code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          #Cylindrical distance
          Rdist = _cylR(R,phi,self._orb.R(t),self._orb.phi(t))
          # Difference vector
          (xd,yd,zd) = _cyldiff(self._orb.R(t), self._orb.phi(t), self._orb.z(t),
              R, phi, z)
          #Evaluate cylindrical radial force
          RF = evaluateRforces(self._pot,Rdist,zd, use_physical=False)
          # Return R force, negative of radial vector to evaluation location.
          return -RF*(nu.cos(phi)*xd+nu.sin(phi)*yd)/Rdist
docs: NAME:
             _Rforce
          PURPOSE:
             evaluate the radial force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the radial force
          HISTORY:
             2011-04-10 - Written - Bovy (NYU)
             2018-10-18 - Updated for general object potential - James Lane (UofT)