What makes a lasso spin? news.sciencemag.org/physics/2014/09/what-makes-lasso-spin Finally, physicists are catching up to cowboys—at least in the lasso department. With the help of a trick roper who works at Disneyland Paris, a custom built “robo-cowboy,” and a researcher who spent hours practicing with a lasso, a team of physicists has mathematically mastered a trick called the flat loop, in which the loop in the lasso rotates horizontally near the roper’s legs. By analyzing high-speed video of both the professional roper and the robo-cowboy (essentially a couple of motorized, rotating joints that mimic the motion of a human arm and wrist) performing the flat loop (*), the scientists isolated the specific forces that govern the lasso’s movement and wrote the first equations that accurately describe the behavior of the rope during the trick. The key to performing the flat loop like a true cowboy? Making sure you have the right amount of loop in your lasso. About 75% of the total length of the lasso needs to be in the loop for the trick to work properly, the team reports today in the Proceedings of the Royal Society A. Any less and the system will collapse, with the loop shrinking down to a point—a common problem for beginners who tend to make their loops too small. Similar mechanics are seen in industrial yarn processing, the researchers say, although for now they plan to concentrate future work on mathematically describing more complex cowboy tricks (**). (*) Lariat Flat Loop Learning: Rope Spinnng https://youtube/watch?v=2NEBQq9IuFA (**) The Texas Skip: Max Reynolds performs the Texas Skip in front of an audience. https://youtube/watch?v=FO_FjLzSIQY Reference An introduction to the mechanics of the lasso Proc. R. Soc. A 8 November 2014 vol. 470 no. 2171 20140512 doi: 10.1098/rspa.2014.0512 rspa.royalsocietypublishing.org/lookup/doi/10.1098/rspa.2014.0512 Abstract Trick roping evolved from humble origins as a cattle-catching tool into a sport that delights audiences all over the world with its complex patterns or ‘tricks’. Its fundamental tool is the lasso, formed by passing one end of a rope through a small loop (the honda) at the other end. Here, we study the mechanics of the simplest rope trick, the Flat Loop, in which the rope is driven by the steady circular motion of the ropers hand in a horizontal plane. We first consider the case of a fixed (non-sliding) honda. Noting that the ropes shape is steady in the reference frame rotating with the hand, we analyse a string model in which line tension is balanced by the centrifugal force and the ropes weight. We use numerical continuation to classify the steadily rotating solutions in a bifurcation diagram and analyse their stability. In addition to Flat Loops, we find planar ‘coat-hanger’ solutions, and whirling modes in which the loop collapses onto itself. Next, we treat the more general case of a honda that can slide due to a finite coefficient of friction of the rope on itself. Using matched asymptotic expansions, we resolve the shape of the rope in the boundary layer near the honda where the ropes bending stiffness cannot be neglected. We use this solution to derive a macroscopic criterion for the sliding of the honda in terms of the microscopic Coulomb static friction criterion. Our predictions agree well with rapid-camera observations of a professional trick roper and with laboratory experiments using a ‘robo-cowboy’.
Posted on: Tue, 30 Sep 2014 20:55:02 +0000
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