1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar

    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

    User Avatar

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook