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Homework Help: Angular Momentum (pendulum) Help

  1. Feb 13, 2012 #1
    1. The problem statement, all variables and given/known data
    a 1.5 kg pendulum bob swings in a vertical circle at the end of a 2m long string with negligible weight. If the bob is dropped from a height in which the string is horizontal and taut, what is the angular momentum at the bob's lowest point (in kgm/s^2)?

    a 40
    b 10
    c 30
    d 20
    e 50

    2. Relevant equations

    3. The attempt at a solution
    I tried using the formula v2=vo2+2aΔy to solve for linear velocity first. It would be v= sqrt(2*9.8m/s/s*2m) because it falls the length of the string to reach the horizontal correct? From there, I calculated v to be 6.26m/s, and then subbed that into L=mrv for a final answer of 18.78kgm2/s, but its a bit off the possible choices and I'm skeptical of my work.

    Any help would be greatly appreciated!
  2. jcsd
  3. Feb 13, 2012 #2


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    Gold Member

    Looks ok to me. Maybe they took g as 10 m/s² which should be a slightly greater velocity and hence angular momentum.
  4. Feb 13, 2012 #3
    Using 10m/s/s comes out to be 18.97, which is closer, but Im still not confident in that. I'm gonna keep trying some stuff, if anyone has anything to contribute please do so!
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