code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          r2= R**2.+z**2.
          rb= nu.sqrt(r2+self.b2)
          dPhidrr= -1./rb/(self.b+rb)**2.
          return dPhidrr*R
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:
             2013-09-08 - Written - Bovy (IAS)