def _Rforce(self,R,z,phi=0.,t=0.):
        """
        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:
           2015-06-15 - Written - Bovy (IAS)
        """
        dPhidrr= -(R**2.+z**2.+self._b2)**-1.5
        return dPhidrr*R
def _Rforce ( self , R , z , phi = 0. , t = 0. ) : dPhidrr = - ( R ** 2. + z ** 2. + self . _b2 ) ** - 1.5 return dPhidrr * R
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:
           2015-06-15 - Written - Bovy (IAS)