Ever walked down the street and as you look down, you see that your shoelace is untied? Again?
Somehow, the knots magically came undone, and now you have to bend down and tie it again— one of life’s little annoyances that we all have to deal with.
But maybe it’s not magic after all. A team of mechanical engineers at UC Berkeley recently published a study examining the physics behind shoe knots unraveling. They found that several forces acted on the shoelace when the person was running, which when combined, caused the knot to come apart. Let’s take a closer look.
Our world is made up of forces. When you push on a box, you exert a force on it. The gravitational force keeps you and that box tethered to the ground. Frictional forces resist your push and make it hard to move the box. So it’s no surprise that forces can explain how shoelaces come undone as well.
In order to find the forces acting on the shoelaces, Christian Gregg, one of the authors of the study, ran on a treadmill with a slow-motion camera watching. In addition, she attached devices called accelerometers to her feet, which could measure the forces acting on the laces. The researchers found that while she was running, the knot was under an immense amount of pressure from at least two different forces. The combined effect of these forces was more than twice the force that astronauts experience during a rocket launch!
Interestingly, Christian ran for a long time before the knot started unraveling. But once it started to fall apart, Christian took only about two steps before it fell apart completely. In essence, when her foot hit the ground, the laces first experienced a downward force. Then, as the leg swung, it pushed forward on the lace and caused the ends of the knot to pull away. Both forces were needed to unravel the knot. Researchers also found that the thicker the material of the shoelace, the easier it gets undone.
Why Shoelace Unraveling Matters
This is a cool problem to solve for the sake of it, but there are plenty of applications. Knots are old (even older than 5,550 years ago, when the first shoelaces were made), and there’s many different types. Shoelace knots are the simplest: called trefoils. But most knots are a combination of trefoils, so we might be able to use this to understand how more complex knots work.
Why does that matter? We could use this knowledge to create stronger fibers for making clothes or other materials. We could also use it to understand why cables become tangled and break, or even examine how DNA breaks when forces are applied to it.
And you can experiment with different knots yourself! Try to tie your laces with different types of knots (after googling ones that work), and see how long it takes till your shoelaces come undone for each knot. Maybe you can even contribute to the next advancement in shoelace physics— but tread carefully!
Do you tie your shoe with a strong knot or a weak knot? Find out in this video and let us know!