1. Not finding help here? Sign up for a free 30min 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!

Rotational Kinetic Energy

  1. Feb 28, 2009 #1
    1. The problem statement, all variables and given/known data
    A uniform rod AB is hinged smoothly at A. The length L of the rod is 50cm. Determine
    a) The angular velocity,


    of the rod when it makes an angle (Theta) =30o with the verticle.

    ( Moment of inertia of rod about axis through one end and perpendicular to the rod = 1/3 mL2 ; g =9.8m s-2


    2. Relevant equations
    Ki + Ui = Kf + Uf


    3. The attempt at a solution

    0 + mghi = 1/2 Iw2 + mghf

    Where hi = 1/2 L

    and hf = 1/2 L ( 1-cos Theta )

    Then plugging in the information given,
    Angular velocity = 7.1 rad s-1

    When the rod is vertical, let the horizontal line which passes through the center of the rod be the reference line where h=0 and U=0.

    Can anybody tell me why the final potential energy is not just 1/2 L cos Theta.

    Or is it 1/2 L ( 1-cos Theta ) ?

    Thanks...
     
  2. jcsd
  3. Mar 1, 2009 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi Victorian91! :smile:

    (have a theta: θ :wink:)

    potential energy can be measured from anywhere

    it's your choice …

    if you chose to measure it from θ = 0, then you have make sure that it's 0 at θ = 0 :wink:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook