What’s going on when your shoelaces come untied?

Christine Gregg, a graduate student in mechanical engineering at UC Berkeley, talks about the forces behind your pesky shoelace problem.

Vocabulary: force, inertial forces, impact, knot, mechanism, friction, structure

Next Generation Science Standards: PS2.A: Forces and Motion, PS3.C: Relationship Between Energy and Forces, CC2: Cause and Effect: Mechanism and Explanation, SEP3: Planning and Carrying Out Investigations, can be used to build towards MS-PS2–2.

Common Core State Standards: CCSS.ELA-LITERACY.RST.9–10.1, CCSS.ELA-LITERACY.RST.6–8.1, CCSS.ELA-LITERACY.RST.11–12.8

Credit: brennaval/flickr/CC BY-NC-ND 2.0

One of the first lessons you learn growing up is how to tie your shoes. Around the same time, you also learn that your shoelaces will eventually come untied. So why do your shoelaces experience ‘catastrophic knot failure’? Christine Gregg, a graduate student in mechanical engineering at UC Berkeley, and a group of researchers investigated the different forces acting on a shoelace knot. The results were published this week in the journal Proceedings of the Royal Society. Gregg describes how the forces created by walking untie a shoelace and the difference between a ‘weak’ and ‘strong’ knot.

Audio Excerpt “Physics Is Untying Your Shoelaces,” 4/14/2017. (Original Segment)

Print this segment transcript.

Questions

  • Based on Christine Gregg’s description, create a diagram of shoelaces (or use one from the original paper). Label the loose ends, loop, and center know. Color the diagram to indicate where friction forces and inertial forces are acting.
  • Although this research may seem silly, Christine discusses applications to surgical sutures and how entangled molecular structures interact. Why do you think it might be useful for researchers and engineers to be able to “predict the behavior of certain knots based only on their structure?”
  • Based on the information in the article what could you do to improve shoelace design, aside from replacing it with Velcro, that might mitigate ‘catastrophic knot failure’ regardless of your tying method? Be sure to use evidence from the interview to justify your design decisions.
A pendulum experiment was used to study the effects of impact and lace dynamics on a knot. Image by Christopher Daily-Diamond, Christine Gregg, and Oliver M. O’Reilly/University of California, Berkeley
  • Look at the experimental setup above. What variables were the researchers trying to control? Why are experimental controls important?

Activity Suggestions

  • Have students evaluate whether they are using the strong form or the weak form of the shoelace knot. Have groups decide what observation method to use when evaluating shoelace knots (e.g. use diagram from the paper, video shoelace tying). This TED talk has a simple test for figuring out which version you have. Encourage students to extend their observations by surveying handedness, likelihood of tying a double knot, and shoe type. Have students develop a testable question based on their observations.
  • Have students design an experiment to extend of the research of Christine and her colleagues. Encourage them to evaluate the potential contribution of their experiment to the body of knowledge on their topic. This can be done as a thinking exercise or you could have students carry out their experiments.

Additional Resources

The original paper features some great images of observations and experimental setup from this research. This article contains more information about the inspiration for this study. This article details more about the methodology.

[Interested in knots? Learn more about knots tied at the molecular level.]