We all go through this almost every day. You put on your shoes, tie the shoelaces firmly, and start running or walking. Soon you realize that the laces have become untied. Why do shoelaces get undone? Now three scientists at the University of California Berkeley have solved the mystery. There is a lot of interaction between your leg’s swinging motion and the impact on the ground. Christine Gregg, a graduate student at the UC Berkeley and co-author of the study, said you need both for the shoelaces to come untied.
Shoelaces get untied within a matter of seconds
Researchers found that the force of your foot hitting the ground stretches and then relaxes the knot. And then there is a second inertial force caused by the swinging of your leg, which acts on the free ends of the laces. Details of the study were described in the journal Proceedings of the Royal Society A. Gregg said in a statement that she has always had this problem as a runner. She would stop and retie the laces at least once on a run.
Christine Gregg, Oliver O’Reilly, and Christopher Daily-Diamond thought it was an important problem to solve. Understanding why and how shoelaces get untied can help scientists understand other structures such as DNA or microstructures that fail under dynamic forces. It also has implications for aerospace engineering and surgery. Christopher Daily-Diamond said it was the first step towards understanding why some knots are better than others.
To conduct the study, Christine Gregg laced up a pair of running shoes and started jogging on a treadmill. Her colleagues videotaped the activity to analyze the forces at work. Researchers also used a mechanical pendulum to identify the movements involved. Using a series of experiments and slow-motion footage, scientists found that the “shoelace knot failure” occurs within a matter of seconds.
Your foot strikes the ground at 7x of gravity
The study showed that your foot strikes the ground at 7x the force of gravity. The knot stretches and relaxes in response to the force. Then an inertial force on the ends of the laces causes a quick unraveling. The forces at work here are inertial forces of your legs swinging back and forth while the repeated striking of the foot on the ground loosens the knot.
Of course, there are a variety of different knots, and some are better than the others. But the same fundamental forces are behind the failure of each of them. Gregg said she has learned to make a square knot instead of the granny knots that most people use to tie their shoelaces. Also, tying the laces tightly requires more cycles of leg-swinging and impact to trigger the knot failure.