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Homework Help: Something weird about Rotational Kinematic Equations

  1. May 12, 2009 #1
    1. The problem statement, all variables and given/known data
    I need to find the average acceleration of a gramophone record.

    Initial angular velocity, W = 0
    Final angular velocity, w = 3.50rad s^-1
    angular displacement, 8 = 0.25rad

    2. Relevant equations
    The equation given by the answer is

    t= 8/w
    a = ( w - W ) / t
    the answer is 49.0 rad s-2

    3. The attempt at a solution
    This is the equation I use

    w2 = W2 + 2a8

    and I get 24.5rad s-2

    But if I multiply it with 2, the angular acceleration will become 49rad s-2

    That mean my equation will become
    w2 = W2 + a8

    Can anyone explain this to me?
    Or I use a wrong equation?
  2. jcsd
  3. May 12, 2009 #2
    You are right:

    t= 8/w is wrong because the angular velocity is not constant

    It should be t= 28/w
  4. May 13, 2009 #3
    ok...thx... ^^
  5. May 14, 2009 #4
  6. May 14, 2009 #5


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    Science Advisor
    Homework Helper

    Hi Stefenng! :smile:

    (have a theta: θ and an omega: ω :wink:)

    You must learn the standard https://www.physicsforums.com/library.php?do=view_item&itemid=204" equations …

    it isn't θ = ωt (or t = θ/ω), it's θ = ωt + 1/2 αt2 :wink:
    Last edited by a moderator: Apr 24, 2017
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