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 I=MR^2 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?