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Homework Help: Sign in Conservation of Angular Momentum Problem

  1. Jan 28, 2008 #1
    1. The problem statement, all variables and given/known data
    A girl of mass M stands on the rim of a frictionless merry-go-round of radius R and rotational inertia I that is not moving. She throws a rock of mass m horizontally in a direction that is tangent to the outer edge of the merry-go-round. The speed of the rock, relative to the ground, is v. Afterward what are (a) the angular speed of the merry-go-round and (b) the linear speed of the girl?

    2. Relevant equations
    L=M(R x V)

    3. The attempt at a solution
    when setting up the conservation of linear momentum I get
    (I+MR^2)w + mRvsin(90) = 0
    although I know I will only get the correct answer by changing the addition sign to a subtraction sign, but still, by the thumb-method the product of r and v should be positive, not negative..so why do I have to make it negative?
  2. jcsd
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