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Natural Science Forum / Physics / Relativity / April 2006Feynman Lectures and Physics Teaching.
I have recently downloaded some very good lectures videotaped by MIT on basic physics and math http://ocw.mit.edu/OcwWeb/index.htm As an aside the following text on Calculus is very good and great for anyone that wants to learn basic physics: http://ocw.mit.edu/OcwWeb/Mathematics/18-013ASpring-2005/CourseHome/index.htm But to get back to the post I started listening to the following lectures on classical mechanics: http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/CourseHome/index.htm In the lecture on Newton's laws Professor Lewin states that while the definition of mass etc implied by Newton's second law may seem circular in realty it is not. He said consider the following - we have two identical bits of cheese - we can combine them together to create double the mass and so check Newton's second law. I thought what. Surely bright MIT students will realize this relies on the fact two equal masses combined together has twice the mass. Very intuitive and of course true but this is science and we do not accept things on face value. The reason two equal masses connected together has twice the mass is because of Newton's third law - push or pull it and one mass reacts on the other with the same force meaning you need twice the force to get the same acceleration hence it has twice the mass. It is not a test for the second law it is a test for the third law. Of course Feynman in his lectures goes to great care to explain exactly what is really going on - and concludes - IMHO correctly - the second law is still a law - but not quite what it appears on the surface. It is sort of a partial law - a statement of how we are to view mechanical problems - a prescription that says - get to the forces. What I don't understand is why lecturers, who probably should know better, still do not take Feynmans lead and teach it correctly. And even further why are the lectures not more widely used as a text? Sure they do not have problems sets and examples of worked out problems - but Schaums outline series is cheap and a does a great job of that. There in no explainer of physics better than Feynman IMHO - some as good but none better. Of course that leaves aside the issue of if it should be taught via Newton's laws in the first place: http://www.eftaylor.com/leastaction.html Thanks Bill > In the lecture on Newton's laws Professor Lewin states that while the > definition of mass etc implied by Newton's second law may seem circular in > realty it is not. He said consider the following - we have two identical > bits of cheese - we can combine them together to create double the mass and > so check Newton's second law. Discovering the addition of two masses to get the sum of the masses is not addressing the definition of mass. You need to derive mass from the Lagrangian to appreciate what mass really is. However, when the Lagrangian that yields the field equations is accepted for the face value, it contains the mass term. Thus, the concept of mass becomes circular under the current interpretation to GR. > ...this is science and we do not accept things on face value. What does this 'we' include? In this message, your buddy Bilge wrote the following. http://groups.google.com/group/sci.physics.relativity/msg/6833440fc4e66c2e?dmode =source&hl=en Excerpt: "In other words, you are the lone genius who discovered that general relativity is wrong because general relativity correctly predicts the perihelion shift of mercury and your proof consists of being unable to get the same result that tens of thousands of people have otained over the last 75 years? Saaay - that really does sound convincing..." It sounds like this person is taking everything that the establishment wants him to know as face value. In another words, you don't have to understand physics. Just let the 'experts' understand it for you. Thus, what you wrote also has a circular meaning. As long as the 'experts' can understand it for you, you are not taking it for the face value. Isn't that comforting, Bill? > [...] > > Of course that leaves aside the issue of if it should be taught via Newton's > laws in the first place: > http://www.eftaylor.com/leastaction.html Well, this is another one. The Principle of Least Action is the most powerful principle ever discovered by mankind. And yet, when coming down to GR, it is abandoned for geodesics. Instead, the good old Riemann/Christoffel method of deriving the geodesics through differential geometry is promoted. Man, you are taking everything they want you to believe for the face value. >> In the lecture on Newton's laws Professor Lewin states that while the >> definition of mass etc implied by Newton's second law may seem circular [quoted text clipped - 14 lines] > > What does this 'we' include? People with common sense. > In this message, your buddy Bilge wrote the following. > [quoted text clipped - 10 lines] > It sounds like this person is taking everything that the establishment > wants him to know as face value. Nope - he is asking you to prove your claims - so far you have not - in fact all you do is string buzzwords together in silly ways. Combine that with the commonsense requirement and you have an obvious conclusion. Bill >In another words, you don't have to > understand physics. Just let the 'experts' understand it for you. [quoted text clipped - 15 lines] > differential geometry is promoted. Man, you are taking everything they > want you to believe for the face value. Koobee Wublee, windbag and blowhard: >> In the lecture on Newton's laws Professor Lewin states that while the >> definition of mass etc implied by Newton's second law may seem circular in [quoted text clipped - 5 lines] >not addressing the definition of mass. You need to derive mass from >the Lagrangian to appreciate what mass really is. Well, why don't _you_ derive it for us? >However, when the >Lagrangian that yields the field equations is accepted for the face >value, it contains the mass term. Thus, the concept of mass becomes >circular under the current interpretation to GR. Wrong. You are so dense you could be used for radiation shielding. >> ...this is science and we do not accept things on face value. > [quoted text clipped - 3 lines] > >http://groups.google.com/group/sci.physics.relativity/msg/ 6833440fc4e66c2e?dmode=source&hl=en >Excerpt: > [quoted text clipped - 7 lines] >wants him to know as face value. In another words, you don't have to >understand physics. I'm perfectly willing to compare what I know aout physics to what you know, except that you keep evading your opportunity to prove your point. [...] >Well, this is another one. The Principle of Least Action is the most >powerful principle ever discovered by mankind. So why have you evaded all of the opportunities you've been given to demonstrate your understanding of the lagrangian? > I have recently downloaded some very good lectures videotaped by MIT on > basic physics and math [quoted text clipped - 39 lines] > Thanks > Bill Thanks for the pointers to Lewin's excellent video lectures. I had seen his 8.02 lectures on electricy and magnetism, http://web.mit.edu/smcs/8.02/ but I didn't know he had a 8.01 as well. Excellent freshman stuff. Dirk Vdm >> I have recently downloaded some very good lectures videotaped by MIT on >> basic physics and math [quoted text clipped - 62 lines] > but I didn't know he had a 8.01 as well. > Excellent freshman stuff. For sure. Indeed I sometimes think a very interesting freshman course would be to use the Feynman lectures as a text (including listening to his audio) and Lewin's excellent video lectures. The very mirror point of contention I mention above would form a very interesting class debate. I am generally not that concerned with infrequent mistakes in texts or lectures - picking them up and discussing them I recon is great for students. Thanks Bill > Dirk Vdm > I have recently downloaded some very good lectures videotaped by MIT on > basic physics and math [quoted text clipped - 13 lines] > bits of cheese - we can combine them together to create double the mass and > so check Newton's second law. So first assume that combining two objects creates double the mass, and then build a device to propel the single and double mass objects, and presumably if the acceleration ratio turns out to be 2:1, you can take it as a verification of F=ma. Right? However, how would you go about independently verifying that the propelling device produces the same force regardless of what it propels? E.g. a familiar "device" that doesn't do that is one which releases small objects in the vicinity of a large object. I thought what. Surely bright MIT students > will realize this relies on the fact two equal masses combined together has > twice the mass. Very intuitive and of course true but this is science and [quoted text clipped - 22 lines] > Thanks > Bill >> I have recently downloaded some very good lectures videotaped by MIT on >> basic physics and math [quoted text clipped - 20 lines] > independently verifying that the propelling device produces the same force > regardless of what it propels? Experimentation with the device (eg a spring force producer based on hooks law). If you test it say 1000 times and show it obeys hooks law with an accuracy of such and such then reasonable statistical assumptions allows you to draw a conclusion about it behavior. Very very elementary science - and usually not in question. Thanks Bill > E.g. a familiar "device" that doesn't do that is one which releases small > objects in the vicinity of a large object. [quoted text clipped - 27 lines] >> Thanks >> Bill >>>I have recently downloaded some very good lectures videotaped by MIT on >>>basic physics and math [quoted text clipped - 26 lines] > to draw a conclusion about it behavior. Very very elementary science - and > usually not in question. Well, you're the one who brought up the question Also Hook's Law can't be used to independently verify F=ma, since in order to recognize Hook's Law, the force involved has to be known. If F=ma is to be considered more than a definition there has to be an independent procedure for measuring force, and AFAIK there isn't one. >>>>I have recently downloaded some very good lectures videotaped by MIT on >>>>basic physics and math [quoted text clipped - 32 lines] > than a definition there has to be an independent procedure for measuring > force, and AFAIK there isn't one. Take a standard mass accelerate it to 1m/s/s and call that force the unit of force - a Newton. Take 2 two standards units of mass etc etc. The fact that 2 masses together forms twice the mass depends on the third law does not invalidate it. Springs for example can then be calibrated in units of force. Many other mechanisms are undoubtedly possible and all can be used to measure/produce predictable forces. Bill >>>>>I have recently downloaded some very good lectures videotaped by MIT on >>>>>basic physics and math [quoted text clipped - 39 lines] > force. Many other mechanisms are undoubtedly possible and all can be used > to measure/produce predictable forces. Give an example of *any* mechanism that can measure or produce a specific force independently of F=ma. Absent that, it isn't possible to "check Newton's second law", i.e. the "law" is simply a definition of the word "force". >>>>>>I have recently downloaded some very good lectures videotaped by MIT >>>>>>on basic physics and math [quoted text clipped - 42 lines] > Give an example of *any* mechanism that can measure or produce a specific > force independently of F=ma. I already have - Hooks law. If you can't see it - fine - no skin off my nose. > Absent that, it isn't possible to "check Newton's second law", i.e. the > "law" is simply a definition of the word "force". It is more than that - it is a prescription that says in analyzing classical mechanical problems get to the forces. Read the Lectures. Bill >>>>>>>I have recently downloaded some very good lectures videotaped by MIT >>>>>>>on basic physics and math [quoted text clipped - 45 lines] > I already have - Hooks law. If you can't see it - fine - no skin off my > nose. Looks like neither of us remembered the correct spelling of his name, but Hooke's Law isn't independent of F=ma, since "F=kx" relies on F=ma to measure the LHS force. (Unlike Newton's "2nd", Hooke's Law does qualify as a law, because it relates measurements, "kx" and "ma"). >>Absent that, it isn't possible to "check Newton's second law", i.e. the >>"law" is simply a definition of the word "force". > > It is more than that - it is a prescription that says in analyzing classical > mechanical problems get to the forces. Well, a "prescription" doesn't amount to a physical law. > Read the Lectures. Offering a 3-volume course of instruction as support for your position on a specific issue is weak. > >>>>>>>I have recently downloaded some very good lectures videotaped by MIT > >>>>>>>on basic physics and math [quoted text clipped - 52 lines] > to measure the LHS force. (Unlike Newton's "2nd", Hooke's Law does > qualify as a law, because it relates measurements, "kx" and "ma"). Interesting! Newton treated m as a constant, which is fine for low speeds: F=ma relates the weight "F" of a body on a scale to its acceleration "a" (of course the scale instead of indicating F calculates m=F/g, which fails on the moon). And Hooke doesn't need acceleration, only force. You will notice that when you squize two springs against a balance, you only have to press them half the distance to obtain the same reading as one spring. Harald > >>Absent that, it isn't possible to "check Newton's second law", i.e. the > >>"law" is simply a definition of the word "force". [quoted text clipped - 8 lines] > Offering a 3-volume course of instruction as support for your position > on a specific issue is weak. >>>"jem" <xxx@xxx.xxx> wrote in message > [quoted text clipped - 107 lines] > you squize two springs against a balance, you only have to press them half > the distance to obtain the same reading as one spring. Nice blend of wrong and irrelevant. Try quantifying force without using F=ma. > Harald > [quoted text clipped - 13 lines] >>Offering a 3-volume course of instruction as support for your position >>on a specific issue is weak. > >>>"jem" <xxx@xxx.xxx> wrote in message > > [quoted text clipped - 110 lines] > Nice blend of wrong and irrelevant. Try quantifying force without using > F=ma. I just did. F=ma defines the conversion from kgf to N, but it's not needed for quantifying force. Perhaps it's the conversion factor between those force units that you mean with "quantifying force". That factor I've always regarded as convention. Harald > >>>>Absent that, it isn't possible to "check Newton's second law", i.e. the > >>>>"law" is simply a definition of the word "force". [quoted text clipped - 11 lines] > >>Offering a 3-volume course of instruction as support for your position > >>on a specific issue is weak. >>>>>"jem" <xxx@xxx.xxx> wrote in message >>> [quoted text clipped - 166 lines] > force units that you mean with "quantifying force". That factor I've always > regarded as convention. What you quantified (measured) were changes in the lengths of some springs. What makes you think you measured a force? To measure force, you need to know what a force is, and it's Newton's laws that provide the definition. > Harald > [quoted text clipped - 16 lines] >>>>Offering a 3-volume course of instruction as support for your position >>>>on a specific issue is weak. SNIP (it became unreadble) > >>>>>>>>>>presumably if the acceleration ratio turns out to be 2:1, you can > >>> [quoted text clipped - 114 lines] > you need to know what a force is, and it's Newton's laws that provide > the definition. Following Bill, I'd say that Hooke's law defines Force in a more fundamental way: How hard one pushes can be determined by the level of deformation. http://www-ccrma.stanford.edu/~jos/pasp/Hooke_s_Law.html Harald >>>"jem" <xxx@xxx.xxx> wrote in message > [quoted text clipped - 139 lines] > Following Bill, I'd say that Hooke's law defines Force in a more fundamental > way: Well, that's not following Hobba (unless you meant chronologically), but suppose you demonstrate how you're going to use this "more fundamental" definition of force to (e.g.) describe the dynamics of planetary motion. How hard one pushes can be determined by the level of deformation. > http://www-ccrma.stanford.edu/~jos/pasp/Hooke_s_Law.html > > Harald > >>>"jem" <xxx@xxx.xxx> wrote in message > > [quoted text clipped - 146 lines] > How hard one pushes can be determined by the level of deformation. > > http://www-ccrma.stanford.edu/~jos/pasp/Hooke_s_Law.html Of course I won't use deformation for the dynamics of planetary motion. >>>>>"jem" <xxx@xxx.xxx> wrote in message >>> [quoted text clipped - 179 lines] > > Of course I won't use deformation for the dynamics of planetary motion. Of course you won't, since you can't. Invest in a dictionary. Restricting the applicability of a term doesn't make it "more fundamental". > >>>>>"jem" <xxx@xxx.xxx> wrote in message > >>> [quoted text clipped - 165 lines] > >>> > >>>Following Bill, I'd say that Hooke's law defines Force in a more fundamental > >>>way: > >> [quoted text clipped - 10 lines] > Of course you won't, since you can't. Invest in a dictionary. > Restricting the applicability of a term doesn't make it "more fundamental". Dictionary: online -> invest in a computer ;-) Note that different dictionaries express different opinions. That *you* (and TvF) want to primarily *define* force by acceleration is *your* choice. The point was that you're not obliged to do so. >>>>>>>"jem" <xxx@xxx.xxx> wrote in message >>>>> [quoted text clipped - 212 lines] > TvF) want to primarily *define* force by acceleration is *your* choice. The > point was that you're not obliged to do so. It's not a matter of what you're "obliged to do", but what was actually done. And on that note, I'm done. Bill Hobba says... >The reason two equal masses connected together has twice the mass is >because of Newton's third law - push or pull it and one mass reacts >on the other with the same force meaning you need twice the force to >get the same acceleration hence it has twice the >mass. It is not a test for the second law it is a test for the third law. Hmm. It took me a little while to see what you were saying here. Let me see if I have your argument right: Suppose you have the following set-up ----[]----[] You have a string connected to one mass, and a second string connecting that mass to another equal mass. You pull on the left string with a certain force, and the two masses start accelerating. Assuming that the string between the two masses doesn't keep stretching, we have: acceleration of the first mass = acceleration of the second mass We can use Newton's second law on each mass separately: First mass: ----[]---- F1 F2 You are pulling to the left on this mass with force F1. The second mass is pulling to the right with force F2. So we have 1. F1 - F2 = m a Now look at the second mass: ----[] F2 This mass has only a single force on it. The first mass is pulling to the left with force F2 (by Newton's third law). So we have 2. F2 = m a Putting 1. and 2. together gives: 3. F1 - F2 = F2 or F1 = 2*F2 So it takes twice the force to give the same acceleration a. Okay, so you are right. -- Daryl McCullough Ithaca, NY  SignatureNewsGuy.Com 30Gb $9.95 Carry Forward and On Demand Bandwidth >Bill Hobba says... >>The reason two equal masses connected together has twice the mass is [quoted text clipped - 56 lines] >Daryl McCullough >Ithaca, NY Finally, we're getting somewhere. >>Bill Hobba says... >>>The reason two equal masses connected together has twice the mass is [quoted text clipped - 59 lines] > > Finally, we're getting somewhere. This stuff has been known for many many years - it is just not often emphasized to students - one possible reason being they have a lot to learn already without a detailed discussion of things that do not really affect the practical application of the Newton's laws. See the Feynman Lectures - all will be clear. And normally students do not see it by themselves without reading something like the lectures (I had read a couple of books on this sort of stuff before the lectures - but that is just me because I am interested in that sort of stuff - but Feynman explains it best of all). But these guys are supposed to be bright MIT students - the cream of the crop. Actually I am with Taylor - a former professor at MIT - who believes it should be taught by the PLA - see http://www.eftaylor.com/pub/FmaAJPguest5.pdf Perhaps one reason is they are bright students and do see the issues with the usual treatment without being told or doing other reading like I did. Thanks Bill > http://www.eftaylor.com/pub/FmaAJPguest5.pdf The Principle of Maximum Aging is not explained very well in the text. Somehow, the Principle of Maximum Aging is simplied into the Principle of Least Action. Since gravitation is solely caused by graviational time dilation where gravitation has nothing to do if space is curved or not, there is no 'sacredness' to the concept of spacetime. Curved space in terms of mathematics should be treated like flat space with a correction factor. In doing so, it is very absurd to say any event obeys the minimum combined of time and space. Thus, the correct interpretation to any event taking place in spacetime is to obey the Principle of Least Time. When observing an object moving from point A to point B in space whether if it is flat or curved, without the laws of physics the event can take any course it can. There are an infinite number of possible paths. However, there is only one possible path that obeys the Principle of Least Time, and that path is indeed what we observe. Just how difficult is it to understand this anyway? >> http://www.eftaylor.com/pub/FmaAJPguest5.pdf > > The Principle of Maximum Aging is not explained very well in the text. > Somehow, the Principle of Maximum Aging is simplied into the Principle > of Least Action. Well - take its negative and guess what you have. > Since gravitation is solely caused by graviational time dilation That is not what GR says. Bill > where > gravitation has nothing to do if space is curved or not, there is no [quoted text clipped - 9 lines] > Principle of Least Time, and that path is indeed what we observe. Just > how difficult is it to understand this anyway? >>> http://www.eftaylor.com/pub/FmaAJPguest5.pdf >> [quoted text clipped - 3 lines] > > Well - take its negative and guess what you have. You are not making any sense! Is this the trait of GR? >> Since gravitation is solely caused by graviational time dilation > > That is not what GR says. That is not what the commonly interpreted GR says, yes. However, that is not what the mathematics says. If you actually accept the mathemtical implication, you will find the modified Schwarzschildm metric that results in the following spacetime also able to explain gravitation. ds^2 = c^2 g dt^2 - dr^2 - r^2 dH^2 +... If you don't accept the mathematical implication, you are better off to follow Hammond and Conrad's teachings. God = Guv, Amen! > Bill Hobba says... >>The reason two equal masses connected together has twice the mass is [quoted text clipped - 52 lines] > > Okay, so you are right. Its nothing new. See the Feynman Lectures - Chapter 10 - Conservation of Momentum and Chapter 12 - Characteristics of Force. He does not use the same argument as me but he reaches exactly the same conclusion (in fact it is knowing the conclusion that allowed me to see Professor Lewin must be wrong) - namely the second law is not really a law in the usual sense - it is a partial law - a paradigm of a way of looking a things. From page 12-1 - In Feynmans own words: 'The real content of Newton's laws is this: that the force is supposed to have some independent properties in addition to the law F=MA but the specific independent properties that the force has were not completely described by Newton or anyone else and therefore the physical law F=MA is an incomplete law. It implies that if we study the mass times acceleration and call the product the force ie if we study the characteristics of force as a program of interest then we shall find that forces have some simplicity, the law is a good program for analyzing nature, it is a suggestion the forces will be simple.' Actually I believe Professor Lewin is doing a service to his students - that way they can nut it out for themselves. I just would have felt more comfortable if they used the Lectures as their text and go home and read Feynmans words and that allowed them to think a bit deeper about it. Do I think it will have a negative impact of their physics/math education? - hardly. Thanks Bill > -- > Daryl McCullough > Ithaca, NY > I have recently downloaded some very good lectures videotaped by MIT on > basic physics and math [quoted text clipped - 39 lines] > Thanks > Bill BH: Some think Feynman is an overly expensive "date". Perhaps, because he says that himself ... in reference to his 200-odd CT freshman students of whom 2-dozen got anything from his lectures. Not proving anything here ... just observing that a circumspect "student" can reasonably chose another way to spend his time. nss ******** |
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