  We discuss vortex-mediated mutual friction in the two-fluid model for
superfluid neutron star cores. Our discussion is based on the general formalism
developed by Carter and collaborators, which makes due distinction between
transport velocity and momentum for each fluid. This is essential for an
implementation of the so-called entrainment effect, whereby the flow of one
fluid imparts momentum in the other and vice versa. The mutual friction follows
by balancing the Magnus force that acts on the quantised neutron vortices with
a resistive force due to the scattering of electrons off of the magnetic field
with which each vortex core is endowed. We derive the form of the macroscopic
mutual friction force which is relevant for a model based on smooth-averaging
over a collection of vortices. We discuss the coefficients that enter the
expression for this force, and the timescale on which the two interpenetrating
fluids in a neutron star core are coupled. This discussion confirms that our
new formulation accords well with previous work in this area.
