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    sfij29 Posts: 2, Reputation: 1
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    #1

    Dec 9, 2007, 02:42 PM
    rotational kinetic energy.
    two uniform solid cylinders, each rotating about its central (longitudinal) axis, have the same mass of 1.25 kig and rotate with the same angular speed of 235 rad/s, but they differ in radus. What is the rotational kinetic energy of a) the smaller cylinder, of raidus 0.25 m, and b) the larger cylinder, of radius 0.75?



    I started part a..
    K=(1/2)Iw^2 [w is omega]
    I= sum (mr^2)
    I don't know how you do the sum or whatever..
    I just put in the mass they gave me and did
    K=(1/2)(1.25)(0.25^2)(2.35)^2
    and I got 4,314.45 J
    the correct answer from the back is 1.1 kJ
    help?
    sfij29's Avatar
    sfij29 Posts: 2, Reputation: 1
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    #2

    Dec 9, 2007, 02:47 PM
    never mind.
    figured it out.
    I forgot to note that because of the shape and how it was rotating, (1/2)mr^2 could be used for I.

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