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Homework Help: Rotation, Power, Inertia

  1. Apr 6, 2004 #1
    Delivery trucks that operate by making use of energy stored in a rotating flywheel have been used in Europe. The trucks are charged by using an electric motor to get the flywheel up to its top speed of 198π rad/s. One such flywheel is a solid, homogeneous cylinder with a mass of 530 kg and a radius of 1.0 m. If the truck operates with an average power requirement of 8.5 kW, how long can it operate between chargings?

    :confused:

    Just a push in the right direction, i've written out so many formulas relating to rotation and Power/Work. And i haven't found a starting point.
     
  2. jcsd
  3. Apr 6, 2004 #2

    Doc Al

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    Staff: Mentor

    Start by figuring how much rotational KE is stored in the flywheel when it rotates at top speed.
     
  4. Apr 6, 2004 #3
    (edit again)
    I solved for I with I=(1/2)mr^2
    I plugged I and w into K= Iw^2 and solved for K.
    Whew..

    I'll keep workin from here, thanks.
     
    Last edited: Apr 6, 2004
  5. Apr 6, 2004 #4

    Doc Al

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    Staff: Mentor

    Rotational KE

    I'm not sure what that equation is supposed to be. Here's another hint: The rotational inertia of a cylinder about its center is I = 1/2 M R2. You'll need it.
     
  6. Apr 6, 2004 #5
    I=(1/2)mr^2
    I=265
    (edited)

    I'm thinkin my next step is to find alpha or torque, i don't see how to do that but there is
    a_r=w^2r
    What is a_r?
     
    Last edited: Apr 6, 2004
  7. Apr 6, 2004 #6
    Oh, I didn't know what a fly wheel was really. : )
     
  8. Apr 6, 2004 #7

    ShawnD

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    I get the "I" as being something completely different.

    [tex]I = \frac{1}{2}mr^2[/tex]

    [tex]I = \frac{1}{2}(530)(1)^2[/tex]

    [tex]I = 265[/tex]


    Once you get the energy

    [tex]E = Pt[/tex]
     
  9. Apr 6, 2004 #8
    Ya shawn, you're right.
     
  10. Apr 6, 2004 #9
    Am so out of it. disregard this reply :)
     
  11. Apr 6, 2004 #10

    ShawnD

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    Science Advisor

    Power is 8500. Time is what you are supposed to find.
     
  12. Apr 6, 2004 #11
    Got it, Thanks Shawn and Doc Al.

    Brain moving slow today.
     
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