  We study the propagation of tension caused by an external force along a long
polymeric molecule in two different settings, namely along a free polymer in 3d
space being pulled from one end, and along a pre-stretched circular polymer,
confined in a narrow circular tube. We show that in both cases, the tension
propagation is governed by a diffusion equation, and in particular, the tension
front propagates as $t^{1/2}$ along the contour of the chain. The results are
confirmed numerically, and by molecular dynamics simulations in the case of the
3d polymer. We also compare our results with the previously suggested ones for
the translocation setting, and discuss why tension propagation is slower in
that case.
