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Homework Help: Rotational Kinetic energy

  1. Aug 4, 2010 #1
    1. The problem statement, all variables and given/known data
    A horizontal 800N merry go round of radius 1.50m is started from rest by a constant horizontal force of 50.0N applied tangetially to the merry go round. Find the kinetic energy of the merry go round after 3.00s. (assume it is a solid cylinder).


    2. Relevant equations
    I = MR^2

    KE (rotational) = I (omega)^2



    3. The attempt at a solution
    I know this is a straight forward question. I don't know where to start. I know there are a few unknowns: omega, angular acceleration, velocity.

    can someone guide me please? thanks
     
  2. jcsd
  3. Aug 4, 2010 #2

    Doc Al

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    Staff: Mentor

    Hint: Use the torque to find the angular acceleration. Then use some kinematics. (That's just one way to go.)
     
  4. Aug 4, 2010 #3
    K. thanks. Here's what I did:

    I found moment of inertia (I = mr^2). For m, i found that using the given weight, 800N.

    I used the torque equation to find angular acceleration. (torque = I * angular acceleration) Where torque is equal to the Force * r. Once i got the angular acceleration, i solved for tangential acceleration (a = r * angular acceleration).

    Then I found v using the equation, v = a*t.

    Once I got v, i found angular velocity from the equation, v = r * omega.

    THen finally I can solve for Kinetic energy! KE = 1/2 * I (omega)^2

    my answer came up to 2.76 x 10^4J, but in the book it's 276J!!! :cry:
     
  5. Aug 4, 2010 #4

    Doc Al

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    Staff: Mentor

    That should be: I = 1/2 mr^2.

    That's OK, but there's no need to convert from angular quantities to linear then back to angular! The kinematic formulas work just fine for angular quantities:
    Use ω = alpha*t instead of v = a*t.

    (The fewer 'conversions' the fewer chances for arithmetic errors.)

    Give it one more shot.
     
  6. Aug 4, 2010 #5
    k. I got the answer
     
  7. Aug 4, 2010 #6
    ooops...i accidentally clicked on post reply.

    In calculating the angular acceleration, i wrote down the wrong Force creating the torque. Instead of 50.0N, I used 500N!! silly mistake!

    Thanks Doc Al =D
     
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