swansont Posted March 13, 2017 Posted March 13, 2017 Do you deny that this is and has been a common demonstration performed by professors for about three hundred years? Do you assert that we might actually achieve 12000rpm when performing this demonstration? The point of my last post was not to pass the buck, but to point out that there is no properly conducted controlled experiment supplied when teaching conservation of angular momentum. There is only the demonstrations. So science education does use an uncontrolled "experiment" in order to convince. Therefore in Argent's words: The science education is not proper. I would like to see a video that purports to show what you are claiming — something starting at 120 rpm and then shortening r to 1/10 of its value.
Argent Posted March 13, 2017 Posted March 13, 2017 Do you deny that this is and has been a common demonstration performed by professors for about three hundred years? Do you assert that we might actually achieve 12000rpm when performing this demonstration? The point of my last post was not to pass the buck, but to point out that there is no properly conducted controlled experiment supplied when teaching conservation of angular momentum. There is only the demonstrations. So science education does use an uncontrolled "experiment" in order to convince. Therefore in Argent's words: The science education is not proper. You have repeated and failed to address my objection to your use of the word convince. The function of such "experiments" is to demonstrate conservation of angular momentum, not to convince people of its reality. Surely that follows in other ways. Is there a reason you have not set up an experiment to demonstrate (and convince) that the postulate is flawed?
Mandlbaur Posted March 13, 2017 Author Posted March 13, 2017 I would like to see a video that purports to show what you are claiming — something starting at 120 rpm and then shortening r to 1/10 of its value. I could point you to my personal youtube videos but I fear that it might give you cause to censor this post as has been the case in other forums. Is there a reason you have not set up an experiment to demonstrate (and convince) that the postulate is flawed? I believe that the logical argument provided initially in this post is perfectly valid and further evidence is therefore unnecessary.
Bender Posted March 13, 2017 Posted March 13, 2017 Do your video's contain quantised measurements? Also, you didn't answer my question: Angular momentum can be mathematically derived from linear momentum. Are you aware of that fact, and as a result, do you also claim that linear momentum is not conserved and Newton's laws of motion are false?
swansont Posted March 13, 2017 Posted March 13, 2017 I could point you to my personal youtube videos but I fear that it might give you cause to censor this post as has been the case in other forums. You said there were a bunch of videos out there from college/university courses. Why so timid about this all of the sudden? 1
Mandlbaur Posted March 13, 2017 Author Posted March 13, 2017 You said there were a bunch of videos out there from college/university courses. Why so timid about this all of the sudden? I have to admit that they seem to be far more difficult to find than they were a year ago when I initially began discussing this problem, but that does not detract from my argument. If you truly wish to see this demonstration in action, it can very easily be performed with some household items and a little imagination. -1
Strange Posted March 13, 2017 Posted March 13, 2017 with some household items and a little imagination. Yes, that is generally acknowledged to be the best way to overthrow centuries of increasingly precise measurements. 1
Phi for All Posted March 13, 2017 Posted March 13, 2017 We can observe this experiment or very similar performed many times by many different people including many professors on youtube and in lecture halls so it can hardly be described as hypothetical. These professors have as yet failed to take measurements which means that the theory has been established due to an oversight. Since the predicted result is so absurdly high and obviously inaccurate, measurements are superfluous. A reasonable person must surely conclude purely from observation that 12000rpm is incorrect by an order of magnitude. ! Moderator Note Since this is no longer a simple classical physics algebra question thread, and is now actively arguing against the mainstream, it's time for all students to stop taking notes. We're moving to Speculations, where the ice is thin and your professor is NOT going to accept any of the answers. Please take the time to read the special rules, Mandlbaur, and please support your idea with evidence. People are going to be asking for it.
swansont Posted March 13, 2017 Posted March 13, 2017 I have to admit that they seem to be far more difficult to find than they were a year ago when I initially began discussing this problem, but that does not detract from my argument. Yes, actually, I think it does.
Strange Posted March 13, 2017 Posted March 13, 2017 I have to admit that they seem to be far more difficult to find than they were a year ago when I initially began discussing this problem, but that does not detract from my argument. If you search for 'experiments conservation angular momentum' then you will find quite a few details of experiments to be performed by students to objectively measure the conservation of angular momentum. Most of these also include estimation of sources of errors and comparison of those with the measured results. That is how science works. Not by criticising youtube videos.
Mandlbaur Posted March 13, 2017 Author Posted March 13, 2017 Yes, actually, I think it does. My argument is the logical argument presented initially in this post. So it does not. If you search for 'experiments conservation angular momentum' then you will find quite a few details of experiments to be performed by students to objectively measure the conservation of angular momentum. Most of these also include estimation of sources of errors and comparison of those with the measured results. That is how science works. Not by criticising youtube videos. You will not find a single measured experiment with a variable radii. My argument has nothing to do with youtube videos, it is the logical argument presented initially in this post.
Strange Posted March 13, 2017 Posted March 13, 2017 My argument has nothing to do with youtube videos, Do you need a hand moving those goalposts, they look heavy.
Bender Posted March 13, 2017 Posted March 13, 2017 My argument is the logical argument presented initially in this post. So it does not. You keep repeating that, but one of the premises in your logical argument is still wrong.
Mandlbaur Posted March 13, 2017 Author Posted March 13, 2017 You keep repeating that, but one of the premises in your logical argument is still wrong. Really? Which premise is that?
Bender Posted March 13, 2017 Posted March 13, 2017 Really? Which premise is that? "momentum and radius are independent"
Mandlbaur Posted March 13, 2017 Author Posted March 13, 2017 That has been discussed and shown to be correct.
Strange Posted March 13, 2017 Posted March 13, 2017 That has been discussed and shown to be correct. No it hasn't.
Phi for All Posted March 14, 2017 Posted March 14, 2017 That has been discussed and shown to be correct. ! Moderator Note In this thread?! I can't find it.
swansont Posted March 14, 2017 Posted March 14, 2017 My argument is the logical argument presented initially in this post. So it does not. Since this failure represents a lack of evidence, it does.
Mandlbaur Posted March 14, 2017 Author Posted March 14, 2017 No it hasn't. ! Moderator Note In this thread?! I can't find it. Prior to the definition of angular momentum, there was no reason to conceive of a dependence between momentum and radius because the concept upon which this supposed relationship is based did not exist. There was no obvious reason to assume a relationship and there was obvious reason, given the clear differences between the two variables, to assume none. Momentum and radius were therefore unrelated and independent at the time of the creation of the definition of angular momentum which means that my premise is true. Since this failure represents a lack of evidence, it does. This example is well known to be the classic demonstration of conservation of angular momentum and we are all aware that it has been performed for centuries and that it is still being performed which is supported by the number of videos which use it as the basis for teaching the concept: P.Anderson https://www.youtube.com/watch?v=hgcudPr73LU Khan Academy https://www.youtube.com/watch?v=CbeCE1HoGfA Prof M.Anderson https://youtu.be/nkVYXHnOPKUH Also, just because it is difficult to find an example of the demonstration being performed, does not mean there aren't any. There is one here for example at 28:04: Dr M.Young https://www.youtube.com/watch?v=kJyI7IFamK0 My personal videos on the subject, some of which contain examples of me performing it can be found here: https://www.youtube.com/channel/UCBX7PVP9_KHnsmHubr8siOg So your claim of a lack of evidence is actually false. -2
Bender Posted March 14, 2017 Posted March 14, 2017 (edited) Prior to the definition of angular momentum, there was no reason to conceive of a dependence between momentum and radius because the concept upon which this supposed relationship is based did not exist. There was no obvious reason to assume a relationship and there was obvious reason, given the clear differences between the two variables, to assume none. Momentum and radius were therefore unrelated and independent at the time of the creation of the definition of angular momentum which means that my premise is true. They become dependent because you apply them to an isolated system, not because of angular momentum. In fact, you could skip the angular momentum completely and do the calculations entirely without the concept of angular momentum and end up with the same result. My personal videos on the subject, some of which contain examples of me performing it can be found here: https://www.youtube.com/channel/UCBX7PVP9_KHnsmHubr8siOg So your claim of a lack of evidence is actually false. I didn't see any evidence, only very poorly performed demonstrations. The friction between the wire and the tube is enormous, the diameter of the hole in the tube is not negligible, and the centre of the circular motion does not remain stationary. All of this contribute to a torque on your system, which means it is not isolated and conservation of momentum does not apply. As has been pointed out, the only purpose of this demonstration is to qualitatively show that there is indeed some change in angular velocity. EDIT: perhaps you should try calculating/measuring the linear momentum, which you claim should be independent of the radius. Edited March 14, 2017 by Bender 1
Mandlbaur Posted March 14, 2017 Author Posted March 14, 2017 They become dependent because you apply them to an isolated system, not because of angular momentum. In fact, you could skip the angular momentum completely and do the calculations entirely without the concept of angular momentum and end up with the same result. I didn't see any evidence, only very poorly performed demonstrations. The friction between the wire and the tube is enormous, the diameter of the hole in the tube is not negligible, and the centre of the circular motion does not remain stationary. All of this contribute to a torque on your system, which means it is not isolated and conservation of momentum does not apply. As has been pointed out, the only purpose of this demonstration is to qualitatively show that there is indeed some change in angular velocity. If your claim was correct that they are dependent, we would see something close to 12000rpm in those demonstrations. That's what the professors have taught. The demonstrations I have provided show very similar results to the one provided by DR. M Young in the previous video which he claims indicates that angular momentum is conserved. There is indeed some change in angular velocity but since angular velocity is equal to velocity divided by radius, we would expect this in any event even if angular momentum is not conserved. So the change in angular velocity is not the evidence that the professors claim it is. This applies also to the turntable professor with weights in his hands demonstration.
Bender Posted March 14, 2017 Posted March 14, 2017 If the speed remains constant, as you claim, why don't you measure it? Could you also please answer this question: Angular momentum can be mathematically derived from linear momentum. Are you aware of that fact, and as a result, do you also claim that linear momentum is not conserved and Newton's laws of motion are false?
swansont Posted March 14, 2017 Posted March 14, 2017 Prior to the definition of angular momentum, there was no reason to conceive of a dependence between momentum and radius because the concept upon which this supposed relationship is based did not exist. There was no obvious reason to assume a relationship and there was obvious reason, given the clear differences between the two variables, to assume none. Momentum and radius were therefore unrelated and independent at the time of the creation of the definition of angular momentum which means that my premise is true. The way you set up the OP, you preclude applying it to a conserved quantity. It was bait-and-switch. For a = b*c, b and c can take on any value, but if you constrain a to be a constant the plot of b vs c must be a hyperbola. Also, just because it is difficult to find an example of the demonstration being performed, does not mean there aren't any. There is one here for example at 28:04: Dr M.Young https://www.youtube.com/watch?v=kJyI7IFamK0 I never said they don't exist. I challenged you to link to some, as you claimed they were ubiquitous. Now you just need to show the analysis that shows that this supports your claim. It is by no means obvious that this does; the rotation obviously speeds up as the radius is shortened. (be sure to account for the fact that there is a torque present, which will reduce the angular momentum over time) So your claim of a lack of evidence is actually false. Don't be obtuse. I claimed you hadn't presented any videos and at that time you hadn't.
Bender Posted March 14, 2017 Posted March 14, 2017 This applies also to the turntable professor with weights in his hands demonstration. I'd like to see your analysis of this demonstration, which is significantly better than the sling thing, since it actually approaches an isolated system. How would you calculate the speed after the professer pulls the weights to his chest? The body of the professor clearly speeds up, yet its radius remains constant.
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