Two of the most promising analytical models of unsteady friction in tu
rbulent pipe flows are based on sharply contrasting hypotheses. One us
es the history of the flow; the other uses instantaneous conditions. T
he purposes of this paper are to present an analysis using the former
approach and to indicate how to determine which of the two methods is
appropriate. A weighting function model of transient friction is devel
oped for flows in smooth pipes by assuming the turbulent viscosity to
vary linearly within a thick shear layer surrounding a core of uniform
velocity and is thus applicable to flows at high Reynolds number. In
the case of low Reynolds number turbulent flows and short time interva
ls, the predicted skin friction is identical to an earlier model devel
oped by Vardy et al (1993). In the case of laminar flows, it gives res
ults equivalent to those of Zielke (1966, 1968). The predictions are c
ompared with analytical results for the special case of flows with uni
form acceleration. It is this case that enables clarifying comparisons
to be drawn with ''instantaneous'' methods of representing transient
skin friction.