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Homework Help: Pendulum and Oscillating Motion

  1. Sep 28, 2008 #1
    1. The problem statement, all variables and given/known data

    A pendulum is 0.8 m long and the bob has a mass of 1.0 kg. At the bottom of its swing, the bob's speed is 1.3 m/s. What is the tension in the string at the bottom of the swing?

    2. Relevant equations

    3. The attempt at a solution

    F= 1.0kg((1.3 m/s^2)/.08)
    F= 2.1125

    I must be using the wrong equation or numbers.. Any help is appreciated.
  2. jcsd
  3. Sep 28, 2008 #2
    The velocity is squared so your force would be F= 1.0kg((1.3 m/s)^2/.08)
  4. Sep 28, 2008 #3
    Oh, i meant 21.125, but I am still getting the wrong answer.
  5. Sep 28, 2008 #4
    I'm getting F= 2.1125N...maybe its trying to keep track of significant figures...in that case I'm thinking it would be F = 2N
  6. Sep 28, 2008 #5
    Yeah, I've tried all those answers, am I even using the right equation? Would the radius really be 0.8 m since the string is 0.8 m and not perpendicular to the earth's gravitational pull?
  7. Sep 28, 2008 #6
    yup...the problem says the string is .8 m. Looks like I missed something too. The centripetal force is 2N, which is the force of the object's "inertia" on the string. At the bottom the object's weight also contributes to the tension...so you'll have to add the two.
  8. Sep 28, 2008 #7
    So it would be 11.9225N?
  9. Sep 28, 2008 #8
    The centripetal force will be the Tension minus the weight. You should then be able to solve for the tension.
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