  We use Smoothed Particle Hydrodynamic simulations of cold, uniform density,
self-gravitating filaments, to investigate their longitudinal collapse
timescales; these timescales are important because they determine the time
available for a filament to fragment into cores. A filament is initially
characterised by its line-mass, $\mu$, its radius, $R$ (or equivalently its
density $\rho\!=\!\mu/\pi R^2$), and its aspect ratio, $A\;\,(\equiv Z/R$,
where $Z$ is its half-length). The gas is only allowed to contract
longitudinally, i.e. parallel to the symmetry axis of the filament (the
$z$-axis). Pon et al. (2012) have considered the global dynamics of such
filaments analytically. They conclude that short filaments ($A\! < \!5$)
collapse along the $z$-axis more-or-less homologously, on a time-scale
$t_{_{\rm HOM}} \sim 0.44\,A\,(G\rho)^{-1/2}$; in contrast, longer filaments
($A\! > \!5$) undergo end-dominated collapse, i.e. two dense clumps form at the
ends of the filament and converge on the centre sweeping up mass as they go, on
a time-scale $t_{_{\rm END}} \sim 0.98\,A^{1/2}\,(G\rho)^{-1/2}$. Our
simulations do not corroborate these predictions. First, for all $A\! > \!2$,
the collapse time satisfies a single equation \[t_{_{\rm
COL}}\;\sim\;(0.49+0.26A)(G\rho)^{-1/2}\,,\] which for large $A$ is much longer
than the Pon et al. prediction. Second, for all $A\! > \!2$, the collapse is
end-dominated. Third, before being swept up, the gas immediately ahead of an
end-clump is actually accelerated outwards by the gravitational attraction of
the approaching clump, resulting in a significant ram pressure. For high aspect
ratio filaments the end-clumps approach an asymptotic inward speed, due to the
fact that they are doing work both accelerating and compressing the gas they
sweep up. Pon et al. appear to have neglected the outward acceleration and its
consequences.
