Comparing these formulae shows that E < 0 corresponds to an ellipse (all solutions which are closed orbits are ellipses), E = 0 corresponds to a parabola, and E > 0 corresponds to a hyperbola. In particular, E = − − k 2 m 2 L 2 for perfectly circular orbits (the central force exactly equals the centripetal force requirement, which determines the required angular velocity for a given circular radius).