Dancing with Physics Richard P

Santa Clara University Scholar Commons Physics College of Arts & Sciences 1-2008 Dancing with physics Richard P. Barber Jr. Santa Clara University, [email protected] David Popalisky Follow this and additional works at: http://scholarcommons.scu.edu/physics Part of the Physics Commons Recommended Citation Barber, Richard P. Jr. and Popalisky, David, "Dancing with physics" (2008). Physics. 7. http://scholarcommons.scu.edu/physics/7 This Article is brought to you for free and open access by the College of Arts & Sciences at Scholar Commons. It has been accepted for inclusion in Physics by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. Physics World Archive Dancing with physics Richard Barber, David Popalisky From Physics World January 2008 © IOP Publishing Ltd 2014 ISSN: 0953-8585 Institute of Physics Publishing Bristol and Philadelphia Downloaded on Mon Sep 29 21:26:18 GMT 2014 [129.210.14.118] physicsworld.com Feature: Teaching physics through dance Dancing with physics Teaching physics by way of the forces on the human body experienced during dance can be an effective way to bring Newton’s laws to life, describe Richard Barber and David Popalisky Tristram Kenton One of the biggest challenges when teaching physics is them both the scientific tools to measure and under- The art of physics to bridge the perceived chasm between the abstract stand as well as the personal experience of forces and In 2005 the Rambert “scientific” world and the “real” world of experience. motion. The physics involved is simply the mechanics Dance Company Newton’s first law of motion, for example, states that of a moving body under the influence of gravity; the developed a an object remains in motion in a straight line unless goal is to understand the physical principles that gov- performance called acted on by an external force. However, our experience ern the dancer’s motion. Constant Speed based on Einstein’s tells us that most objects keep moving only as long as The body senses motion and the forces acting on it groundbreaking work something pushes them. The key to mastering physics via our “kinesthetic sense”, whereby information from on relativity, lies in the ability to abstract the “law” from a reality that accelerometers such as the semicircular canals in our Brownian motion and includes friction and other effects. ears and “proprioceptors” located in our muscle tissue the photoelectric There are many ways in which this can be achieved. allow us to correct our movement when we stumble so effect. But dance can In a lecture we might drop a coin and a feather to show that we do not fall over. To ensure that our bodies be- be an effective way to that both accelerate at the same rate. However, this have consistently, this internal “black box” effectively teach physics, too. demonstration will only be successful if we use an eva- processes the same physical laws that we might also cuated chamber, since the real world tells us that the analyse as physicists. By blending physics with dance, feather falls much more slowly. Laboratory sessions are non-science students can therefore gain an insight into another option, whereby students carry out experi- the scientific approach while honouring both the cre- ments designed to reinforce what they have learned in ative and analytical aspects of both disciplines. class. But this approach relies critically on how much Richard Barber preparation the student has done, since in practice it is Noble origins is in the department of physics and often possible to do the experiments without under- The scientific understanding of dance can be traced to David Popalisky is in standing the science behind them. the royal court of France in the early 17th century, when the department of A novel approach, which we have recently developed people began to study the human body during ex- theatre and dance, into a college course for non-science undergraduates, pressive movement. In 1672 Louis XIV established the both at Santa Clara is to teach physics through the art of dance. The key Académie Royale de Danse, where regular training University, US, theme of the course is to personalize the physics of and the systematic study of ballet challenged the range e-mail rbarber@ motion by making each student the object, and to give of human motion. Individual dancers with specific scu.edu Physics World January 2008 29 Feature: Teaching physics through dance physicsworld.com 1 The laws of jumping garet H’Doubler, who in 1926 set up the world’s first university dance course at the University of Wisconsin. 2000 She viewed a solid understanding of anatomy – and thus of the mechanical possibilities for human movement – as essential before students could actually explore expressive movement. H’Doubler understood 1000 that body mechanics needed to be first practised lying force (N) down, where the pull of gravity would not hinder effi- cient muscular action; her classroom experiments, in 0 turn, informed the emerging science of kinesiology. Physicists have tended to take a more fundamental 1.8 approach to understanding dance than the anatomical and physiological starting point of kinesiology. In 1989, for instance, Ken Laws of Dickinson College in Penn- 1.4 sylvania explored why a dancer’s motion does not fol- height (m) low a parabola during a large jump or leap. He reasoned 1.0 that the dancer’s head and torso do not execute a parabola because the centre of gravity of the body changes during the jump: the arms and legs – and thus ) 1 2 – the centre of gravity – are raised to their highest loca- tion at the midpoint of the leap, which makes the dancer 0 appear to travel horizontally for a brief time. Despite –2 obeying the fundamental laws of physics, this creates a velocity (m s wonderfully effective illusion for the audience. Another example of a common illusion in dance occurs during an “assemblé”. In this airborne movement, the trailing leg is brought forward into alignment 2000 with the leading leg (i.e. assembled) while the dancer is still in the air. But in doing so the trailing leg “robs” some forward momentum from the rest of the dancer, force (N) 0 so the parabolic shape of the leap appears to be abruptly cut off as the dancer returns to the floor. 4.0 4.5 5.0 5.5 6.0 Through a series of such investigations, culminating in time (s) his book Physics and the Art of Dance, Laws probed the Vertical forces and kinematic data can be measured simultaneously beauty and illusion of the dance using the basic prin- during a simple vertical jump using force plates and a video recorder. ciples of mechanics. While the student is in the air, the force plates register zero force, as As well as providing a framework for appreciating expected (top), while the video data allow the vertical height to be the motion of a dancer, physics can be used as a tool for calculated (second from top). From the slope of the displacement improving dance. For example, a successful pirouette data, students can calculate the vertical velocity (third from top), and requires an external torque, which, due to friction, is a linear fit during the free-fall portion of the jump then gives an just a reaction to the dancer’s torque against the floor. estimate for the acceleration due to gravity of –10.1 m s–2 (red). One way for a dancer to increase the torque, and thus Finally, the net force (red) on the student can be calculated by their rotational velocity, is to move their “pushing” leg subtracting their weight from the vertical-force measurement, which further from the axis of rotation, but this shift also can then be compared with the result of multiplying the slope of the increases the body’s moment of inertia and therefore velocity graph by their mass (green). Although the “noise” in the slope slows the initial rotation. By optimizing these two com- calculations is large, the fact that the two curves are similar provides a peting variables, the number of rotations performed by direct visualization of Newton’s second law. a dancer can be increased. natural skills, such as jumping or turning, continually Kinesthetic education raised the technical bar, thus forcing the teaching regi- The idea for a “physics of dance” course at Santa Clara men to be revised. University was born in 2002, when one of the present For instance, an unnamed dancer – perhaps the late authors (RB), a professor of physics, turned up at a 18th-century August Vestris – mastered multiple pirou- weekly workshop called “dads don’t dance” that was ettes by continually tightening the circle of his arms being run by the other author (DP), a professor of (thereby reducing his rotational inertia) instead of theatre and dance. We quickly realized the creative and maintaining the fully classical open circle of the arms analytical processes common to both physics and in first position throughout the turn. Multiple pirou- dance. A successful dance requires extensive prelim- ettes became more desirable than the previous clean inary organization as well as spontaneous decisions; maintenance of arm shape, and thereafter this “trick” likewise, a hypothesis worthy of analysis is often tested became taught as official turning technique. in a simple experiment first to see if the idea is robust By the 20th century the study of human movement enough to merit a more detailed experiment. Noting had begun to mature into a scientific discipline called that the kinesthetic sense is the body’s way of sensing kinesiology.

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