code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          sqrtRz= numpy.sqrt(R**2.+z**2.)
          return -R/self.a/sqrtRz/(1.+sqrtRz/self.a)**2./2./self.a
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)