Have you ever had the experience of gliding effortlessly across a dance floor with a partner of rugby-player proportions, yet endured a swollen ankle minutes later from the quickstep of someone half the size? Why does one twin sister seem to be "hard" on shoes while the other can make a pair last twice as long? The explanation for counterintuitive phenomena like these lies in the study of friction and wear - a field known as tribology - and an increasingly rich source of tribological data come from the science of sport

Stack, Margaret

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Stack, M.M.

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There's something in the way you move - horse racing and skiing are two sports where a better understanding of friction could work

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Strathprints Institutional RepositoryStack, M.M. (2004) There’s something in the way you move - horse racing and skiing are two sportswhere a better understanding of friction could work. Physics World, 11. pp. 24-25. ISSN 0953-8585Strathprints is designed to allow users to access the research output of the University of Strathclyde.Copyright c© and Moral Rights for the papers on this site are retained by the individual authorsand/or other copyright owners. You may not engage in further distribution of the material for anyprofitmaking activities or any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or study, educational, ornot-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to Strathprints administrator:mailto:strathprints@strath.ac.ukhttp://strathprints.strath.ac.uk/  Stack, M.M.* (2004) There's something in the way you move - horse racing and skiing are two sports where a better understanding of friction could work. Physics World, 11. pp. 24-25. ISSN 0953-8585      http://eprints.cdlr.strath.ac.uk/6290/      Strathprints is designed to allow users to access the research output of the University of Strathclyde. Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (http://eprints.cdlr.strath.ac.uk) and the content of this paper for research or study, educational, or not-for-profit purposes without prior permission or charge. You may freely distribute the url (http://eprints.cdlr.strath.ac.uk) of the Strathprints website.  Any correspondence concerning this service should be sent to The Strathprints Administrator: eprints@cis.strath.ac.uk  From Margaret Stack in the Department ofMechanical Engineering, University ofStrathclyde, Glasgow, UKHave you ever had the experience of glidingeffortlessly across a dance floor with a part-ner of rugby-player proportions, yet endureda swollen ankle minutes later from the quick-step of someone half the size? Why does onetwin sister seem to be “hard” on shoes whilethe other can make a pair last twice as long?The explanation for counterintuitive phe-nomena like these lies in the study of frictionand wear – a field known as tribology – andan increasingly rich source of tribologicaldata come from the science of sport.Take, for example, horse-riding. For cen-turies, horseshoes have been used to protecthorses’ hooves from wear and tear. But tra-ditional steel horseshoes reduce the naturalshock-absorbing properties of the hoof andcan therefore cause damage to the animal’slower limbs, especially on hard surfaces suchas roads. As a result, modern-day “black-smiths” have been developing an alternativeto steel shoes based on polymers, which arelighter and more elastic. Polymer horse-shoes can therefore accommodate the de-formation of the hoof during movement,but they have one major drawback: theywear much more quickly than steel shoes.To increase the performance and lifetimeof polymer shoes, tribology researchers havebeen studying the rate at which wear occursin relation to the way horses move. The re-sults, which were presented at a meeting on tribology in sport in London last month,are surprising.Equine experimentsNormally engineers turn to the “Archard”equation to study the friction and wear ofsurfaces. Here, the wear rate of a surface isdirectly proportional to the applied load:V = kW/H, where V is the wear volume perunit sliding distance, k is a constant, W is theload and H is the hardness of the surface.In other words, the heavier the object, thelarger the impact it has on a stationary sur-face and the quicker it deteriorates.But by measuring the wear performanceof polymer horseshoes for a given exercisesuch as cantering, Stefano Mischler of theSwiss Federal Institute of Technology inLausanne and co-workers have found thatthe wear rate is independent of the mass of the horse. This is in stark contrast to the wear rate of components in industrialmachines, which follows the Archard equa-tion (at least in the absence of significantfrictional heating).There are some indications that a similarsituation might occur in humans. Studies of the biomechanics of running by ColinWalker and colleagues at the University ofStrathclyde show that the “heel strike” andcontact time of running shoes on the groundchanges depending on the running style of the individual athlete. These factors willThere’s something in the way you moveHorse-racing and skiing are two sports where a better understanding of friction could helpP H Y S I C S W O R L D N O V E M B E R 2 0 0 424 p h y s i c s w e b . o r gFighting friction – waxing your skis is not the onlyway to increase your speed on the slopes.P H Y S I C S W O R L D N O V E M B E R 2 0 0 4 25p h y s i c s w e b . o r gultimately affect the wear rate of the outersurface of the running shoe.It seems as if animals adjust their move-ment so as to reduce the overall impact on a surface. For example, early studies of thebiomechanics of running revealed that thebody limits the vertical force on a limb byquickly rotating the lower limb on impact. Asthe time over which this force is applied andthe area of contact may vary depending onrunning style, the wear rate of running shoes– and, indeed, of joints such as the knee –may also vary accordingly. This has inter-esting consequences for the design of mater-ials for sports shoes, since using harder ortougher materials may not be the only factorthat improves the longevity of the product.Looking at these results from the oppo-site end of the spectrum – i.e. with a view todesigning engineering processes to reducewear – suggests that machines could bedesigned to automatically change speed ormotion at high applied loads in the sameway that animals do. Using knowledge-based routines, such as artificial intelli-gence, may therefore be a better way ofreducing component wear than regularlyreplacing materials.Slippery surfacesThe wear and tear of horseshoes is one ofmany examples of how tribology can im-prove sports performance. Another, per-haps more obvious example, is skiing. Skiersand snowboarders often wax the surfaces of their equipment to reduce friction onsnow surfaces, but what is less well known is that the temperature of the snow canaffect how quickly they make it down aslope. Moreover, the faster you go, the lessfriction you experience.Recent measurements of the frictionalperformance of ski surfaces made by Sté-phane Ducret and co-workers at the EcoleCentral de Lyon in France have demon-strated that friction is highest at very lowtemperatures of about –15 °C, but is at a minimum at –5 °C. This minimum isthought to be due to an “elasto-hydrody-namic layer” of melted snow, which formsdue to frictional heating. As work is con-verted into heat at high speeds and largeloads, it means the faster the skier goes (atlow temperatures), the easier it will be toform this melted surface layer. So, when youwonder how your ski guide skims the slopesin a matter of seconds, when it takes minutesfor you to reach the bottom, you should beartribology in mind. The high speed at whichthe skier is travelling indirectly helps them to go even faster due to a self-lubricatingaction on the ski surface.The combined effects of mass and speed,in addition to the friction and wear resist-ance of the materials in contact with a sur-face, all play a significant role in how youand your sports kit will fare during the com-ing winter months. So when you are check-ing the soles of your running shoes before amorning run, or lugging skis up a chairlift,remember that your performance may notonly be due to the friction and wear charac-teristics of the materials; it may also havesomething to do with the way you move.Passive-flight simulatorAs anyone who has watched a leaf or a piece of paper flutter to the ground is aware, it is impossible toaccurately predict where it will land. But this has not prevented physicists and mathematicians fromtrying. This simulation shows the vortices created by a plate (white disc) as it tumbles through a fluid,which was obtained by Umberto Pesavento and Jane Wang of Cornell University by solving theNavier–Stokes equation (Phys. Rev. Lett. 93 144501). In particular, their calculation reveals that theaerodynamic lift that causes a falling object such as a leaf to momentarily rise (as can be seen on theright of the image) is due to the coupling between translational and rotational motions. The differentcolours represent the “vorticity” of the fluid (the curl of the velocity): red indicates high vorticity, whileblue indicates low vorticity. Pesavento and Wang are now using their model to investigate the transitionbetween fluttering and tumbling motion. The rich dynamics of such passive flight represents more thanjust interesting physics; it could also help researchers learn more about insect flight (see Physics WorldApril 1999 pp21–25). MCH IGHL IGHTS FROMPHYS ICSWEBSuperstructures and superconductorsThe discovery of oxygen “superstructures”in cuprate materials by two independentteams of physicists could help shed newlight on the origins of high-temperaturesuperconductivity. X-ray scatteringexperiments have shown that oxygendefects and vacancies form an orderedsuperstructure with a periodicity of fourunit cells. The results suggest that the“stripes” of charge found in some cupratesmight not be responsible for their ability tocarry currents without resistance. Theexistence of superstructures in the cuprateswas first predicted by theorists in 1990.Earthquake shakes up gravityGeophysicists in Japan have detected achange in the Earth’s gravitational fieldcaused by an earthquake for the first time. The team used an array ofsuperconducting gravimeters to detect anincrease of less than 10–8 ms–2 in theacceleration due to gravity near theepicentre of an earthquake that occurredin September 2003. The results agree withtheoretical predictions and could help withthe interpretation of measurements of theEarth’s gravity made by satellites.Fingerprint model makes impressionEveryone has a unique set of fingerprints,yet scientists are still unsure about howthese patterns form. Now two appliedmathematicians in the US have developeda model that is able to reproduce real-lifefingerprint patterns. The model suggeststhe patterns have their origins in thestresses that build up in the basal layer of skin (the layer between the outerepidermis and the inner dermis) while weare still in the womb. These stresses causethe basal layer to buckle inwards, creatingridges on the surface of the skin.Nanotubes shape up for spintronicsScientists at IBM have shown thatnanotubes made of vanadium oxide are magnetic at room temperature.Moreover, the magnetic properties of thenanotubes can be controlled by dopingthem with electrons or holes. The workcould have applications in “spintronics”,and the IBM team now plans to developdevices in which spins can be controlledby a voltage rather than by doping.Read these articles in full, and sign up forfree e-mail news alerts, at physicsweb.orgP H Y S I C S  I N  A C T I O N

There's something in the way you move : horse racing and skiing are two sports where a better understanding of friction could work

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There's something in the way you move : horse racing and skiing are two sports where a better understanding of friction could work

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