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Homework Help: Rotational Kinematics

  1. Feb 28, 2017 #1
    1. The problem statement, all variables and given/known data
    A wheel rotates with a constant angular acceleration of π rad/s^2. During a certain time interval its angular displacement is π rad. At the end of the interval its angular velocity is 2π rad/s. What is its angular velocity at the beginning of the time interval?

    2. Relevant equations
    angle displacement = (initial rotational velocity * time) + 1/2(rotational acceleration)time^2
    rotational velocity = rotational acceleration * time
    rotational velocity^2 = (initial rot. velocity^2) + 2(rotational acceleration)(angle displacement)

    3. The attempt at a solution
    When plugging in the values into the third equation, I'm able to solve the problem and get the correct answer (π*sqrt2).

    But I'm wondering what's wrong with my other way of doing it.
    I tried solving for the time by plugging in the final velocity and acceleration into the 2nd equation (2π = π*t), which gives t = 2.
    Then I plugged the values into the first equation to try to solve for the initial angular velocity (π = (initial rotational velocity x 2) + 1/2(π)2^2
    But that didn't work out. Is there a reason why?

    Thanks for any help!
  2. jcsd
  3. Mar 1, 2017 #2


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    Your formula for the rotational velocity is not complete. You have ignored the initial rotational velocity in it.
  4. Mar 1, 2017 #3
    I see, thanks!
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