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Homework Help: Rotational Kinematics Problem - find the angular displacement

  1. Jun 7, 2009 #1
    1. The problem statement, all variables and given/known data

    The drive propeller of a ship starts from rest and accelerates at 2.90e-3 rad/s^2 for 2.1e3 seconds. For the nest 1.4e3 seconds, the propeller rotates at a constant angular speed. Then it decelerates at 2.3e-3 rad/s^2 until it slow (without reversing direction) to an angular speed of 4.0 rad/s. Find the total angular displacement of the propeller.

    2. Relevant equations

    a) w(t) = w0 + (alpha*time)

    3. The attempt at a solution

    Basically i used equation a) to solve for displacement for the first 2100 seconds, then for the next 1400 seconds I used equation a) and first found w0, and then plugged that value into theta= theta0+ w0*t +(1/2 alpha*time)

    I took both these values and added them together.
  2. jcsd
  3. Jun 7, 2009 #2


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    Welcome to PF!

    Hi grandprix! Welcome to PF! :smile:

    (have an omega: ω and a theta: θ and an alpha: α and try using the X2 tag just above the Reply box :wink:)
    You seem to have the equations the wrong way round …

    (and anyway it should be theta= theta0+ w0*t +(1/2 alpha*time-squared) :wink:)

    just use the standard constant acceleration equations in the usual way, but with θ ω and α instead of s v and a :smile:
  4. Jun 7, 2009 #3
    oh thank you so much! I tried to make the solution a lot more complicated than it is i guess.. and thank you for letting me know about where to find all the proper notations!
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