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Rotational motion, energy conservation problem please help

  1. Nov 16, 2011 #1
    rotational motion, energy conservation problem URGENT please help

    1. The problem statement, all variables and given/known data

    A uniform rod of mass M=2 kg and length L = 2m is pivoted from its end. It has a point mass m=1 kg on its tip. It stars from rest at an angle of 30 degrees above horizontal. It falls under the influence of gravity. How fast is the tip m moving when the rod is horizontal at θ
    = 0
    2. Relevant equations

    mgh = 1/2mv^2

    3. The attempt at a solution

    h= Lsinθ = 1
    m cancels from both sides
    so you're left with

    BUT, the actual answer given is 4.8

    Help please! I have a test in just a few hours and have been trying to figure this problem out for awhile! Thanks
  2. jcsd
  3. Nov 16, 2011 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Re: rotational motion, energy conservation problem URGENT please help

    How would you write the rotational KE of the system? Also, what's the total change in gravitational PE as it falls?
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