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Homework Help: The angular velocity of a process control motor is (21−(1/2t^2)) rad/s

  1. Sep 20, 2013 #1
    1. The problem statement, all variables and given/known data
    The angular velocity of a process control motor is (21−(1/2t^2)) rad/s, where t is in seconds.

    1.) At what time does the motor reverse direction?

    2.) Through what angle does the motor turn between t =0 s and the instant at which it reverses direction?

    2. Relevant equations
    angular velocity of a process control motor is (21−(1/2t^2)) rad/s

    3. The attempt at a solution
    For part 1 I just set the equation equal to 0 and got a time of 6.5 sec, which is correct

    For part 2, I have no idea what to do. Nothing is working, so if you could help, that would be great! I have tried taking the derivative to find acceleration, which is -t, and then plugging that into θf = θi + ωi(t) + 1/2a(t^2). I set θi = 0 because I figured there would initially be no angle. I set ωi = 21 because at t=0, ω = 21. I set t= 6.5, and a=-6.5...so my equation was θf = 0 + 21(6.5s) + 1/2(-6.5)(6.5s^2), but the answer came to like -0.8125 rad? is that right? it seems odd to me...
    Last edited: Sep 20, 2013
  2. jcsd
  3. Sep 20, 2013 #2


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    Staff: Mentor

    Hi zapzapper, Welcome to Physics Forums.

    It appears that you're familiar with calculus since you mentioned taking a derivative.

    Suppose this were a linear motion problem and you were given a velocity function v(t). How would you go about finding d(t) using calculus? You can apply the same method to rotational motion where velocity is represented by the variable ω and distance by θ.
  4. Sep 20, 2013 #3
    ok, I'll integrate the function
    Last edited: Sep 20, 2013
  5. Sep 20, 2013 #4
    oh ok, so I just integrate the function then?, If I integrate it then I get (21t - (1/6t^3) + c), what do i do with the constant c though?
  6. Sep 20, 2013 #5


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    Staff Emeritus
    Science Advisor
    Homework Helper

    You are trying to find the total angular displacement between two known times. What happens to the constant of integration when you evaluate a definite integral?
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