code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          Rz= R**2.+z**2.
          sqrtRz= numpy.sqrt(Rz)
          return R*(1./Rz/(self.a+sqrtRz)-numpy.log(1.+sqrtRz/self.a)/sqrtRz/Rz)
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:
             2010-07-09 - Written - Bovy (NYU)