding of algebra. Why do you say that? Is your real problem accepting that the Lorentz transformations are part of physics? If that is the case the I am happy to discuss that subject with you. There really is no point in discussing accelerations until you are completely happy with inertial motion > I suspect this posting still won't result in any physics answers to my > two cylinder questions I posted - this one and the one 4/10/2006. I note you have slipped the word 'physics' in. Does that mean that you will not allow any mathematics to be used? One further comment. You have been asking your particular brand of question here for many years yet I have never seen you say, 'Yes I see, that answers my question'. The obvious inference is that you are deeply suspicious of SR and do not accept any of the answers you get. This means that you would be much better going back a bit and finding out where your problem with SR lies. On the other hand, your questions might be useful for undergraduate examinations. Martin Hogbin >> >If he could demonstrate a sound understanding of Einstein's >> >postulates, [quoted text clipped - 19 lines] >I have not come across this statement before. Can you tell me >where he says this. This is the statement in the common translated version of his original paper on special relativity titled "On the Electrodynamics of Moving Bodies" published in 1905. >> >what an inertial frame is, > [quoted text clipped - 16 lines] >That surprises me somewhat. I suspect that you are trying >to make some oblique point. You should quit reading motives into what I post in simply read what I say. I really don't know what train experiment you are referring to. I don't see why you should be surprised and think I'm making some oblique point. Shouldn't you be more surprised you haven't read Einstein's original paper on relativity (or at least you say you don't remember the time definition statement from it). >> To >> demonstrate that one inertial frame is identical to every other frame [quoted text clipped - 20 lines] > >Is it coming back to you? No, I haven't read that particular thought experiment. Can you give me the reference where Einstein describes this? >> >and the pole and barn paradox the others would be much >> >more willing to help him. [quoted text clipped - 14 lines] >point in discussing accelerations until you are completely happy >with inertial motion I follow what those particular equations are saying but as to whether I am "completely happy" with them as you put it - I haven't enough knowledge yet to determine whether or not they accurately describe our physical universe. >> I suspect this posting still won't result in any physics answers to my >> two cylinder questions I posted - this one and the one 4/10/2006. > >I note you have slipped the word 'physics' in. Does that mean that >you will not allow any mathematics to be used? The mathematics show that rigid rods can change shape in one frame and not another. The mathematics show rigid rods can be stretched and change length in one frame but not another. I don't understand the physics concept that a rod that is stretched to its breaking point can have its length increased in some frames. I don't understand the physics concept that a rod that breaks when it is twisted can be twisted without breaking in some frames. As in the posting of the problem in this thread I don't see how increasing the tension on a straight wire causes the wire to curve, but only if its moving (that's what the math shows), nor do I understand how with a maleable wire that is in the shape of a spiral why stretching the wire doesn't cause the wire to straighten out. That's what the math shows - the wire doesn't straighten. I don't understand the physics part. >One further comment. You have been asking your particular brand >of question here for many years yet I have never seen you say, 'Yes >I see, that answers my question'. The obvious inference is that you >are deeply suspicious of SR and do not accept any of the answers >you get. This means that you would be much better going back a bit >and finding out where your problem with SR lies. In general when a post answers my questions I say Yes. But as you know this group hates that. In one post I asked David McAnally a math question. He replied with a simple, clear understandable answer that made the answer incredibly obvious. When I responded with a Thank You very much at how simple he made things, there were rash of posts about with the standard comments about stupidities, alterior motives, trolling, oblique points, etc. But in general I want to apply the answers in a consistant manner and as you know if I post a problem and a reply seems to answer the posted question but conflicts with some other question that had been answered, if I ask well why doesn't it apply to this other situation the response typically is you are changing the problem or posters start making comments about trolling or being an idiot or whatever - that leaves me with conflicting and unaswered questions so I re-examine my posting and see if I can simplify the problem so that I get a single clear answer to a concept. David >On the other hand, your questions might be useful for undergraduate >examinations. > >Martin Hogbin > >> >If he could demonstrate a sound understanding of Einstein's > >> >postulates, [quoted text clipped - 13 lines] > >> points cannot be defined at all unless the "time" required to travel > >> from A to B equals the "time" required to travel from B to A. > >I have not come across this statement before. Can you tell me > >where he says this. > This is the statement in the common translated version of his original > paper on special relativity titled "On the Electrodynamics of Moving > Bodies" published in 1905. Where in the paper does Einstein your quote come from? > >> > the train experiment, > > [quoted text clipped - 21 lines] > > > >Is it coming back to you? > No, I haven't read that particular thought experiment. Can you give > me the reference where Einstein describes this? I believe that it was published in the popular press rather than in a scientific journal. Search Google for Einstein's train experiment. > >> >and the pole and barn paradox the others would be much > >> >more willing to help him. [quoted text clipped - 5 lines] > >> doors open and close at different times and this is compensated > >> precisely by the length contraction? That is a good start. What about the case where there is a brick wall at the far end of the barn? > >> Maybe you want me to plug in the > >> length contraction, and then add the product of the time difference & [quoted text clipped - 6 lines] > >point in discussing accelerations until you are completely happy > >with inertial motion > I follow what those particular equations are saying but as to whether > I am "completely happy" with them as you put it - I haven't enough > knowledge yet to determine whether or not they accurately describe our > physical universe. Where have you looked for this knowledge? The evidence is overwhelming. The physics FAQ has a list of experiments done specifically to test SR. In all cases the experimental results were what was predicted by theory. There are also some everyday uses of relativity (CRT design, GPS). There has never been a reproducible experiment that disagrees significantly with relativity. > >> I suspect this posting still won't result in any physics answers to my > >> two cylinder questions I posted - this one and the one 4/10/2006. > > > >I note you have slipped the word 'physics' in. Does that mean that > >you will not allow any mathematics to be used? > The mathematics show that rigid rods can change shape in one frame and > not another. The mathematics show rigid rods can be stretched and > change length in one frame but not another. That is correct. > I don't understand the > physics concept that a rod that is stretched to its breaking point can > have its length increased in some frames. Sticking to inertial frames, the length of a rod is measured to be different in a relatively moving frame. Length itself is measured to be different. In the rod's rest frame nothing changes - no stretching no bending, no stress. Are you happy with that. > I don't understand the > physics concept that a rod that breaks when it is twisted can be [quoted text clipped - 5 lines] > the wire to straighten out. That's what the math shows - the wire > doesn't straighten. What maths are you using? You need to bear in mind that rotation involves non-inertial motion. > I don't understand the physics part. I suggest that you make sure that you fully understand the physics of inertial frames before considering rotation. > In general when a post answers my questions I say Yes. But as you > know this group hates that. In one post I asked David McAnally a math [quoted text clipped - 11 lines] > unaswered questions so I re-examine my posting and see if I can > simplify the problem so that I get a single clear answer to a concept. You must do things in the right order. I say again, make sure that you understand all the physics relating to inertial frames first. From your comments above you obviously do not. Martin Hogbin | > >> >If he could demonstrate a sound understanding of Einstein's | > >> >postulates, [quoted text clipped - 22 lines] | | Where in the paper does Einstein your quote come from? Too dumb to read, Hogbin? Section f.cking I, paragraph 6, you stupid c.nt. Androcles. Too dumb to read, Hogbin? Section f.cking I, paragraph 6, you stupid c.nt. Thank you, you are too kind. Martin Hogbin | Too dumb to read, Hogbin? | Section f.cking I, paragraph 6, you stupid c.nt. | | Thank you, you are too kind. | | Martin Hogbin I know I am. You fit right in with that other detestable c.nt Van de merde. He can't read either. http://www.androcles01.pwp.blueyonder.co.uk/Dork/shit.htm Poe can't read, "Uncle Al" couldn't read. Proof Poe cannot read: "Where in that equation did (c+v) occur?" -- Poe the blind man. Proof "Uncle Al" cannot read: "BTW, you f.ck-faced baboon, "(c+v) appears nowhere in the paper, nor could it. Hey Androcyst, you are an ineducable idiot. Your high school should be leveled and replaced by an abandoned bowling alley." --Schwartz the f.cking imbecile. Illiteracy is a common trait amongst relativist shitheads. Someone like David tries to educate you but as usual you've got your head up your arse trying to appear clever; the only morons that will think you are clever are similar imbeciles to yourself. "Where in the paper does Einstein your quote come from?" - Hogbin. Hurry up and snip, illiterate jerk, some might read it. Androcles. >> >> >If he could demonstrate a sound understanding of Einstein's >> >> >postulates, >> >> I cannot demonstrate a clear "understanding" of Einstein's >> >> postulates. I know the two main hypotheses stated in relativity [quoted text clipped - 53 lines] > a scientific journal. Search Google for Einstein's train > experiment. It's in the book "Relativity", by Albert Einstein, available from Dover for about two bucks (used). He introduces the train and embankment pretty early and uses it off and on throughout the book, but I think the passage that's closest to what you're talking about is Chapter IX, "The Relativity of Simultaneity". The book is an elementary introduction to SR with a few words on GR. It's old, but IMHO it's still worthwhile for anyone who wants a reasonably simple introduction to the subject which doesn't use (any!) advanced math. (David, please take note!) Einstein was quite a clear writer when he put his mind to it (and when he wasn't writing for a scientific journal). It's not oversimplified, and aside from the presentation of Minkowski's geometry (which is outdated) he doesn't leave you with a lot of baggage you'll need to "unlearn" later. [NB -- The text is also available for free download from Project Gutenberg, but their version doesn't have the illustrations and the equations are a mess IIRC. So, don't go that way -- if you've got any interest in the text at all buy the physical book.] >> >> >and the pole and barn paradox the others would be much more >> >> >willing to help him. [quoted text clipped - 95 lines] > > Martin Hogbin  SignatureNospam becomes physicsinsights to fix the email I can be also contacted through http://www.physicsinsights.org > >> >> >If he could demonstrate a sound understanding of Einstein's > >> >> >postulates, [quoted text clipped - 77 lines] > equations are a mess IIRC. So, don't go that way -- if you've got any > interest in the text at all buy the physical book.] Fwiw, imo the HTML version is quite okay. I compared it with an official edition of the real book and it looks really good. Download from http://www.gutenberg.org/etext/5001 Dirk Vdm >>> >> >If he could demonstrate a sound understanding of Einstein's >>> >> >postulates, [quoted text clipped - 77 lines] >equations are a mess IIRC. So, don't go that way -- if you've got any >interest in the text at all buy the physical book.] Thank You. David >>> >> >and the pole and barn paradox the others would be much more >>> >> >willing to help him. [quoted text clipped - 95 lines] >> >> Martin Hogbin >> >> >If he could demonstrate a sound understanding of Einstein's >> >> >postulates, [quoted text clipped - 68 lines] >That is a good start. What about the case where there is a brick wall >at the far end of the barn? You need to describe this problem in more detail. I don't see the point in repeatedly applying the same equations and getting the same results. THE EQUATIONS RELATING TIME-SPACE IN DIFFERENT INERTIAL FRAMES ARE SELF-CONSISTENT. I DON'T HAVE A PROBLEM WITH THAT. >> >> Maybe you want me to plug in the >> >> length contraction, and then add the product of the time difference & [quoted text clipped - 42 lines] > >Are you happy with that. I apparently wasn't clear enough in my statement as you did not address the difficulty I have. What I don't understand is that the length in one frame can be a constant L = C for all time while in another frame the length doesn't remain constant. It could be L' = K1 at one time while L' = K2 at some other time where K1 does not equal K2 while all the time in another frame the length remains constant. I don't know of a case where this occurs without accelerations, so I agree with you that SR equations are self-consistant when objects do not undergo an acceleration. ALTHOUGH YOU KEEP SAYING I NEED TO WORK ON ONLY INERTIAL FRAMES, I DON'T SEE CONFLICTS WITH THE PHYSICS CONCEPTS WHEN OBJECTS DO NOT ACCELERATE. >> I don't understand the >> physics concept that a rod that breaks when it is twisted can be [quoted text clipped - 8 lines] >What maths are you using? You need to bear in mind that rotation >involves non-inertial motion. Well thank you for clarifying that a change of velocity is non-inertial. The problem has to do with rotations or other accelerations which you feel are verboten for someone with my limited skills. But anyway here's the type of problem. If I have a rotating cylinder with longitudinal axis along the x-axis, and I have a straight rigid rod near the surface of the cylinder and parallel to the x-axis when all points of the rod are attached to the surface at time t0 as measured in the rest frame of the rod and cylinder, the rod rotates with the cylinder, but the rod remains a straight line. In a frame moving with velocity V along the x-axis, this straight line is a spiral that wraps itself around the cylinder. Now in the rest frame I can increase the tension (stretch the rigid rod) and it remains in a straight line. But from the moving frame point of view this is contrary to what happens if we try to stretch a coiled rod that has no motion in the x direction relative to our frame. Why do the moving frame observers say the coiled rod doesn't try to straighten into a straight line as a stretching force is applied to the coiled rod. And the other version of this (as I originally posted here) is where all the points of the rod are attached to the cylinder simultaneously as measured in the moving frame. In this case the rod is measured in the moving frame to be a straight line on the surface of the cylinder (and a spiral wrapped around the cylinder in the rest frame of the cylinder). Now as we increase the tension of the rod (we stretch the rod), the center point of the rod moves toward the center of the cylinder, cutting into the cylinder. How do moving frame observers explain this. In their frame the rod is a straight line but stretching the rod causes it to curve. So we see that whether a straight rod remains on the surface of a rotating cylinder in the moving frame depends on what materials the rod and cylinder are made of. An even more puzzling thing is when the moving frame observer simultaneously attaches all points straight rod to the surface of a cylinder along the cylinder's longitudinal axis. Let the rod continue to be a straight line as measured in this frame. The rod rotates with the cylinder. Now we do the identical thing with another cylinder and another rod. The same thing occurs. But now we cut this second straight rod into N very short segments. The two rods are identical in terms of length and shape and mass but one is composed of segments just touching each other where as the other has chemical bonds at each of this same corresponding points. At time t0 as measured in the moving frame we release all points of contact of the rigid rod from its rotating cylinder, and we release all points of contact of the segmented rod from the rotating cylinder. The segmented rod segments go flying off the cylinder. Whereas the solid connected rod does not. It continues to rotate the x axis just as if it were still attached to the cylinder. As viewed in the moving frame, what is the difference between these two situations. That's what I don't understand, but those involve accelerations David. >> I don't understand the physics part. > [quoted text clipped - 22 lines] > >Martin Hogbin [David] > >> >> I cannot demonstrate a clear "understanding" of Einstein's postulates. [Martin] > >> >So, in what way do you not 'understand' Einstein's postulates? [David] > >> >> Although stated as a > >> >> definition and not as a hypothesis Einstein states that "time" at two [quoted text clipped - 3 lines] > >What about the case where there is a brick wall > >at the far end of the barn? > You need to describe this problem in more detail. OK. A man with a 20m foot long pole runs into a 11 m barn with a brick wall at the far end and closes the door behind him. In the frame of refernce of the barn, the pole is measured to be 10 m long and thus fits in the barn, allowing the door to be closed. Explain how this is achieved from the point of view of the man with the pole. [David] THE EQUATIONS RELATING TIME-SPACE IN DIFFERENT INERTIAL FRAMES ARE SELF-CONSISTENT. I DON'T HAVE A PROBLEM WITH THAT. [Martin] > >> >Is your real problem accepting that the > >> >Lorentz transformations are part of physics? If that is the case > >> >the I am happy to discuss that subject with you. There really is no > >> >point in discussing accelerations until you are completely happy > >> >with inertial motion > >> I follow what [the Lorentz transformations] are saying but as to whether > >> I am "completely happy" with them as you put it - I haven't enough > >> knowledge yet to determine whether or not they accurately describe our > >> physical universe. [Martin] > >Where have you looked for this knowledge? The evidence is > >overwhelming. [David] > I apparently wasn't clear enough in my statement as you did not > address the difficulty I have. What I don't understand is that the [quoted text clipped - 8 lines] > ON ONLY INERTIAL FRAMES, I DON'T SEE CONFLICTS WITH THE PHYSICS > CONCEPTS WHEN OBJECTS DO NOT ACCELERATE. So let me get this clear. You are saying here that you fully understand relativity when applied to inertial frames and objects? I find that hard to takein view of your comments above. If you really are happy with the relativity of inertial objects in inertial frames then you should study linear accelerating objects in inertial frames. Try this link: http://math.ucr.edu/home/baez/physics/Relativity/SR/spaceship_puzzle.html Then look at rotating discs. Perhaps you will then be able to solve your own problems. > >> I don't understand the > >> physics concept that a rod that breaks when it is twisted can be [quoted text clipped - 7 lines] > > > >What maths are you using? You have not answered that question. Martin Hogbin | If you really are happy with the relativity of inertial objects in | inertial frames then you should study linear accelerating | objects in inertial frames. ahahahaha.. HAHAHAHA.. hahaha.. http://www.androcles01.pwp.blueyonder.co.uk/Dork/inertialacc.htm Try this link: | http://math.ucr.edu/home/baez/physics/Relativity/SR/spaceship_puzzle.html ahahahaha.. HAHAHAHA.. hahaha.. "To begin, a statement of the paradox--and if you notice some ambiguities in my formulation, that's the point! (That's always the point in SR paradoxes.) " Thanks for the laughs, Pigbin. Hey dork... show me where it says "inertial" in http://www.fourmilab.ch/etexts/einstein/specrel/www/ Androcles >> >If he could demonstrate a sound understanding of Einstein's postulates, >> I cannot demonstrate a clear "understanding" of Einstein's >> postulates. I know the two main hypotheses stated in relativity are [quoted text clipped - 18 lines] > I have not come across this statement before. Can you tell me where > he says this. C'mon, Martin, lighten up, everybody who reads this group regularly knows this one. It's from Electrodynamics, 1905, section 1.1, definition of simultaneity, about halfway through the section. It's near the top of P. 40 in the frequently quoted Dover edition. Einstein says: "We have not defined a common 'time' for A and B, for the latter cannot be defined at all unless we establish _by_ _definition_ that the 'time' required by light to travel from A to B equals the 'time' it requires to travel from B to A." (Einstein's emphasis.) On the other hand D.Seppala still hasn't responded to post http://groups.google.com/group/sci.physics.relativity/msg/c870593c186fe533?hl=en& (that's message ID <pan.2006.03.20.20.10.44.536824@nospam.org> in case the link doesn't work) which is kind of a disappointment; I thought we were getting somewhere. Maybe he thought my tone was too insulting. (And maybe it was...) >> >what an inertial frame is, > [quoted text clipped - 77 lines] > > Martin Hogbin SignatureNospam becomes physicsinsights to fix the email > >> Although stated as a definition and not as a hypothesis Einstein > >> states that "time" at two points cannot be defined at all unless [quoted text clipped - 17 lines] > the 'time' required by light to travel from A to B equals the 'time' > it requires to travel from B to A." (Einstein's emphasis.) Thanks for that, I did not recognise the quote. David missed out the context and the word 'light' which makes his words somewhat nonsensical. I can now attempt to address his difficulty. > On the other hand D.Seppala still hasn't responded to post > [quoted text clipped - 6 lines] > somewhere. Maybe he thought my tone was too insulting. (And maybe > it was...) As you probably know, David has been posting the same kind of question for ten years or so. It is not unknown for him to suddenly break off when you try to test him on his basic understanding. He is still something of a mystery to me. Martin Hogbin >>> >> Can anyone explain this rotating disk problem from the point of view >>> >> of a moving observer? [quoted text clipped - 70 lines] > different times and this is compensated precisely by the legnth > contraction? Yes, this would be a great start. Actually do the calculation and post the result, thus demonstrating that you've gotten to first base. You frequently request others to post calculations for you. Do one yourself, and post it. If you already understand it, it won't be hard to do and won't take much time. If you _don't_ already understand it, you'll actually learn something useful from it. So, the exercise will be either easy or valuable (or both) and it'll impress everybody who's currently throwing tomatoes at you, and give you a chance to say "See I told you all along I understood (at least) this much of it!". So, yeah, do it, Dave! > Maybe you want me to plug in the length contraction, and > then add the product of the time difference & V (distance traveled) to [quoted text clipped - 14 lines] >> >>Martin Hogbin SignatureNospam becomes physicsinsights to fix the email [snip] > > The pole and barn paradox. What am I supposed to do here? Plug in values > > in the time and length separation formulas to show in one inertial frame [quoted text clipped - 17 lines] > > So, yeah, do it, Dave! You are one h*ll of an optimist. Good for you :-) Dirk Vdm >>>> >> Can anyone explain this rotating disk problem from the point of view >>>> >> of a moving observer? [quoted text clipped - 85 lines] > >So, yeah, do it, Dave! Actually in the train problem I posted you said to show some numbers so I did and you did not reply. In the battery problem I posted my explanation Harald replied saying he found my error but it appeared he mis-interpreted what I had said. So I clarified the response and he no longer replied. I don't know how to search for archived threads and my reader/service only shows the past 30 days or so, so I can't get the full thread to post as a reference for you. David >> Maybe you want me to plug in the length contraction, and >> then add the product of the time difference & V (distance traveled) to [quoted text clipped - 14 lines] >>> >>>Martin Hogbin >>>>> >> Can anyone explain this rotating disk problem from the point of >>>>> >> view [quoted text clipped - 101 lines] > no longer replied. > I don't know how to search for archived threads Easily fixed http://groups.google.com/group/sci.physics.relativity?lnk=srg Now no more excuses. Bill >and my > reader/service only shows the past 30 days or so, so I can't get the [quoted text clipped - 21 lines] >>>> >>>>Martin Hogbin [snip] > >>So, yeah, do it, Dave! > > Actually in the train problem I posted you said to show some numbers [quoted text clipped - 8 lines] > > Now no more excuses. And for instance: http://groups.google.com/groups/search?q=author%3Adseppala@austin.rr.com+harald Now even less excuses ;-) Dirk Vdm > >>>> >> Can anyone explain this rotating disk problem from the point of view > >>>> >> of a moving observer? [quoted text clipped - 94 lines] > full thread to post as a reference for you. > David Hmm, your reply didn't show up in my newsreader. I now do see your reply, but it doesn't appear as if I significantly misinterpreted what you said. I'm not sure to recall the whole story correctly, but Bill's above remark seems to be spot-on, for you wrote (if I'm not mistaken) about an *accelerating* point: "From the moving end's point of view [...] "time" is measured to be slower at the other end". That's no good: such statements are valid for inertial frames, but not in general for accelerating points. Harald >> >>>> >> Can anyone explain this rotating disk problem from the point of >view [quoted text clipped - 117 lines] >*accelerating* point: "From the moving end's point of view [...] "time" is >measured to be slower at the other end". In that problem the rates were constant, and then there was an acceleration as in the twin's paradox and constant rates were resumed. >That's no good: such statements are valid for inertial frames, but not in >general for accelerating points. > >Harald SNIP > >> Actually in the train problem I posted you said to show some numbers > >> so I did and you did not reply. In the battery problem I posted my [quoted text clipped - 5 lines] > >> full thread to post as a reference for you. > >> David > >Hmm, your reply didn't show up in my newsreader. I now do see your reply, > >but it doesn't appear as if I significantly misinterpreted what you said. > >I'm not sure to recall the whole story correctly, but Bill's above remark > >seems to be spot-on, for you wrote (if I'm not mistaken) about an > >*accelerating* point: "From the moving end's point of view [...] "time" is > >measured to be slower at the other end". > In that problem the rates were constant, and then there was an > acceleration as in the twin's paradox and constant rates were resumed. OK, thanks for refreshing my memory! Then here we face *again* the problem that you made no real calculation, leaving only place for vague discussions. If I understand well, you switched back to the more general problem of mutual time dilation, and you seem to think that that doesn't work out for the case of passengers entering a moving train and leaving it on one end. If you repost that problem *with a full calculation* (according to SRT!) and still obtain a contradiction then I and others are very likely to be willing to have another look at it. And in case you manage to solve it, that will be worth a posting too - just to show us that all this talking wasn't in vain! Harald > >That's no good: such statements are valid for inertial frames, but > >not in general for accelerating points. > > > >Harald >>>>> >> Can anyone explain this rotating disk problem from the point >>>>> >> of view of a moving observer? [quoted text clipped - 89 lines] >> >>So, yeah, do it, Dave! [David said:] > I don't know how to search for archived threads and my > reader/service only shows the past 30 days or so, so I can't get the > full thread to post as a reference for you. David Aha, this comment explains a lot. Two things. a) If your news service has retention extending back just 30 days, chances are it's missing a lot of messages right from the start, as well; that may be why you don't always see them. SO.... you should use Google Groups to see the full set of messages (because they're more reliable than your usual news server), and if you want to be sure everybody sees what you post, you should post through Google as well. b) You can access archived messages for (at least) several years through Google. You've been given a couple links but maybe you need something more. Here's a link + some directions: Go here: http://groups.google.com/advanced_search Enter "sci.physics.relativity" in the "Group" field Enter words from the subject, in the "Subject" field Enter the Author if you want. Then click the _UPPER_ "Google Search" button (the one at the page bottom which says "Lookup Message" is only for message ID searches). Once you get a message whose thread you're interested in from the search results, go to the top of the web page, find the link "view as tree" just below the bold-faced topic header, and click it. That will give you the whole thread. And that's that. [And David said:] > Actually in the train problem I posted you said to show some numbers > so I did and you did not reply. If you mean the "rubber train" problem ... If I recall correctly, I asked you to show, mathematically, that you could build a train that could be stretched arbitrarily (like, to 10x its original length, maybe?) without breaking, yet which would collapse if one additional passenger stepped on board. As far as I can recall you made no attempt to do so, but just asserted that you could. Since I very much doubt that it's even possible, showing it seems important. You also seemed to have lost track of whether you had the front and back of the train glued together or not, and you never clarified that. Frankly, that problem was pretty ridiculous, and I was kind of losing interest anyway. However, if you think you showed that it made sense, post a link to the message, but I don't recall ever seeing any calculations from you on that one, just assertions. [David:] > In the battery problem I posted my explanation Harald You replied to Harald, but you ignored _my_ last reply to you on the battery problem, which problem is actually a lot more interesting than the rubber train, IMHO. Maybe you didn't see it. You should, again, use Google if you really want to know whether someone replied to you or not. They seem to be well-connected to the whole web; they hardly ever drop messages. > replied saying he found my error but it appeared he mis-interpreted > what I had said. So I clarified the response and he no longer > replied. >>> Maybe you want me to plug in the length contraction, and >>> then add the product of the time difference & V (distance traveled) to [quoted text clipped - 14 lines] >>>> >>>>Martin Hogbin SignatureNospam becomes physicsinsights to fix the email I can be also contacted through http://www.physicsinsights.org > >> >> Can anyone explain this rotating disk problem from the point of view > >> >> of a moving observer? [quoted text clipped - 39 lines] > that the speed of light is constant and independent of the motion of > the emitting source. The you may be interested to read a paper by Ives, in which he actually *derived* the LT's from conservation of energy and momentum - and by which those "two main hypotheses" follow (I'll have to scan it, but will be happy to do so). > The translated text I read actually used the > word "velocity" of light instead of speed. We all know the velocity > of light (speed and direction) must vary with the motion of the light > source but the speed can possibly be constant. Just see the dictionary: "velocity" can also be synonym for "speed" - only in recent years it has increasingly been used as distinguished from speed. Language is full of such tricks... > Although stated as a > definition and not as a hypothesis Einstein states that "time" at two [quoted text clipped - 3 lines] > as "true by definition" seems to me more like a hypothesis than > something true by definition. He simply adopted the then existing convention as published by Poincare, who apparently agreed with Lorentz that this convention is generally *untrue* in reality. There have been endless discussions about this as some people can't understand that a convention is not required to be "really true", and on top of that, Einstein's view of "reality" was very peculiar... Harald >>> Can anyone explain this rotating disk problem from the point of view of >>> a moving observer? [quoted text clipped - 33 lines] > not understand the answers. If the problems are so uninteresting as one > person has posted here why does he read David's postings? Two reasons, really, or perhaps three. First, once in a while David posts a problem which I hadn't thought about, and which I find interesting. Quite some time back (last year, I think) David posted a contorted question about moving ammeters, and thinking about it led to some issues I found very interesting. Among other things, it was in the pursuit of that in which I learned that a resistor's value, as measured by a "stationary" observer, changes when the resistor is in motion. Second, David doesn't get abusive, so one can read and respond to his posts without getting showered with s*** in payment. Third, David constantly gives the (possibly deceptive) impression that he's _about_ to actually learn something from the responses. That is hard to resist... As to finding some of David's posts very "ho hum", as I think I've put it in the past, that's because David has shown a tendency to post the _same_ _thing_ over again, apparently without having made any progress at all in delving into the particular issue raised by the problem. The moving ammeter problems, while they were interesting to start with, have been hashed out pretty thoroughly in this NG in the past, partly as a result of David posting them. But David does not seem to have learned anything about them from those earlier discussions, and seems to want to start over at the beginning again every time. If, as has been suggested repeatedly, he tried harder to work things out for himself and to take replies to his posts and carry them forward by adding his own calculations and deductions rather than stopping at the first difficult spot, he might get farther. > If the answers > are so obvious why not simply post the correct interpretation and help > everyone? Why do you care about David's motives? After a while, observing David's apparently self-defeating behavior, and observing his lack of any apparent progress in understanding the issues, one must start to wonder about his motives. And besides, guessing motives is part of politics and "politics" among primitive primate groups, it has been suggested, is what led to the runaway evolution of the enormously overdeveloped human brain. So, guessing motives is among the most important traits which distinguishes human beings from the lower animals. If you expect people to interact with you without guessing at your motives you will consistently be disappointed. > David >> >>Martin Hogbin SignatureNospam becomes physicsinsights to fix the email > Can anyone explain this rotating disk problem from the point of view > of a moving observer? [quoted text clipped - 30 lines] > as viewed in the moving frame why the center of this straight wire > cuts the wax all the way through to x-axis as the wire is stretched? You may not realize just how complex this problem is. You are asking about the forces on and tension in a wire which is in motion, where the forces and tension are measured by a _stationary_ observer. For starters, you need to at least think about how a stationary observer would even _measure_ the tension in a wire that's in motion. The measurement is surely not going to be the same as the value measured in the wire's rest frame -- but we need to go beyond that simple assertion of what it _won't_ be. What _will_ the tension be, as viewed from the stationary frame? And what does it even _mean_? Until you've determined how you can measure the tension within an object which is in motion, without having the measurement apparatus co-move with the object, you don't have a working definition for "tension" in a moving object. The rotating lever paradox is difficult, and it involves just a couple of torques; what you've described here is even more complex than that. So I would suggest backing up and taking a running start. Go back to a simpler problem, and work out an analytic solution to that. Then tackle the more complex problem. As a general rule, what you need to do is begin by analyzing the problem completely in the most convenient frame-of-reference you can find. Typically, that's the center of mass frame. Here's an example of a simpler problem, similar to something you've posted in the past; I'll just sketch it (I'm sure you can fill in the details): Start with a spinning rod with a (straight) stripe on it. Viewed in a moving frame the stripe looks like a spiral. In the center of mass frame, let the stripe "fall off" and fly away. Now, describe _exactly_ what appears to happen in the moving frame. Post the answer. Working that through completely will help a lot with your later problems, I think. Then take the problem you've posted here, but instead of pulling the wire through the wax, release it so it can fly off. Figure out what happens in the center of mass frame (this may prove harder than you expect, even though it's "just" Newtonian mechanics!). Then map that into the frame in which the cylinder is moving, and tell us what the moving observer would see. Finally, figure out how to transform _tension_ and _force_ between frames. Figure out the tension and 4-force on the taught wire in the center of mass frame, and transform that to the moving frame. Tell us what you found: post the transformation equations. The latter is going to be difficult but would be a very useful exercise for you to do, and is a prerequisite for solving the problem you actually posted. Once you've done that, you can apply your transformations to the "straight" wire in the moving frame and see if you can correctly predict that it will cut through the wax. I, personally, have no plan to do this for you :-) but I'd be more than happy to look at (and, if possible, help with) any attempt at a solution you can come up with. > If you post an explanation, does the same explanation work when the > straight wire is simultaneously attached as measured in the rest > frame and then the wire is slowly stretched? After you post your explanation, we'll be glad to determine if it so applies. > That is, the wire is > stretched and stretched but it never cuts into the wax eventhough it > spirals around the cylinder as viewed in the moving frame. > > This physics result of SR seems non-sensical to me. Lots of results in SR seem nonsensical. Unfortunately for our "common sense" every such prediction which has been tested has been born out. God apparently doesn't care tuppence for whether reality behaves according to your -- or my -- common sense. > Thanks, > David Seppala >> Can anyone explain this rotating disk problem from the point of view >> of a moving observer? [quoted text clipped - 96 lines] >> straight wire is simultaneously attached as measured in the rest >> frame and then the wire is slowly stretched? The rotating cylinder problem is easy to visualize. I don't need an extensive math explanation just some simple verbage. Here's the same problem without using rotations. This version doesn't have the graphic visualizations of the rotating cylinder problem. I'll provide the simple physics verbage for the effect in the rest frame and perhaps you can provide the simple physics (or math) verbage in the moving frame which I will start. In the rest frame I have a long rectangular steel rod (like a long two by four) aligned on the x-axis. The two end surfaces of this rod are perpendicular to the x-axis. Perpendicular to the x-axis is a wide conveyer belt (as wide as the rod is long). The belt is moving with some low speed along the y-axis. Now I have a moving frame that has velocity V=0.866c along the x-axis with respect to this rest frame. At time t0 moving frame observesr simultaneously place all points of the rod on to the conveyer belt. The placement is parallel to the x-axis. Rest Frame View After the accelerations have stopped and the rod is moving at the same rate as the conveyer belt the rest frame observers note that the rod is no longer parallel to the x-axis. They say this occurred because one end of rod was placed on the conveyer belt before the other end (SR view). They also measure that the end surfaces of the rod are also no longer perpendicular to the x-axis. They note that the acceleration of the rod was so small that chemical bonds remained intact and the rod did not change shape. Moving Frame View Now what is the verbage the moving frame uses for the same events? Observers in the moving frame measure that long edges of the rod remains parallel to the x-axis. But they observe that the rod has changed shape. The end surfaces of the rod are no longer perpendicular to the x-axis, nor to the sides of the rod. What explanation is given by these observers to explain why the rod changed shape? That is what I'm trying to understand. David >After you post your explanation, we'll be glad to determine if it so >applies. [quoted text clipped - 13 lines] >> Thanks, >> David Seppala >>> Can anyone explain this rotating disk problem from the point of >>> view of a moving observer? [quoted text clipped - 101 lines] > The rotating cylinder problem is easy to visualize. I don't need an > extensive math explanation just some simple verbage. <g> OK, then let me ask you a simple question about it. Let's suppose the cylinder lies on the X axis. So, if the wire runs straight from one disk to the other, we would say that it makes an angle of 0 degrees with the X axis. If, on the other hand, it is in a circle around the cylinder -- not a spiral at all, just a single loop -- then it makes an angle of 90 degrees with the X axis. _IF_ the wire is to appear _STRAIGHT_ in the moving frame, what's the _MAXIMUM_ angle it can make with the X axis? Obviously, it can't cross the X axis at 90 degrees, since in that case it's just a circle, not a spiral. But can it cross the X axis at angles arbitrarily close to 90 degrees? In other words, can it form a really _tight_ spiral? Or is there some limit to the "tightness" of the spiral it can make, and still appear to be completely straight in the moving frame? Please post your answer. :-) Actually the problem of the spinning spiral is pretty cool, and I hope to have more to say about it later. The wax cylinder makes it more graphic but you don't really need it in order to have a very confusing problem -- the spiral of wire alone will do the job! > Here's the same problem without using rotations. No, no, let's stick with one problem at a time. Besides, the spiral problem is quite interesting all by itself, as I already said. The acceleration on the spiral of wire is the real killer -- the problem below hasn't got that aspect to it (which certainly makes it easier to deal with). > This version doesn't have the graphic visualizations of the rotating > cylinder problem. I'll provide the simple physics verbage for the [quoted text clipped - 29 lines] > What explanation is given by these observers to explain why the rod > changed shape? That is what I'm trying to understand. While I think about this one, you think about it too, and see if you can answer this simple question: Given that the rod appears parallel to the X axis in the "moving" frame, what is the _maximum_ angle the rod can (appear to) make with the X axis in the "stationary" frame? Can the rod appear to be arbitrarily close to an angle of 90 degrees with the axis, or is there some smaller upper bound on the angle? It's the same question as in the spiral case, of course, and once again it points up the fact that just imagining the objects is insufficient to understand what's going on. You should also think about ways you could answer _all_ questions about the rod's orientation and apparent shape. How would you go about it, if you _had_ to find the answer and nobody was here to work it out for you? Could you apply the Lorentz transforms in some way? Please think about that and try to post a procedure for doing it. SignatureNospam becomes physicsinsights to fix the email I can be also contacted through http://www.physicsinsights.org >>>> Can anyone explain this rotating disk problem from the point of >>>> view of a moving observer? [quoted text clipped - 124 lines] > >Please post your answer. :-) If the wire was arbitrarily thin and the cylinder's diameter was arbitrarily small, and the rotation rate of the cylinder was arbitrarily high then the angles can get arbitrarily close to 90 degrees. >Actually the problem of the spinning spiral is pretty cool, and I >hope to have more to say about it later. The wax cylinder makes it [quoted text clipped - 50 lines] >frame, what is the _maximum_ angle the rod can (appear to) make with >the X axis in the "stationary" frame? Don't have a calculator or paper here but basically to solve the problem for a given L there is a given time difference found by simply using the standard SR transform equations. We can then set the speed of the conveyer belt close to c to compute the perpendicular leg of the triangle to find the angle you seek. David >Can the rod appear to be arbitrarily close to an angle of 90 degrees >with the axis, or is there some smaller upper bound on the angle? > >It's the same question as in the spiral case, of course, and once >again it points up the fact that just imagining the objects is >insufficient to understand what's going on. The problem with the spiral wire is that I can cut the wire into arbitrarily short segments with the same shape and position as the one long continuous wire. The segments just touch each other but they don't have chemical bonds linking each touching segment. These segments can be held in place by some force that keeps the segments attached to the rotating cylinder. But when the force that keeps the wire and segmented wire attached to the cylinder disappears the segments go flying off whereas the same shaped continuous wire continues rotating about the cylinder as if it were still attached. That is what I don't understand. >You should also think about ways you could answer _all_ questions >about the rod's orientation and apparent shape. How would you go >about it, if you _had_ to find the answer and nobody was here to work >it out for you? Could you apply the Lorentz transforms in some way? > >Please think about that and try to post a procedure for doing it. [ snip ] >>> The rotating cylinder problem is easy to visualize. I don't need >>> an extensive math explanation just some simple verbage. [quoted text clipped - 21 lines] >> >>Please post your answer. :-) > If the wire was arbitrarily thin and the cylinder's diameter was > arbitrarily small, and the rotation rate of the cylinder was > arbitrarily high then the angles can get arbitrarily close to 90 > degrees. Then prove it. Pick a velocity, pick a rotation rate, write down the parameters, run the transformations, and show that there is a case where the wire can be wrapped such that it makes and angle of, say, 89 degrees in the center of mass frame but appears to make an angle of 0 degrees in the moving frame. If you're right, this should be easy. [David, on the rod-on-conveyor belt:] >>> In the rest frame I have a long rectangular steel rod (like a long >>> two by four) aligned on the x-axis. The two end surfaces of this [quoted text clipped - 32 lines] >>frame, what is the _maximum_ angle the rod can (appear to) make with >>the X axis in the "stationary" frame? > Don't have a calculator or paper here but basically to solve the > problem for a given L there is a given time difference found by > simply using the standard SR transform equations. We can then set > the speed of the conveyor belt close to c to compute the > perpendicular leg of the triangle to find the angle you seek. David I'm asking for an analytic answer, not a numeric one. You should be able to find it without using a calculator. >>Can the rod appear to be arbitrarily close to an angle of 90 degrees >>with the axis, or is there some smaller upper bound on the angle? >> >>It's the same question as in the spiral case, of course, and once >>again it points up the fact that just imagining the objects is >>insufficient to understand what's going on. > The problem with the spiral wire is that I can cut the wire into > arbitrarily short segments with the same shape and position as the one > long continuous wire. The segments just touch each other but they don't > have chemical bonds linking each touching segment. These segments can be > held in place by some force that keeps the segments attached to the > rotating cylinder. That's the force that makes the problem interesting, really. > But when the force that keeps the wire and segmented > wire attached to the cylinder disappears the segments go flying off > whereas the same shaped continuous wire continues rotating about the > cylinder as if it were still attached. That is what I don't > understand. Yeah it's a toughie. Haven't got a good answer to it (yet). >>You should also think about ways you could answer _all_ questions >>about the rod's orientation and apparent shape. How would you go [quoted text clipped - 3 lines] >> >>Please think about that and try to post a procedure for doing it. Please do! SignatureNospam becomes physicsinsights to fix the email I can be also contacted through http://www.physicsinsights.org > [ snip ] >>>>> [quoted text clipped - 38 lines] > >If you're right, this should be easy. In the velocity between the moving a |
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