Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Conservation of Angular Momentum from Newton's third law

  1. Dec 12, 2007 #1
    I am reading Introduction to Mechanics by Kleppner and Kolenkow. I am now reading the articles dealing with conservation of angular momentum. I am not satisfied with the articles dealing with how the third law does not lead to conservation of angular momentum. Could anyone please throw some light there?
     
  2. jcsd
  3. Dec 13, 2007 #2

    Shooting Star

    User Avatar
    Homework Helper

    I don't have the book right now with me, so I'm a bit unsure of exactly what you don't like. I presume that you are talking about system of particles. The third law states that the action and reaction forces are equal and opposite, but does not say that they lie along the line joining the particles. Only in the latter case will the total angular momentum of a system be conserved if the external torque is zero.
     
  4. Dec 16, 2007 #3
    I think I got it now. I read into the book incorrectly. :D
     
  5. Dec 16, 2007 #4

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Of course, in the special (&idealized) case of CONTACT forces, the joining line segment is of zero length, and hence, the conservation of total angular momentum is preserved anyhow we look at it...
     
  6. Dec 16, 2007 #5

    Shooting Star

    User Avatar
    Homework Helper

    So, the case you are talking about is more pertinent for collisions, it seems to me?
     
  7. Dec 17, 2007 #6

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    That's a typical case, yes.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?