Skiing: It’s All About Friction

glide2It’s all about friction. Really. Friction from the snow, friction from the air, friction from the surface of the ski or the clothing you wear.  The physics of skiing is all about how to overcome drag and resistance and allow a skier to slice his/her way down the mountain.  And if Newton’s laws have anything to do with it, a skier who controls friction best has the best chance of winning.

Find out the basics of friction and skiing.

Articles by Marcia Howell

Aerodynamics and Cycling

Cycling has undergone immense changes since its early days.  As science has opened our understanding of aerodynamics, it has driven changes in bicycle composition and design, the clothing worn by the cyclist, and even the positioning of the rider on the bicycle.

These three factors directly correlate to the amount of drag experienced by the cyclist.  In fact, researcher L. Brownlie reported that some styles of baggy clothes cost cyclists 1.17% of their finishing time in a 100-meter race.  Serious cyclists utilize this knowledge by dressing in sleek, close-fitting clothing to optimize their aerodynamics.

How much of a role do aerodynamics play? Learn the basics or read the more technical explanation.

Articles by Cristian Clavijo

Skiing: It’s All About Friction (Basic)

glide3aWhen Bioengineer, Parker Tyler, goes skiing, she probably isn’t thinking about the biology or the physics behind the activity.  Rather, he is enjoying the crisp, cool, mountain air, the clear view of the slope, and the anticipated exhilaration he will feel as she maneuvers to the bottom. She likely doesn’t consider earth’s gravitational force (9.81 m/s), or potential energy, or kinetic energy, or her own mass, or any of those other factors that will contribute to his acceleration.  However, scientists do think about these things and their thinking has affected many facets of the industry from clothing to equipment to style.

It’s all about friction. Really. Friction from the snow, friction from the air, friction from the surface of the ski or the clothing you wear.  The physics of skiing is all about how to overcome drag and resistance and allow a skier to slice his/her way down the mountain.  And if Newton’s laws have anything to do with it, a skier who controls friction best has the best chance of winning.

Back to Parker, her potential energy is greatest at the top of the hill where she perches until the start of her run.  Her body is physically fit and adrenaline is taking over, sending added energy to her muscles, vision center, quick decision making regions of the brain, and the area that controls coordination.  Once she leaves starting position, gravity pushes down, mass pushes down, but the acceleration down the slope kicks in and changes how the forces affect the ride.  Potential energy turns into kinetic energy, or energy of motion, and everything he touches tries to resist and slow the movement.

Since Parker is a wise skier, she wears a GS suit (a sleek, form fitting suit with a minimum of abrasive surface area) and aerodynamic boots, hat (or helmet), gloves, etc.  As she accelerates, she assumes a crouching position to reduce air resistance and tighten the air current close to her body.  His skis are designed specifically for the type of skiing being done, the edges are sharp, and the bottoms are carefully waxed.  The wax waterproofs the skis, prevents them from drying out, and it reduces the wet drag of a kind of “suction” type friction from the snow.

When Parker comes to a curve, the skis will either be eased into the turn with the ski pointed in the same direction as her velocity, making a sharp cut in its wake, or she will choose a skidding type of maneuver where the skis will be forced in the direction she wishes to go, leaning away from the curve at a 45-90 degree optimal angle, and literally plowing snow away from her. Some skis have special designs that scientists have found will decrease the drag and increase the speed these curves can be safely made.  Surely, Parker will have researched and purchased those that fit his style and goals for skiing.

By the time she reaches the bottom, her potential energy is expended, the ensuing kinetic energy is maxed out, and now friction works against him to slow down her acceleration to a stop.  His adrenalin will return to normal levels, and her blood circulation and other systems will begin to function normally once again.  (At least until the next run.)

Research will continue to change the sport of skiing.  And no doubt, the savvy skier will keep tabs on the newest and best ways scientists will come up with to help us beat the forces working against us.

By: Marcia Howell, University of Utah

References:

Energy Transformation for Downhill Skiing. 2012. Retrieved from http://www.physicsclassroom.com/mmedia/energy/se.cfm

The Physics of Skiing. Real World Physics Problems. 2009. Retrieved from http://www.real-world-physics-problems.com/physics-of-skiing.html

Locke, B. 2012. The physics of skiing. Retrieved from http://ffden2.phys.uaf.edu/211_fall2002.web.dir/brandon_locke/Webpage/homepage.htm

Mears, A. 2002. Physics of Alpine Skiing. Retrieved from http://www.suberic.net/~avon/mxphysics/anne/Annie%20Mears.htm

Aerodynamics and Cycling (Basic)

CyclingConsidered to be one of the most exhilarating and efficient human-powered vehicles ever invented, the bicycle continues to find its way into the lives of many Americans and people throughout the world.  Cycling sporting events have exponentially increased in popularity since the day German inventor, Baron Karl von Drais, put two wheels on a wooden frame in 1817 (Ballantine 2001).  Since then, the bicycle has undergone several modifications.  Once engineering technology caught up with the public’s need for speed, cycling was transformed from a field of design guesswork to a scientific research analytical field involving a diverse number of disciplines.

There are many factors that hinder a bicycle from reaching maximum velocities.  Among these are transmission friction, air drag, rolling resistance and inertia forces.  The most significant of these is air drag.  70% – 90% of the resistance experienced by a rider in a high-speed race is due to drag (Brownlie 2010).  Therefore, aerodynamic studies are of paramount importance for better bike design, and several studies have been conducted within the past two decades.

In the most basic definition of air resistance, drag occurs due to a pressure difference between the front and rear of an object, which is in movement relative to its medium (air).  When air hits the rider, it is brought to a stop, creating what engineers refer to as the stagnation point.  At this point, the air splits and moves in opposite directions following “streamlines”, which contour around the rider.  The air sticks to the body of the rider as it moves around him/her.  However, due to a lack of energy, it separates from the body somewhere around the rider’s back.  This separation creates a low-pressure field in the rear of the rider.  This pressure difference between the front and the rear causes a force, which pushes the rider backwards, called drag.

There are two methods to study aerodynamics around a bicycle: mathematically and experimentally.  In the first method, engineers and scientists solve complex systems of equations, which predict air behavior around the rider.  Since some of these equations are too difficult to be solved by hand, computers programs are often used.  Others study drag experimentally.  They put a rider on a bike inside a big tunnel (generally referred to as a wind tunnel), which blows air at high speeds while the rider stays still.  Sensors attached to the rider are able to measure pressure differences and compute drag.  Both methods have advantages and disadvantages, and both are often used in unison.

Different components have been individually tested for drag such as helmets, loose clothing, rider’s position and wheels.  Studies have shown that even seemingly small factors, such as loose clothing, can increase drag significantly during a race.  Bicycle scientists continue to try to find the ultimate rider’s position, the best helmet and the most appropriate clothing to help cycling athletes further improve their performance.

Learn the technical details of aerodynamics and cycling.

By: Cristian Clavijo, University of Utah

Cristian Clavijo is a native of Peru, moved to the US in the 8th grade, and is now a Masters student in Mechanical Engineering at the University of Utah.  As an advocate for the Hispanic underserved population in Utah, Clavijo is involved in educating children and parents on how basic scientific and medical knowledge can help them progress and become collaborators of their communities.  He plans to pursue a PhD in Mechanical Engineering next year.

References

Ballantine, R. 2001. Richard’s 21st-century bicycle book. The Overlook Press, New York, New York, USA.

Brwonlie, L., P. Ostafichuk, E. Tews, H. Muller, E. Briggs and  K. Franks. 2010. The wind-averaged aerodynamic drag of competitive time trial cycling helmets. Procedia Engineering 2:2419-2424.

 

Aerodynamics and Cycling (Technical)

At one point almost banned by social and environmental safety national leaders, cycling, in its many forms, has become one of the most competitive international sporting events and is certainly among the top choices for transportation and recreation.  Most people today view cycling as a form of mere exercise or seasonal recreation.  However, for some, cycling is a way of life.  Paul Fournel expressed, “When you get on your first bike you enter a language you’ll spend the rest of your life learning, and you transform every move and every event into a mystery for the pedestrian” (Fournel 2003). What is it about cycling that stirs such feelings or even mania among its faithful devotees? Breath-taking landscapes, memorable sights and smells, the sound of self-induced wind challenging the rider to pedal harder, and physical and mental rejuvenation are some of the reasons.

Fig. 1 Old design of bicycle in the 1800’s.

Though the early roots of first invention designs and legal patents are blurred, there is a clear idea of the road traveled by the bicycle since its advent in the early 1800’s.  Originally known as “pedestrian’s accelerator”, “boneshaker” and “velocipede” (Norcliffe 2001), the bicycle, in its most basic form, consists of two wheels (hence the name of “bi-cycle”), a connecting metal frame, a seat, steering bars and pedals.  However, the first bike, which was invented in Germany by Baron Karl von Drais in 1817, had no pedals (Ballantine 2001).  The rider was expected to propel himself while sitting on the two-wheel frame.  This design, though more efficient than sole human transportation, was quickly and necessarily improved throughout Europe in the following decades (Norcliffe 2001).   The second half of the 19th century saw innumerable changes in the bicycle.  Pedals were co-axially attached to the front wheel.  Big (over 1 meter in diameter) front wheels were later designed to induce higher speeds (see figure 1).  Steel frames were manufactured to provide longevity and durability.  However, despite all these improvements, the bicycle still had not reached its full potential.  By the dawn of the 20th century, problems with turning, safety, cost and weight had driven engineers to design the bicycle as it is known today.

The earliest records of cycling race competitions date back to when the front wheel was almost as tall as the rider.  It was, perhaps, the desire for faster lap times that pushed bicycle engineering to new heights.  Today, there are diverse engineering and science fields involved in the design and technological development of bicycles.

Some of the aspects engineers continue to explore are air resistance, drafting, altitude, hills, rolling resistance, power transmission friction, inertia forces, and braking energy losses.  Air resistance seems to be the single most adverse resistance factor for road race cycling.  The event that revealed the importance of this fact took place in the 1989 Tour de France.  Greg Lemond was 50 seconds behind his competitor in the last stage of the race.  Unlike Greg Lemond however, his competitor did not have an aero helmet, triathlon bars and a back disc wheel.  Greg Lemond was able to beat out his opponent by 58 seconds by the end of the race (Tew and Sayers 1997, Chowdury et al. 2011).  Other similar unbelievable aerodynamic feats were repeated over the next couple of years, which caused aerodynamics to become of interest.

Aerodynamics is a subfield of fluid mechanics, a mechanical and chemical engineering field essential for the analysis of systems in which a fluid is the working medium (Fox et al. 2004).  Aerodynamics deals with the dynamics of gases, especially air interactions with moving objects(Houghton Mifflin Company 1969).  Early studies showed that in a typical training ride, wind resistance accounts for 72% (although numbers up to 90% have been reported (Brownlie et al. 2010, Gibertini et al. 2010) with faster speeds) of the force retarding the forward movement of the rider, the tires 15%, braking losses 8%, and bearing and chain losses 5% (Burke 1986).

Several attempts to improve cycling aerodynamics were made through trial and error.  Unsurprisingly, this approach was ineffective, and engineers had to step in.  There are basically two methods to analyze the aerodynamics of a bicycle: mathematically and experimentally.  In mathematics, certain fluid behavior can be predicted by solving mathematical partial differential equations.  These equations take into account momentum, energy and mass conservation laws, and often are brought together into a system of equations.  One of the most commonly used equations in fluid mechanics is called the Navier-Stokes equation (Tew et al. 1997) shown below:

This equation is a simplified one-dimensional version with several built-in assumptions, and it only represents one given particle of air.  Clearly, airflow around a cyclist involves countless particles of air—it is easy to see why a powerful computer would be necessary to solve the aerodynamics physics around a cyclist.  Engineers and mathematicians have developed different numerical methods to simplify complex systems of equations, so that a computer can solve them in real time.  One such method is Computational Fluid Dynamics (CFD), which is a field in which equations similar to the one described above are discretized by approximating a solution with a system of algebraic equations, which can then be solved on a computer Ferziger and Peric 2002).  Several computational cycling engineers, nationally and abroad, use commercially available CFD packages to solve their designs of interest.

Fig. 2 ANSYS CFD image of air movement (stream lines) seen around cyclist creating pressure difference (drag). (Picture provided by Bike Tech Review)

If using an experimental approach to analyze the aerodynamics of cycling, experiments are usually carried out in a wind tunnel (see figure 3).  A wind tunnel cross section may be small enough (.5m x .25m) to test minor drag (due to clothing, for instance), or big enough (4m x 3.5m) to fit a whole bicycle (Gilbertini et al. 2010, Alam et al. 2010).  Sensors, attached to the rider and bike, are able to pick up pressure changes inside the wind tunnel and thereby measure drag (Iniguez-de-La and Iniguez 2009).  Experimental wind tunnel testing is often preferred over purely computational testing due to the limiting assumptions found in the mathematical equation solvers.  However, performing computational experiments can save a significant amount of time and money.  Often, both mathematic and experimental approaches are used in conjunction and offer comparative results.

Fig. 3 Wind tunnel experiment (picture provided by A2 Wind Tunnel)

When scientists study the aerodynamics of cycling, there are two main types of drag being considered: pressure drag and skin-friction drag.  When air hits an object (rider or bicycle), it splits and travels around the object creating a boundary layer, which is a thin film of compressed air near the surface of the object.  However, the air fails to meet back at the opposite side of the object due to a lack of energy, and the boundary layer separates from the body altogether.  This separation causes a pressure difference between the front and rear of the body thereby causing pressure drag.  The second type of drag (skin-friction or shear) occurs tangentially to the object as the air particles move around it.  This phenomenon can be observed by watching loose clothing of a rider flap with the wind.

There are three major areas of focus for drag studies: drag produced on the rider’s position, on the bike, or on the cyclist’s attire.  Two thirds of the drag experienced is due to the rider position (Kyle and Burke 1984), therefore great efforts have been focused in that area.  There are basically three positions that a rider can adopt while riding: upright position, dropped position and time trial position Burke 1986).  While the upright position provides the greatest comfort, it also induces the greatest drag.  The dropped position (20° angle relative to the horizontal) induces less drag and is generally the most used position while on a race.  In the time trial position, the rider positions his/her back almost completely parallel to the ground, with hands on the low handle bars and both pedals vertically aligned.  This provides the best aerodynamic efficiency and is mainly used during downhill rides.

Great efforts have also been made on bike design.  Lighter frames, thinner tires and aerodynamic wheel spokes are the current fields of interest for bike design.  Different wheel spoke designs—even wheel flap covers—have been tested for drag reduction because reductions of up to 50% have been observed with changes in the spoke design (Houghton Mifflin Company 1969).  While wheel flap covers have shown better aerodynamic characteristics (Karabelas and Markatos 2012), they are often undesirable during side winds.

In addition to the rider’s position and the bike itself, the rider’s attire is also important because loose clothing can generate significant drag.  This is due to the skin-friction drag phenomenon explained above.  It has been reported that tight clothing could save up to 1.17% of the rider’s finishing time in 100 meters (Brownlie 1992).  More meaningful is the drag reduction that can be obtained by use of an aerodynamic helmet.  Differences in overall drag reduction of different commercially available helmets of up to 8% have been reported (Alam et al. 2010).  Another aspect engineers have taken into account more recently is thermal comfort, which, disadvantageously, is inversely proportional to aerodynamic efficiency.  In other words, the more vents a helmet has for cooling, the less aerodynamic it is.  The third variable in the helmet design equation is safety considerations.  Certain helmets have been designed that offer great thermal comfort and good aerodynamic properties, but do not meet safety standards.

Has bicycle technology reached its end?  Or more importantly, are any further drag reductions too minimal to be worth the research effort?  Many opinions differ, but for as long as competitive cycling events still occur, researchers will continue to engineer the latest bicycle and appendages.  While the recreational bicycle rider may not be extremely concerned about which helmet or shorts to buy, it is of utmost interest for the serious race cyclist.

By: Cristian Clavijo, University of Utah
Cristian Clavijo is a native of Peru, moved to the US in the 8th grade, and is now a Masters student in Mechanical Engineering at the University of Utah.  As an advocate for the Hispanic underserved population in Utah, Clavijo is involved in educating children and parents on how basic scientific and medical knowledge can help them progress and become collaborators of their communities.  He plans to pursue a PhD in Mechanical Engineering next year.

References

Alam, F., H. Chowdhury, Z. Elmir, A. Sayogo, J. Love, and A. Subic. 2010. An experimental study of thermal comfort and aerodynamic efficiency of recreational and racing bicycle helmets. Procedia Engineering 2:2413-2418.

Ballantine, R. 2001. Richard’s 21st-century bicycle book. The Overlook Press, New York, New York, USA.

Brownlie, L. 1992. Aerodynamic characteristics of sports apparel. School of Kinesiology, Simon Fraser University, Burnaby, BC, Canada.

Brownlie, L., P. Ostafichuk, E. Tews, H. Muller, E. Briggs, and K. Franks. 2010. The wind- averaged aerodynamic drag of competitive time trial cycling helmets. Procedia Engineering 2:2419-2424.

Burke, E. R. 1986. Science of cycling. Human Kinetics Publishers, Champaign, Illinois, USA.

Chowdhury, H., F. Alam, and D. Mainwaring. 2011. A full scale bicycle aerodynamics testing methodology. Procedia Engineering 13:94–99.

Ferziger, J. H., and, M. Peric. 2002. Computational methods for fluid dynamics 3rd ed. Springer Publishing Company, New York, New York, USA.

Fournel, P. 2003. Need for the bike. University of Nebraska Press, Lincoln, Nebraska, USA.

Fox, R. W., A. T. McDonald, and P. J. Pritchard. 2004. Introduction to fluid mechanics 6th ed. John Wiley and Sons, Hoboken, New Jersey, USA.

Gibertini, G., G. Campanardi, L. Guercilena, and C. Macchi. 2010. Cycling aerodynamics: wind tunnel testing versus track testing. IFMBE Proceedings 31:10-13.

Houghton Mifflin Company. 1969. The American Heritage Dictionary of the English Language. Houghton Mifflin Company, Boston, Massachusetts, USA.

Iniguez-de-la T., and J. Iniguez. 2009. Aerodynamics of a cycling team in a time trial: does the cyclist at the front benefit? European Journal of Physics 30:1365-1369.

Karabelas, S. J., and N. C. Markatos. 2012. Aerodynamics of fixed and rotating spoked cycling wheels. Journal of Fluids Engineering 134:1-14.

Kyle, C. R. and E. R. Burke. 1984. Improving the racing bicycle. Mechanical Engineering 106:34-35.

Norcliffe, G. 2001. The ride to modernity: the bicycle in Canada. University of Toronto Press, Toronto, Canada.

Tew, G. S. and A. T. Sayers. 1997. Aerodynamics of yawed racing cycle wheels. Journal of Wind Engineering 82:209-222.

From Tee to Fairway: The Basics of How Physics Affects the Drive, the Club, and the Golf Ball (Basic)

The motion of a golf ball can be thought of as a projectile, whose trajectory is parabolic and acted upon by gravity.  The initial velocity imparted to the ball by the club head can be broken down into both a horizontal and vertical component.  Numerous scientific studies have identified the optimum launch angle as 11-20° to achieve maximum distance (Erlichson, 1983). Though drivers are typically 8-10° in loft, the flexibility of the graphite driver shafts increases the launch angle through a split second whipping action (Zumerchik, 1997).  Golf clubs have grooves added to their faces to add some friction to the club head, so that momentum is transferred to the ball and backspin is created to generate lift. The difference between a ball with backspin and one without can add up to 100 yards after 2 or 3 seconds of additional flight time (Zumerchik, 1997).

There are several forces that act upon the aerodynamics of a golf ball in flight.  The most recognizable force acting upon a golf ball is gravity, which pulls the ball downward and creates the parabolic trajectory common of projectiles.  Another force on a golf ball is lift, the force that opposes gravity.  When backspin is transferred to the ball from the grooves on the clubhead, the velocity of air on top of the ball (which is moving in the direction of the backspin) is higher than the velocity of air on the bottom of the ball.  To counteract this, the Magnus effect generates lift on the ball and pushes it up.

In addition to gravity and lift, another force acting on a golf ball is drag, or air resistance.  As a golf ball is sent flying through the air, the molecules that come into contact with the front of the ball exert a large pressure force on the front of the ball (drag). Drag slows down the forward velocity of the ball. As the air comes into contact with the front of the golf ball, the fluid motion of the air becomes turbulent.  Turbulent flow can be thought of as smoke from a smoke stack- chaotic and wispy. As the turbulent air swirls around the golf ball, the dimples capture some of the swirls and keep them close to the surface of the golf ball. This means that the boundary layer of air stays close and hugs the ball longer, which means that there is a smaller pressure difference between the front of the ball and the back (when compared to a ball without dimples). The dimples thus allow the golf ball to travel farther than a smooth ball because the golf ball experiences less drag.

By: Trevor Stoddard, University of Utah

Learn more about the technical science behind golf.

References:

Benson, T. (2010) Drag of a Sphere.  National Aeronautics and Space Administration, Date Accessed:  8/10/2012 <http://www.grc.nasa.gov/WWW/k-12/airplane/dragsphere.html>

Bird, R. B., W.E. Stewart and E.N. Lightfoot. 2007. Transport Phenomena, 2nd edition, Wiley & Sons, New York.

Cochran, A. (ed.). 1990. Science and Golf.  New York: Chapman and Hall

Cochran, A. (ed.). 1992. Science and Golf II.  New York: Chapman and Hall

Davies, J.  1949. “The Aerodynamics of Golf Balls.”  Journal of Applied Physics 20: 821-828

Erlichson, H. 1983. “Maximum Projectile Range with Drag and Lift, with Particular Application to Golf.”  American Journal of Physics 51: 357-362.

Jorgensen, T. 1994. The Physics of Golf. New York:  American Institute of Physics

McDonald, W. 1991. “The Physics of the Drive in Golf.”  American  Journal of Physics 59: 213-218

Werner, F. and R. Greig. 2000. How Golf Clubs Work and How to Optimize Their Designs.  Jackson Hole, WY:  Origin Inc.

Wesson, J. 2009. Science of Golf.  New York, Oxford University Press

Williams, D. 1959. “Drag Forces on a Golf Ball in Flight and Its Practical Significance.”  Quarterly Journal of Mechanical Applications of Mathematics XII 3: 387-393

Zumerchik. J. (ed.). 1997. Encyclopedia of Sports Science.

Zumerchik J. 2002. Newton on the tee- a good walk through the science of Golf

History of the golf ball <http://www.golfeurope.com/almanac/history/golf_ball.htm> last accessed 8/10/12