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Homework Help: Grindstone angular acceleration

  1. Oct 23, 2007 #1
    [SOLVED] Grindstone angular acceleration

    1. The problem statement, all variables and given/known data
    Object: a disk-shaped grindstone of mass 1.69 kg and radius 7.95 cm that operates at 707 rev/min. When the power is shut off, you time the grindstone and find it takes 32.1 s for it to stop rotating. What is the angular acceleration of the grindstone? (Assume constant angular acceleration.)


    2. Relevant equations
    W=.5mv^2
    V=distance/t
    W=Wo+at
    a=alpha, which is angular acceleration

    3. The attempt at a solution
    V=[707*(2pi/60)]/32.1 = 2.31m/s
    W=0
    Wo=.5(1.69)(4.50^2) = 4.50J
    0=4.50+a(32.1)
    therefore, a=-.140 rad/s^2

    however, this is the wrong answer according to the program. It didn't have a problem with the units though, so I don't know what I am doing wrong. Please point out any wrong assumptions or completely wrong or silly mistakes. THANK YOU!
     
  2. jcsd
  3. Oct 23, 2007 #2

    hage567

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    Homework Helper

    You seem to have W representing two different things here. I don't know what you are trying to do with the first equation you have, but it's not for rotational motion. Also, Wo is your initial angular velocity, I don't understand why you are trying to make it into energy?:confused:

    All you need for this is your third equation. Change the 707 rev/min into rad/s, that is your Wo. Solve for the angular acceleration.
     
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