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Homework Help: Angular Acceleration help

  1. Jul 18, 2006 #1
    This problem states:

    A human centrifuge takes 1.0 min to turn through 20 complete revolutions before reaching final speed.

    They are asking for angular acceleration (a) and angular speed in rpm (b).

    The answer to a is 40 rev/min^2
    The answer to b is 40 rev/min
    -I'm have the formulas for angular acceleration but most of them won't work b/c I don't have any velocity attached to this problem.

    I don't know how to set the problem up to receive the right answer. I've tried converting from radians to rev/min^2 and end up with the wrong answer every time. This isn't a very difficult problem but I don't understand how to set the problem up to find the right answer.

    I end up with 2.09 rad/s but that answer doesn't take me in the right direction (at least I don't think so).

    Thanks in advance,
  2. jcsd
  3. Jul 18, 2006 #2
    Why not try some Kinematics equations? Since angular acceleration is presumed to be constant, you can convert these equations to the relevant form for rotational motion.

    For example, a=(v-u)/t will now be: angular acceleration=(final angular velocity-initial angular velocity)/time

    Note: Choose units for angular displacement, angular velocity and angular acceleration wisely! Also, remember that the centrifuge starts from rest.
    Last edited: Jul 18, 2006
  4. Jul 18, 2006 #3
    when i use that equation, i end up with .333.

    i'm doing (20rad/s-0rad/s)/60s = 0.333 ??? The answer needs to be in rev/min^2 and I don't know how to get the answer in the form that they want it. If i double the amount that I have, I'll have the right answer, but I don't understand why.
  5. Jul 18, 2006 #4
    20 isn't the final angular velocity, it is the displacement. You don't yet know what the final velocity is, but since you know the initial velocity, the time, and the displacement, you can use a kinematics equation like displacement = vi*t + 1/2*a*t^2 to solve for the acceleration. Now you have the initial velocity, the time, and the acceleration, so you can easily solve for the final velocity. Since the answer is in rpm, you don't need to convert any of the numbers to a different form, ie leave the time in minutes and dont use radians.
  6. Jul 18, 2006 #5
    thank you dzza...finally got the right answer.

    i don't think i understood exactly what information i had. i misinterpreted the 20 rev/min as final velocity.

    thank you very much!
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