Abstract
The accidental untying of a shoelace while walking often occurs without warning. In this paper, we discuss the series of events that lead to a shoelace knot becoming untied. First, the repeated impact of the shoe on the floor during walking serves to loosen the knot. Then, the whipping motions of the free ends of the laces caused by the leg swing produce slipping of the laces. This leads to eventual runaway untangling of the knot. As demonstrated using slow-motion video footage and a series of experiments, the failure of the knot happens in a matter of seconds, often without warning, and is catastrophic. The controlled experiments showed that increasing inertial effects of the swinging laces leads to increased rate of knot untying, that the directions of the impact and swing influence the rate of failure, and that the knot structure has a profound influence on a knot's tendency to untie under cyclic impact loading.
Highlights
Introduction and motivationWhile most people have experienced accidental untying of their shoelaces, little is known and even less is documented about the physical mechanisms responsible for this ubiquitous annoyance
Tests conducted with 3 g weights experienced acute knot failure, which occurred for approximately half of the 3 g tests (53% or 8/15 tests)
Observations point to a failure driven by the complex interplay between impact-induced deformation of the centre of the knot, dynamic swinging of the walking motion, and the inertial forces of the laces and free ends of the knot
Summary
While most people have experienced accidental untying of their shoelaces, little is known and even less is documented about the physical mechanisms responsible for this ubiquitous annoyance. Stomping the foot on the ground the same number of cycles did not lead to untying These observations suggest that the knot failure involves an interplay between the swing and stance phases (cf figure 1) of the walking motion. There appeared to be two time scales upon which untying took place: little change to the knot was observed for many strides until some untying began, after which the speed of untying was remarkable (often in less than two running strides).. There appeared to be two time scales upon which untying took place: little change to the knot was observed for many strides until some untying began, after which the speed of untying was remarkable (often in less than two running strides).1 These observations informed both our hypothesized failure mechanism and experimental design. Our work presents some challenges to the computational mechanics community which we hope will inspire future work
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