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SHM of Pendulum

  1. Apr 19, 2008 #1
    1. The problem statement, all variables and given/known data
    A pendulum's angle is given by (.10 rad)cos(5t + pi)
    where t is in sec. What is the initial angle?

    2. Relevant equations

    3. The attempt at a solution
    Do I plug in t= 0 to get this angle, or is the phase constant this angle? I read that the phase constant specifies initial conditions, but I'm not entirely clear on what it means.
  2. jcsd
  3. Apr 19, 2008 #2


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    Sounds good to me :approve:. The initial conditions is simply a set of conditions which fix the values of a function at time t=0.
  4. Apr 19, 2008 #3
    According to the equation above pi is my phase constant, but if I plug in t=0, I don't come up with pi as initial angle, so I'm not sure if phase constant and initial angle are the same thing
  5. Apr 19, 2008 #4


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    They're not, the phase angle is chosen such that the initial conditions, i.e. the angle at t=0, is satisfied. Does that make sense?
  6. Apr 19, 2008 #5
    I still don't understand. Could you describe it to me using the equation Theta(t) = theta max * cos (omega*t+ phase constant) with the values in the equation in the problem up above?
  7. Apr 19, 2008 #6


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    Hi bcjochim07! :smile:

    You're confused because, in this case, the amplitude is an angle, just like the phase constant.

    Usually, the amplitude is a number or a length or a speed, so there's no confusion … the question would ask for the initial value, or the initial distance, or the initial speed … and you couldn't get that confused with the phase constant!

    The initial angle is θ(0), which is θmax * cos (phase constant). :smile:
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