1. The problem statement, all variables and given/known data Please find it attached. 2. Relevant equations 3. The attempt at a solution For the first part of the question I managed to show that the vector is a constant of the motion by differentiating it with respect to time and showing that it yields a value of zero. As for the second part where I need to calculate the magnitude and direction of the vector then I'm not sure how to go about doing that explicitly. I know that the way vector is defined is such that it is equivalent to the eccentricity vector and so for the circle part it would necessarily be equal to zero. But to show that explicitly I'm not to sure. Any ideas?