1. The problem statement, all variables and given/known data A n object with mass 5e23 kg travels around a star in a nearly circular orbit in the xy plane. Its speed is nearly constant at 4.1e4 m/s. Diagram accompanying the question http://img197.imageshack.us/img197/7682/68232428.jpg [Broken] It asks to find the momentum at point D and also it asks to find the change in momentum going from D to A. 2. Relevant equations change in momentum = pf - pi where pf = final momentum and pi = initial momentum momentum = (gamma) x (mass) x (velocity) 3. The attempt at a solution I was stuck right from the beginning because the example in the textbook had the exact same diagram but gave atleast one position vector of any of the points. In this question, a position for the object is not given. I then tried to do trigonometry (long shot?), by making each of the points contained within a right angled triangle. With a 45-90-45 triangle, I assumed that the vector D had a postion vector of (1,0,0) and therefore A had a position vector of (0,-1,0). This is obviously not the answer because I had not even utilized the given information in the question. Maybe there is enough information given and I am just missing something? -Thanks in advance.