def _force(self,z,t=0.):
        """
        NAME:
           _force
        PURPOSE:
           evaluate the force
        INPUT:
           z
           t
        OUTPUT:
          F_z(z,t;R)
        HISTORY:
           2010-07-13 - Written - Bovy (NYU)
        """
        return self._Pot.zforce(self._R,z,phi=self._phi,t=t,use_physical=False)\
            -self._Pot.zforce(self._R,0.,phi=self._phi,t=t,use_physical=False)
NAME:
           _force
        PURPOSE:
           evaluate the force
        INPUT:
           z
           t
        OUTPUT:
          F_z(z,t;R)
        HISTORY:
           2010-07-13 - Written - Bovy (NYU)