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Pendulum bob on a string

  1. Feb 7, 2012 #1
    A 2 kg pendulum bob on a string that is 3m long is released with a velocity of 1 m/s when the support string makes an angle of 30 degrees with the vertical. What is the tension at the highest point of its motion?

    3. The attempt at a solution
    Ok so using Newton's second Law, the equation should be Force of tension =mg cos θ. Adding the numbers in =2 kg (9.80 m/s)cos 30°=16.97 N. However its telling me its wrong and the answer should be 16.6 N. I'm confused as to where I've gone wrong. I realize that I skipped over length of the string but I was told that because we're dealing the the vertical component, its not needed.
     
  2. jcsd
  3. Feb 7, 2012 #2

    Doc Al

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    Staff: Mentor

    What angle does it make when it's at its highest point?
     
  4. Feb 8, 2012 #3
    Wouldn't the angle still be 30° at the highest point?
     
  5. Feb 8, 2012 #4

    Doc Al

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    It would be if it were released from rest at that point, but it was given a push.
     
  6. May 4, 2012 #5
    I would suggest you to use energy instead.
     
  7. May 5, 2012 #6

    Doc Al

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    Staff: Mentor

    One would use energy conservation to find the highest point, if that's what you mean.
     
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