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Angular momentum

  1. Apr 29, 2007 #1
    Hi! Please, could someone tell me who defined angular momentum as r x P? Thank you.

  2. jcsd
  3. May 1, 2007 #2


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    Sorry, not me. This is a weird question. What is it that you actually want to know ?
  4. May 2, 2007 #3

    Meir Achuz

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    Newton used it and no one came before Newton.
  5. May 6, 2007 #4
    I want to know who defined angular momentum as r x p. That is all,
  6. May 6, 2007 #5
    Thank you. Could you please give me some reference? It doesn't seem it was Newton who used it first.
  7. May 6, 2007 #6
    It looks like my knowledge of the history of physics turned out to be somewhat useful after all. Before Newton and Galileo, the most commonly believed theory of physics was that of Aristotle and his followers. According to Aristotelian physics, the rest is the natural state and it requires a momentum in order to move an object. The concept of angular momentum came from this context, and the Aristotelian theory stated that it required a constant effort to keep an object in uniform circular motion.
  8. May 6, 2007 #7
    Thank you very much. But would you know if Aristotle himself defined angular momentum as r x p? If he did not do it, somebody else must have done it. I would like to know who did it and why. I have reason to believe this definition is incorrect.
  9. May 6, 2007 #8
    Wait, why do you think this definition is incorrect? It works remarkably well and leads to all manner of useful and accurate predictions.
  10. May 6, 2007 #9

    D H

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    The originator of [itex]\vec l = \vec r \times \vec p[/itex]? It certainly was not Aristotle, Galileo, or Newton. The modern concept of vectors (and the cross product) was not "invented" until the mid 1800s.
  11. May 6, 2007 #10
    I found an experiment where r x p is not conserved when, clearly, no external torque acts on the object.
  12. May 6, 2007 #11
    I think you are right. It is funny, although I have been asking far and wide, it seems nobody knows who was the first to use r x p.
  13. May 6, 2007 #12

    D H

    Staff: Mentor

    The math before vector calculus was a bit convoluted. The concepts we use today existed nonetheless. In particular conservation of angular momentum is quite valid and predates this modern notation. Please explain your experiment.

    Edited to add:
    Remember that "Extraordinary claims require extraordinary proof".
    Last edited: May 6, 2007
  14. May 6, 2007 #13
    Could you please describe the experiment to us so that we can help you clear any confusions?
  15. May 6, 2007 #14


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    Perhaps if you explained the problem/experiment you are working on, we can help you understand the issue. Who originated this concept doesn't have anything to do with whether or no it is valid...
    Last edited: May 6, 2007
  16. May 7, 2007 #15
    I know physics textbooks teach angular momentum is mr^2w, but that is incorrect. For a point mass it is mrw. I wrote a paper on this subject, and a physics journal is now considering its publication.
  17. May 7, 2007 #16
    According to Newton's second law F(net) = dp/dt, only a net force in the direction of motion of an object, can change the magnitude of its velocity. A force perpendicular to the direction of motion of an object, does not change the magnitude of its velocity. If this were not true, a rotating object at the end of a string would increase its speed constantly, because it is subject to a constant centripetal force.
  18. May 7, 2007 #17


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    I'm afraid I disagree with you there. I also must warn you that according to the PF guidelines the discussion of theories which contradict mainstream beliefs is prohibited.
  19. May 7, 2007 #18
    Don't forget to let us know the name of the journal that has published this.
  20. May 7, 2007 #19
    I am sorry to upset you, but I found an experiment that shows the currently accepted definition of angular momentum r x p is incorrect. That is why I was interested in knowing who defined angular momentum as r x p and why. Unfortunately, I have been unable to do it, so far at least. Anyway, I understand that, in physics, the experimental truth has the last word, and that should not be suppressed.
  21. May 7, 2007 #20


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    You haven't upset me in the slightest, I was simply making you aware that such discussions in this forum are prohibited.
    I tend to agree with you there. As such PF has an Independant Research Forum where you can submit your theory for review subject to these guidelines.

    However, I reiterate that the discussion of theories which contradict mainstream beliefs is prohibited in the main forums.
    Last edited: May 7, 2007
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