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Homework Help: A simple pendulum swinging

  1. Dec 6, 2015 #1
    1. The problem statement, all variables and given/known data
    A simple pendulum of length 2.00 m is made with a mass of 2.00 kg. The mass has a speed of 3.00 m/s when the pendulum is 30.0° above its lowest position.

    (a) What is the maximum angle away from the lowest position the pendulum will reach?

    2. Relevant equations
    1/2mv^2(initial) + mgh(initial) = 1/2mv^2(final) + mgh (final)

    3. The attempt at a solution
    1/2mv^2(initial) + mgh(initial) = 1/2mv^2(final) + mgh (final)

    1/2(2)(3)^2 + (2)(9.8)(2)cos(30) = 1/2(2)(0)^2 + (2)(9.8)(2)cos(theta)
    And solving for theta, I get some imaginary number or something. I can't figure out what I'm doing wrong, or maybe I just messed up from the beginning.

    Thanks!!
     
  2. jcsd
  3. Dec 6, 2015 #2

    Simon Bridge

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    Draw a picture: the height is not ##2\cos\theta##.
     
  4. Dec 6, 2015 #3
    Oh, oops! I got it now. Thank you!!
     
  5. Dec 6, 2015 #4

    Simon Bridge

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    Well done. Easy mixup to make.
     
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