1. The problem statement, all variables and given/known data Ok so take r' to be r dot.. i have E = 1/2m (r' ^2 + h^2/r^2) - A/r = E Energy equation for central force (inverse square) and r = l/(1+ecostheta) where l is the latus rectum.. need to derive the expression linking E and e i.e. E= A^2/2mh^2 (e^2-1) but cant seem to get there... any help would be great.. thanks 2. Relevant equations 3. The attempt at a solution well for a start, when you differentiate r, the result includes a term theta dot -- where does that go?!