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Posted by on Thursday, June 25, 2009 at 10:44pm.

A large turntable rotates about a fixed vertical axis, making one revolution in 6.00 s. The moment of inertia of the turntable about this axis is 1200 kg * mg^2. A childe of mass 40.0 kg, initually staning at the center of the turntable, runs out along a radius. What is the angular speed of the turntable when the child is 2.00 m from the center, assuming that you can treat the child as a paarticle?

  • Physics - , Friday, June 26, 2009 at 12:00am

    KE (original) = (1/2)Iω2

    KE (new) = (1/2)I'w'2

    where I'=1200 + mr2
    m=40 kg
    r=2 m
    I' = 1200 + 160 = 1360

    Setting KE (original) = KE (new)
    the only unkown is ω' for which you can solve.

    Note: ω=2π/6 rad/s = π/3 rad/s.

  • Physics - , Friday, June 26, 2009 at 12:06am

    KE (original) = (1/2)IĆ¹2
    (1/2)(1360)(PI/3)^2 = 744.9475

    do i do the same for new... the "w" looks different

  • Physics - , Friday, June 26, 2009 at 12:17am

    I put w' instead of ω', but they are meant to be omega's. ω' is different from ω.
    Note also that I changes to I'.

  • Physics - , Friday, June 26, 2009 at 12:59pm

    KE (original) = (1/2)I(PI/3)2

    KE (new) = (1/2)(1360)w'2

    right??? now im confused because i don't have I and i don't have w^2

  • Physics - , Friday, June 26, 2009 at 2:24pm

    I' is the sum of the turn-table + the child. The contribution of the child is simply mr2, since it can be considered as a particle.
    So I' = 1200 kg * mg^2 + 40 kg * 22 kg * mg^2
    = 1320 kg * mg^2 (as already worked out above).

    The only unknown is now omega2 which can be obtained by equating KE before and after the child's participation, i.e.
    KE original = KE new

  • Physics - , Friday, June 26, 2009 at 2:26pm

    Oops, I' should be 1360 as you had it.
    I is given in the question, 1200 kg * mg^2, so the only unknown is omega (w) in the equation.

  • Physics - , Sunday, March 2, 2014 at 6:04pm

    That is not right at all guys -_-

  • Physics - , Sunday, March 2, 2014 at 6:13pm

    I' w' = I'' w''

  • Physics - , Sunday, November 9, 2014 at 10:22pm

    anyone know the answer?

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