Why do laces untie?

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A well made lace is by definition indissoluble and very difficult to unfasten without a precise external force acting for this purpose. When we are not the ones who undo the laces, it is evident that there must be some other external force to untie them, like two invisible hands that undo the knots with force. A group of Californian engineers has studied the phenomenon with passion, trying to give an answer to a pressing question, why do laces untie?

It often happens to those who practice sports to discover themselves with laces to be redone, or to parents who try to keep their energetic children at bay, or to those who simply go for a walk risking suddenly to fall. A small scientific team from the University of California Berkeley thought about taking care of this question, studying the different forces that act in series and simultaneously on the laces giving the start to a process of dissolution that, once begun, proceeds without stopping.

The mechanic engineer Oliver O’Reilly actually filmed in slow motion two colleagues intent on running on two treadmills with a pair of sneakers on their feet. An accelerometer has been set to analyze the forces acting on the footwear. It has thus been noted how a running foot acts with a force equal to 7 grams on the shoe, that means seven times more than the gravity one, very similar to the one that the astronauts are subjected to when they return on the Earth’s atmosphere.

The knot reacts accordingly, tightening and loosening, until the undulating movement of the leg moves in, which moves back and forth. This acts with inertial force on the two ends of the laces, pulling just as if a pair of invisible hands were trying to untie them. It is the combination of the two forces that succeeds, where singularly other forces would never had succeeded, in undoing the knot of the shoes.

Starting from the laces of the shoes, the discovery can have implications able to understand new ways to practice surgical sutures, to better analyze the structure of DNA, to find a better method to fix the cables of various kinds to the submarine backdrops.

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