1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Moment of Inertia of a System

  1. Mar 30, 2015 #1
    1. The problem statement, all variables and given/known data
    "a uniform steel rod of length 1.20 meters and mass 6.40 kg has attached to each end a small ball of mass 1.06 kg. The rod is constrained to rotate in a horizontal plane about a vertical axis through its midpoint. Find the moment of inertia of the ball-rod system."

    2. Relevant equations
    I = (1/12)ML^2
    I = MR^2

    3. The attempt at a solution
    So, my friend was trying to help explain to me the solution to this, but I'm kind of stuck on it. See, what she did was:

    I(system) = I(ball) + I(rod)
    = MR^2 + (1/12)ML^2
    = M(L/2)^2 + (1/12)M(L)^2

    My question is why you can assume that the radius of the ball is apparently half of the length of the rod. That doesn't really seem like a logical conclusion to make.
  2. jcsd
  3. Mar 30, 2015 #2
    Wait! Never mind. I just looked through my notes again. "R" (as it is defined here) is actually just the perpendicular distance that a particle (in this case, the ball) is from the given rotation axis. In that case, R should be L/2 here.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted