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    heyyalliee's Avatar
    heyyalliee Posts: 37, Reputation: 1
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    #1

    Oct 25, 2010, 03:06 PM
    Rotational and Kinetic Energy for a Cylinder
    GIVEN:A 1.9 kg solid cylinder (radius = 0.20 m, length = 0.50 m) is released from rest at the top of a ramp and allowed to roll without slipping. The ramp is 0.85 m high and 5.0 m long.

    I got total energy by multiplying mass x gravity x h
    that answer is 16.

    I'm having a hard time finding rotational and translational kinetic energy.
    For solid cylinder I know I=(1/2)MR^2
    So I plug that into the equation and solve for velocity.
    mgh=(1/2)mv^2+(1/2)((1/2)MR^2) (v^2/r^2)
    w=(v^2/r^2)
    So when I solve for v I get 4.6?
    Is this right because I know this must be right to get Rotational.
    Rot-KE=(1/4mv^2)

    I could be way off. Any help is appreciated!
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #2

    Oct 26, 2010, 10:26 AM
    Quote Originally Posted by heyyalliee View Post
    GIVEN:A 1.9 kg solid cylinder (radius = 0.20 m, length = 0.50 m) is released from rest at the top of a ramp and allowed to roll without slipping. The ramp is 0.85 m high and 5.0 m long.

    I got total energy by multiplying mass x gravity x h
    that answer is 16.

    I'm having a hard time finding rotational and translational kinetic energy.
    For solid cylinder I know I=(1/2)MR^2
    So I plug that into the equation and solve for velocity.
    mgh=(1/2)mv^2+(1/2)((1/2)MR^2) (v^2/r^2)
    w=(v^2/r^2)
    So far so good. But I don't see how you got v = 4.6. This collapses to:

    mgh = 1/2 mv^2 + 1/4mv^2

    Rearrange:
    v^2 = 4gh/3 = 4(9.8)(0.85)/3 = 11.1 m^2/s^2
    v = 3.33 m/s
    heyyalliee's Avatar
    heyyalliee Posts: 37, Reputation: 1
    Junior Member
     
    #3

    Oct 26, 2010, 03:35 PM
    That is where I messed up. I tried to multiply both side by 2 to get rid of (1/2)s.
    When I set it up your way I got it right.

    Thanks!

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