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Friction as a conservative force

  1. Jan 24, 2008 #1
    friction as a nonconservative force

    I was wondering, can the friction force be split up? Suppose you have a friction force working under an angle alpha, can you just say Fx = Ffric*cos(alfa), Fy = Ffric*sin(alfa)

    Suppose you're working in a flat horizontal plane, and you launch a ball in 45° direction, what are the equations of the ball in x and y?
    mx" = -Fx = -Ffric cos (alfa)
    my" = -Fy - -Ffric sin(alfa)

    It seems that this does not works since the friction force is a nonconservative force ... Is Lagrange method still valid?

    Last edited: Jan 24, 2008
  2. jcsd
  3. Jan 24, 2008 #2


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    If the frictional force is the same in all directions ( isotropic) then there's no reason to decompose it as you suggest. Just rotate your frame.

    As far as the Lagrangian goes, clearly the frictional force is dissipative. There are good lecture notes here -

    http://tabitha.phas.ubc.ca/wiki/index.php/Dissipative_Forces [Broken]
    Last edited by a moderator: May 3, 2017
  4. Jan 25, 2008 #3
    When dealing with lagrangian mechanics it is always best to minimise the number of dependant variables, i.e. just make your situation 1D such that it is up and down the slope. So then you dont have to break up the force, you just have to alter (thru trig) the force from gravity.
  5. Jan 25, 2008 #4
    And when it is very inconvenient to consider it 1D, is it still correct to split the forces into Fx and Fy?
  6. Jan 25, 2008 #5
    It seems ok to do so. Give it a go. See where you end up.
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