code: def _Rforce(self,R,phi=0.,t=0.):
          
          return self._Pot.Rforce(R,0.,t=t,use_physical=False)
docs: NAME:
             _Rforce
          PURPOSE:
             evaluate the radial force
          INPUT:
             R
             phi
             t
          OUTPUT:
            F_R(R(,\phi,t))
          HISTORY:
             2010-07-13 - Written - Bovy (NYU)