code: def potential_energy(self, potential=None):
          r
          if self.hamiltonian is None and potential is None:
              raise ValueError("To compute the potential energy, a potential"
                               " object must be provided!")
          if potential is None:
              potential = self.hamiltonian.potential
  
          return super(Orbit,self).potential_energy(potential)
docs: r"""
          The potential energy *per unit mass*:
  
          .. math::
  
              E_\Phi = \Phi(\boldsymbol{q})
  
          Returns
          -------
          E : :class:`~astropy.units.Quantity`
              The potential energy.