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Rotational kinematics and energy

  1. Mar 26, 2008 #1
    [SOLVED] rotational kinematics and energy

    1. The problem statement, all variables and given/known data
    A 190 kg woman stands at the rim of a horizontal turntable with a moment of inertia of 1.3x10^3 kg.m^2 and a radius of 0.68 m. The system is initially at rest and the turntable is free to rotate about a frictionless vertical axle through its center. The woman then starts walking clockwise (when viewed from above) around the rim at a constant speed of 0.86 rad/s relative to the Earth.
    With what angular speed does the turntable rotate? Answer in units of rad/s.


    2. Relevant equations
    w(angular velocity)=v/r
    I=mr^2
    Conservation of angular momentum I(woman)W(woman)=I(turntable)W(turntable)


    3. The attempt at a solution
    FIRST, I figured out the angular velocity of the woman.
    w=v/r=0.86/0.68=1.2647 m/s

    SECOND, I figured out the moment of inertia of the woman.
    I=mr^2=(190)(0.68^2)=87.856

    LAST, I used this information to solve for the angular speed of the turntable using the Conservation of Angular Momentum equation.
    I(woman)W(woman)=I(turntable)W(turntable)
    Rearranging this equation, I get W(turntable)=I(woman)W(woman)/I(turntable)
    W(turntable)=(87.856x1.2647)/(1.3x10^3)
    W(turntable)=0.08547


    However, when I punch this answer in, it says the answer is wrong but I don't know what I'm doing wrong. HELP!!!
     
  2. jcsd
  3. Mar 26, 2008 #2

    hage567

    User Avatar
    Homework Helper

    This isn't correct.
    You were given this w in the question. w is angular speed (rad/s) and v is linear speed (m/s). You seem to have the two confused. There is no need to change the 0.86 rad/s to anything else.

    See if that solves your problem.
     
  4. Mar 26, 2008 #3
    Oh okay thanks. I see where I went wrong. I redid the problem using 0.86 as the angular speed of the woman and came out with the right answer. Thanks!
     
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