For this problem we take the potential energy to be zero as r approaches infinity, so the potential at the origin must be infinitely less, i.e., negative infinity. Note that the motion is confined to a plane so we can display the results on a flat screen. The motion is in fact quite similar to the bouncing falling ball. The green area on the r plot shows the classically allowed region. The above diagram plots various the energies involved as a function of r .
Total energy is the sum of kinetic energy and potential energy.