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Simple angular problem but answer is wrong

  1. Oct 20, 2008 #1
    When a turntable rotating at 33 1/3 rev/min is shut off, it comes to rest in 26 s. Assume constant angular acceleration.
    (a) Find the angular acceleration.
    rad/s2

    (b) Find the average angular velocity of the turntable.
    rad/s

    (c) Find the number of revolutions it makes before stopping.
    rev

    For Part A I did:
    omega = omega initial + alpha * t
    omega = alpha * t
    omega/t = alpha

    (33.3 rev/min) / (26s) * (2Pi rad) / (1 rev) * (1 min) /(60s)
    = .1341218402 rad/s^2 = approx .13 rad/s^2
    but this is wrong.

    For Part C:
    displacement = omega initial + 1/2 alpha * t^2
    = 1/2(.1341218402 rad/s^2)(26s)^2
    =45.33318199 approx = 45 revolutions.

    For Part B is it taking the average of initial omega velocity and final omega velocity?
     
  2. jcsd
  3. Oct 20, 2008 #2

    LowlyPion

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    Isn't acceleration negative? (V goes to 0.)

    Your equation for c) yields radians not revolutions.

    b) yes. Initial angular velocity over 2.
     
  4. Oct 20, 2008 #3
    Hmm, I put -.13 rad/s^2 but it was still wrong and as for part C I calculated to approx 71 revolutions and it was wrong. what else I did wrong?

    Initial angular velocity / 2
    initial angular velocity is = angular acc. * 1s correct?
     
  5. Oct 20, 2008 #4

    LowlyPion

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    33.33 = 100/3
    My calculation for angular acceleration is w = 2*π*f = 2*π*100/(3*60)

    Since w = a*t => a = w/t = 2*π*100/(3*60*26) = -.13426 r/s2

    Total radians = 1/2*a*t2 = 45.38 radians

    Revolutions = radians/(2*π) = 7.22 revolutions

    It couldn't have been 71 revolutions because it only was going 33/min. At full speed it can't be over 33/2 since 26 sec is < 1/2 min.

    w = 3.49 rad/s and half is 1.75 rad/sec
     
  6. Oct 20, 2008 #5
    Oh, I guess I should've put more decimal places rather than just -.13 rad/s^2
    and for part B, you are right it was 7.22, I saw it as 72.

    Thanks for the clarification!
     
  7. Oct 20, 2008 #6

    LowlyPion

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    Not sure what the number of significant digits are for your teacher. Perhaps there is something from class?

    They ask for rad/sec2 and that has to be a small fractional number.
     
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