Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G14: Invited Session: Toys and Mechanisms |
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Sponsoring Units: GSNP Chair: James Hanna, Virginia Polytechnic Institute and State University Room: 301-303 |
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G14.00001: Rotation with zero angular momentum: Demonstrations of the falling cat phenomenon go sour Invited Speaker: Andy Ruina It is well known that a system with zero angular momentum can, by appropriate deformations, rotate while preserving the condition of zero angular momentum. This effect explains how a cat that is dropped while upside down can turn over and of how certain gymnastic maneuvers are performed. These rotations are taken as a demonstration of the ``non-integrability'' of a ``non-holonomic'' constraint. There is a simple demonstration of this rotation-with-zero-angular-momentum effect with a rotating platform. But the demonstration often doesn't work because most floors are not perfectly flat. I found a simple better demonstration experiment. Unfortunately, the experiment came out all wrong for different reasons. But I figured out why and did a second demonstration experiment. And that came out wrong exactly in the opposite way. The talk presents the four puzzles: a) how can you turn while having zero angular momentum? b) Why does a rotating platform demonstration often not work. c) Why does a simple demonstration not work? d) Why does almost exactly the same demonstration not work in the opposite way? The talk starts with various personal stories about non-holonomic constraints and their relation to locomotion, like bikes skates and walking, and then gets into the 4 rotation puzzles. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G14.00002: Physics of Toys: The Joy of Asking Questions Invited Speaker: Beverley Taylor Children are natural scientists. They ask questions, they observe, they try things to see what happens. Often school-based science does little to nurture the young scientist and, in fact, may do just the opposite with thick textbooks, fact heavy lessons, and too many equations. The exploration of common toys produces deep learning by emphasizing concepts and connections before formal definitions and mathematics. It also connects the classroom to the familiar world outside of school and gets students writing and talking about physics ideas. At the university level, investigating what toys do and how they do it can be a challenging application of undergraduate physics from the introductory course up through senior mechanics. Toys provide an ideal system for the kind of open-ended inquiry that introduces students to what scientists really do. They can pose their own questions, explore the behavior of the system sufficiently to create a hypothesis, use their theoretical knowledge to make a simplified model of the system and predict an outcome, design an experiment, discover that the real world is messy, think about what they haven't taken into account with their simple model and try to improve it. I have spent close to 30 years thinking about how to use toys to enhance physics education from 4th grade through college. In the process I have collected hundreds of toys the majority of which relate to mechanics, but also to sound, light, electricity and magnetism. I will discuss the pedagogical reasons for using toys in physics education and the many different ways to use them from demonstrations to laboratory experiments to discussion starters as well as how it is possible to use the same toy with many different age levels by approaching the analysis differently. I will share a number of my favorite toys, but focus particularly on those related to energy concepts. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G14.00003: Water Bouncing Balls: how material stiffness affects water entry Invited Speaker: Tadd Truscott It is well known that one can skip a stone across the water surface, but less well known that a ball can also be skipped on water. Even though 17th century ship gunners were aware that cannonballs could be skipped on the water surface, they did not know that using elastic spheres rather than rigid ones could greatly improve skipping performance (yet would have made for more peaceful volleys). The water bouncing ball (Waboba\textregistered) is an elastic ball used in a game of aquatic keep away in which players pass the ball by skipping it along the water surface. The ball skips easily along the surface creating a sense that breaking the world record for number of skips could easily be achieved (51 rock skips Russell Byers 2007). We investigate the physics of skipping elastic balls to elucidate the mechanisms by which they bounce off of the water. High-speed video reveals that, upon impact with the water, the balls create a cavity and deform significantly due to the extreme elasticity; the flattened spheres resemble skipping stones. With an increased wetted surface area, a large hydrodynamic lift force is generated causing the ball to launch back into the air. Unlike stone skipping, the elasticity of the ball plays an important roll in determining the success of the skip. Through experimentation, we demonstrate that the deformation timescale during impact must be longer than the collision time in order to achieve a successful skip. Further, several material deformation modes can be excited upon free surface impact. The effect of impact velocity and angle on the two governing timescales and material wave modes are also experimentally investigated. Scaling for the deformation and collision times are derived and used to establish criteria for skipping in terms of relevant physical parameters. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G14.00004: Snap, crack and pop: What elastic instabilities in toys can teach us Invited Speaker: Dominic Vella The mechanism of many modern toys rely on some form or other of elastic instability, from the locomotion of the ``Hexbug nano'' to the snapping of a ``Hopper popper.'' In this talk I will discuss some fundamental mechanical problems that are inspired by the mechanism of such toys. A particular focus will be on the ``snap'' and ``pop'' phases of the Hopper popper but I will also discuss the ``crack'' of a whip and other examples of dynamic elastic instabilities. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G14.00005: The mechanics of trick roping Invited Speaker: Pierre-Thomas Brun Trick roping evolved from humble origins as a cattle-catching tool into a sport that delights audiences the world over with its complex patterns or ``tricks,'' such as the Merry-Go-Round , the Wedding-Ring, the Spoke-Jumping, the Texas Skip... Its implement is the lasso, a length of rope with a small loop (``honda'') at one end through which the other end is passed to form a large loop. Here, we study the physics of the simplest rope trick, the Flat Loop, in which the motion of the lasso is forced by a uniform circular motion of the cowboy's/cowgirl's hand in a horizontal plane. To avoid accumulating twist in the rope, the cowboy/cowgirl rolls it between his/her thumb and forefinger while spinning it. The configuration of the rope is stationary in a reference frame that rotates with the hand. Exploiting this fact we derive a dynamical ``string'' model in which line tension is balanced by the centrifugal force and the rope's weight. Using a numerical continuation method, we calculate the steady shapes of a lasso with a fixed honda, examine their stability, and determine a bifurcation diagram exhibiting coat-hanger shapes and whirling modes in addition to flat loops. We then extend the model to a honda with finite sliding friction by using matched asymptotic expansions to determine the structure of the boundary layer where bending forces are significant, thereby obtaining a macroscopic criterion for frictional sliding of the honda. We compare our theoretical results with high-speed videos of a professional trick roper and experiments performed using a laboratory ``robo-cowboy.'' Finally, we conclude with a practical guidance on how to spin a lasso in the air based on the results of our analysis. [Preview Abstract] |
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