1. The problem statement, all variables and given/known data 2. The attempt at a solution I would use the arctan (Position j / Position i) and set it equal to the position vector. Then I would substitute values from the graph to find the variables. The solution manual, however, takes the derivative of the position vector and then sets it equal to the arctan (Position j / Position i). I don't understand the reasoning behind taking the velocity vector (derivative of position vector) instead of the position vector itself.