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: Conservation of energy with rotational motion problem

  1. Nov 13, 2012 #1
    1. The problem statement, all variables and given/known data
    Alright so I am confused about why the solution does not include the kinetic energy due to rotational energy from I_g2 as indicated in red in the following image. I don't understand why they would neglect it... I calculated the value and it makes a big difference.

  2. jcsd
  3. Nov 13, 2012 #2
    The rotational kinetic energy measures energy of the body due to rotation around its center of gravity.

    The person is not spinning around his center of gravity (located somewhere in his stomach); presumably he stays upright during the ride.
  4. Nov 13, 2012 #3
    But the person spins around the pivot? OH is it because the center of gravity for the bar INCLUDES the person on it???

    Are we suppose to make an assumption?
    Last edited: Nov 13, 2012
  5. Nov 13, 2012 #4
    I just saw the question says 'neglect the size of the passenger' which means IG2 = 0. Notice that IG2 << IG1 anyway.

    But just to continue... the person and bar are definitely treated separately here. If the person were rigidly attached to the bar so that he does not move with respect to the bar, you would be right to say the person has rotational kinetic energy 0.5*IG2*w^2. This is because the person would be like he is part of the bar and would posses the same w.

    I assumed that this ride is like most rides where the person is seated in a carriage that is pinned to the end of the arm so that the person always stays upright. Then the person does not spin and has no rotational kinetic energy.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook