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Angular Acceleration question

  1. Mar 12, 2015 #1
    1. The problem statement, all variables and given/known data
    Dario, a prep cook at an Italian restaurant, spins a salad spinner 20.0 times in 5.00 seconds and then stops spinning it. The salad spinner rotates 6.00 more times before it comes to rest. Assume that the spinner slows down with constant angular acceleration.

    What is the angular acceleration of the salad spinner as it slows down? Express your answer numerically in radians per second per second.

    2. Relevant equations

    ω2f = ω20 + 2α(θ)

    3. The attempt at a solution

    ω = θ / t
    ω = 20 * 2 * π / 5.00
    ω = 8π
    ω = 25.13272 radians/s

    now I can solve for α using a kinematic equation:

    ω2f = ω20 + 2α(θ)
    02 = 25.132722 + 2 * α * (2 * π * 6.00)
    0 = 631.6536 + 75.39816α
    75.39816α = -631.6536
    α = -8.37757 radians/s2

    apparently This answer is wrong...where did I go wrong?
     
  2. jcsd
  3. Mar 12, 2015 #2
    Significant figures??

    Chet
     
  4. Mar 12, 2015 #3
    -8.0 didnt work as well.
     
  5. Mar 12, 2015 #4
    -8.38??

    Chet
     
  6. Mar 12, 2015 #5

    SammyS

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    I used a somewhat different method and got -8π/3 rad/s2, which agrees with you.
     
  7. Mar 12, 2015 #6
    α =
    −8
    radians/s2

    ANSWER 2: Deduction: -3%
    α =
    −8π3
    radians/s2

    The question only asks for the magnitude of the angular acceleration.
    ANSWER 3: Deduction: -3%
    α =
    −8.4
    radians/s2

    The question only asks for the magnitude of the angular acceleration.
    ANSWER 4: Deduction: -3%
    α =
    −2.67π
    radians/s2

    The question only asks for the magnitude of the angular acceleration.


    These were my answers. All marked wrong.
     
  8. Mar 12, 2015 #7

    SammyS

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    After the first message stating that only the magnitude was asked for, it seems reasonable to give the magnitude. (The statement in the OP clearly asks for angular acceleration, no mention of magnitude.)
     
  9. Mar 14, 2015 #8
    What would be the magnitude though? I am confused as to why my answer of -8.38 radians/s^2 is wrong
     
  10. Mar 14, 2015 #9
    When they ask for the magnitude of a vector, they want it without the sign. The magnitude is always taken to be positive, and the sign is absorbed into the direction of the unit vector. Pretty silly to require that in the answer to this problem, however.

    Chet
     
  11. Mar 14, 2015 #10

    SammyS

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    I agree with Chet on this point, especially since there was no mention of magnitude in the problem statement.
     
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