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Need Help For Daughter

  1. Jul 6, 2009 #1
    1. The problem statement, all variables and given/known data

    A horizontal 800 N merry-go-round of raidus 1.5 meters is started from rest by a constant horizontal force of 50 N applied tangentially to the merry-go-round find the kinetic energy of the merry-go-round after 3 seconds. Assume it is a solid cylinder.

    Hello all,

    My daughter, who is a senior in HS, is currently having trouble with the above problem for her AP Physics class. I haven't been to school in ages, so I was hoping you folks could help me out. I have no idea how to go about this, but if you guys could at least help me with setting up the formula or equation I would be eternally grateful. If you want to include the answer that would be great as well. I'll make sure she works out the problem with the formula first before having her check her answer against yours.

    Thanks again!
  2. jcsd
  3. Jul 6, 2009 #2


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    Homework Helper

    First of all find the mass of the merry-go-round.
    Find moment of inertia by using the formula for cylinder I = 1/2*M*R^2, where R is the radius of the merry-go-round.
    A tangential force is applied. Find the torque T = F*R = Iα where α is the angular acceleration.
    Find the angular velocity ω after time t seconds by using ω = ωo + αt.
    Finally the kinetic energy KE = 1/2*I*ω^2.
  4. Jul 6, 2009 #3


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    Homework Helper

    How far did she get in attempting this question? Does she have no idea how to begin? In that case, think about the problem this way:

    (1) What's the formula for rotational kinetic energy? Hint: it contains ω, the merry-go-round's angular speed.

    (2) We're given the force applied to the merry-go-round, so we can find the torque. What's the formula relating torque, ω, and time?
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