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Comparing 2 formula derivations

  1. Oct 6, 2009 #1
    Here is how my book goes about defining the moment of inertia.
    Now here's how my book goes about deriving an expression for the angular momentum of a rigid body.
    From this the book derives that L = Iw
    My question is in regards to the bolded part. How come in one derivation we assume r_i is a measure of the perpendicular distance from the axis of rotation, and in the other we made no such distinction, but rather considered r_i a 3-D vector?
  2. jcsd
  3. Oct 9, 2009 #2
    My physics teacher answered this for me.

    The only reasons the derivations are different regarding r_i is because inertia isn't a vector whereas angular momentum is.
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