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Forces on pendulum

  1. Jul 21, 2010 #1
    I find that the forces acting on a pendulum bob are treated in two different (contradictory?) ways.
    Let the angle that the cord makes with the vertical be x, the mass of the bob m and the tension in the cord T.
    1. In one case, the weight of the bob is resolved into components so that mgcos(x) is along T, so T = mgcos(x).
    2. In another case, the tension is resolved into ITS components, so that mg = Tcos(x).
    Obviously only one of them can be correct at one time. I can't figure out which of them to use and when.
    I am a physics teacher, by the way.
    Thanks in advance.
  2. jcsd
  3. Jul 21, 2010 #2
    Why do you think that statement is true?
    "mgcos(x) is along T" does not imply "T = mgcos(x)"
    Draw a free body diagram.
  4. Jul 21, 2010 #3


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    Science Advisor

    Neither of these two are correct.

    [tex]ma_r = mgcos(x) - T[/tex]
    [tex]a_r = -L\dot{x}^2[/tex]
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