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AP Physics Multiple Choice Question

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  1. Mar 20, 2012 #1
    5. A force of 12 N is applied tangentially to the rim of a wheel with radius 0.70 m and rotational inertia 3.0 kg-m2. What angular acceleration results?
    a) 2.6 rad/s2 b) 2.8 rad/s2 c) 3.2 rad/s2 d) 3.4 rad/s2 e) none of these
    ____6. If the force in question #5 acts for 20 s, what will be the kinetic energy of the spinning wheel?
    a) 4.7 x 103 j b) 4.4 x 102 j c) 4. 7 x 10 j d) 84 j e) none of the above answers is within 10 % of the correct value

    I figured out number 5 to be B.) 2.8 rad/s by rF/I from Torque=Ia
    For 6, when I solve using K=1/2Iw^2, and using 2.8 rad/s^2 divided by 20 seconds to get the angular velocity of .14 rad/s, I get a really small number, like .03. What am I doing wrong?
     
  2. jcsd
  3. Mar 20, 2012 #2

    gneill

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    Staff: Mentor

    Suppose this was a regular linear kinematics problem where you have an acceleration a and you want to find the velocity v after some time t, what's the formula?
     
  4. Mar 20, 2012 #3
    you cant divide acceleration/time to get velocity?
     
  5. Mar 20, 2012 #4

    gneill

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    Nope. What would the resulting units be?
     
  6. Mar 20, 2012 #5
    (rad/s^2 )/ s
    so rad/s which is velocity?
    I dont know something in my brain isnt connecting
     
  7. Mar 20, 2012 #6
    No it would be rad/s^3. duh omg
     
  8. Mar 20, 2012 #7

    gneill

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    :smile:

    So, better to multiply acceleration by time then... :wink:
     
  9. Mar 20, 2012 #8
    thank you! I got A I hope its right!
     
  10. Mar 20, 2012 #9

    gneill

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    Looks good.
     
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