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Drawing free body diagrams for pendulum

  1. Dec 4, 2006 #1
    1. The problem statement, all variables and given/known data

    I have to do this for a lab. Draw and label the free body diaggram of a pendulum when it is at its maximum amplitude of 30 degrees. The magnitudes of the vectors must be correctly scaled and the directions correct.

    2. Relevant equations

    3. The attempt at a solution

    I drew the free body diagram with T pointing diagonally up and to the left. The arc angle is 30 degrees. I then draw gravity mg straight down with two components, mgcos(30) and mgsin(30). I reasoned that mgcos(30) is smaller than T in magnitude because the pendulum is in motion. The tension and centripetal force are in the same direction, and the pendulum is not at rest. Is my reasoning correct?
  2. jcsd
  3. Dec 4, 2006 #2
    Seems okay from your description, but we aren't going to know whether you fulfilled all the criteria for the free body diagram.

    That first statement is confusing. An amplitude of 30 degrees?
  4. Dec 4, 2006 #3

    It means that at most the pendulum travels 30 degrees in either direction from equilibrium point.
  5. Dec 5, 2006 #4


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    At 30º the pendulum is momentarily at rest. There is no centripetal acceleration at that point.
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