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Homework Help: Time at a certain reference line

  1. Oct 17, 2006 #1
    At t=0, a flywheel has an angular velocity of 4.7 rad/s, a constant angular acceleration of -0.25 rad/s^2, and a reference line at θ(t=0)=0 rad.
    Assuming the motion proceeded similary at times before t=0, at what negative time was the reference line at θ=-10.5 rad?

    Based on this question I'm assuming that acceleration is still -0.25 rad/s^2 and angular velocity is still 4.7 rad/s.

    Θf = θi + ωit + 1/2αt^2 is the equation I was planning on using to find time. However I'm confused as to what I should consider the final radians and final velocity. Should it be at θ=0 with final velocity at 4.7 rad/s?
  2. jcsd
  3. Oct 17, 2006 #2


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    When you write
    Θ = θi + ωit + 1/2αt^2
    you are effectively saying that α is a constant and that
    ω = ωi + αt
    θi is the value of Θ when t = 0
    ωi is the value of ω when t = 0
    Putting those values into
    Θ = θi + ωit + 1/2αt^2
    allows you to find Θ at any other time (positve or negative)
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