1. The problem statement, all variables and given/known data The problem is related with central forces. In the problem I am given the equation of the orbit of a particle subjected to a central force (with an angular momentum "l"), r=a/(phi+1)^2 (where "r" is de distance to the center of forces and "a" a constant). I am asked for the potential energy of the particle, the force applied to it and then I am asked to discuss qualitatively the characteristics of the orbit of a particle subjected to this force, depending on its energy. 2. Relevant equations E=(1/2)m(dr/dt)^2 + (l^2)/(2mr^2) + U(r) F=-dU/dr 3. The attempt at a solution Getting to the potential energy of the particle and force applied to it is rather easy, and I am sure the following expressions are correct: U(r)=E-(l^2/(2mr^2))(1+(4r/a)) and F=-(l^2/(mr^2))(2/a+1/r). I think the graph I am looking for is something like this: http://l27.imgup.net/adasdad3f46.jpg [Broken]. However now I don't know what levels of energy to use, and what to say about them. My guess would be that I have to simply use one positive and one negative level of energy. Could someone help please?