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

Curling: The Friction Sport

pic_curlingCurling was first created in 16th century Scotland, where river bottom rocks were slid across ice-covered lochs to a target. In modern times, the “roaring game,” named for the sound of the “stone” sliding across the ice, is more refined. It consists of a 41 pound granite stone sliding across 42 m of ice to a target called a house, utilizing pebbled ice rinks, brooms, curling shoes, and carefully formed stones.

The ice rinks used for Curling are not smooth, but have a pebbled surface made by spraying the rink with water and allowing the tiny droplets to freeze on its surface. This surface is necessary for the stone to have suction and slide across the ice. Two players with “brooms” vigorously sweep the ice immediately in front of the stone to influence the trajectory, or direction it travels. These players wear a “gripping” shoe and a “sliding shoe,” which allow them to use friction to move along the ice. As the stone travels, the interaction between the stone, ice, and sweeping changes the sources of friction, and cause the stone to “curl” in a curved trajectory, giving the sport its name.

Although there are many theories, it is poorly understood as to what causes the stone to curl as it travels along the ice. As the sport gains popularity, further scientific inquiry can be expected to explore the role of friction and different sweeping styles.

Learn the basics or read about the technical aspects of friction in curling.

Articles by Jessica Egan

Curling: The Friction Sport (Basic)

The modern game of curling consists of a 41 pound granite rock sliding across some 42 m of ice to a target called a house. The rock, technically termed a “stone,” is typically preceded on the ice by two players with “brooms,” who vigorously sweep the ice immediately in front of the stone to influence its trajectory (Willoughby et al. 2005, Bradley 2009, Esser 2011). This unique winter sport, affectionately termed the “roaring game” because of the sound the stone makes as it slides on the ice, has its roots in 16th century Scotland, where river bottom rocks were the stones which slid across ice-covered lochs to a target (Clark 2008). Now, play is more refined, utilizing pebbled ice rinks, brooms, curling shoes, and carefully formed granite stones.

The ice rinks are pebbled by spraying water onto the ice to make a slightly bumpy surface as the droplets freeze into little protrusions on the ice surface, without which the stone could not curl (Shegelski 2001). Brooms are now typically made of synthetic materials such as nylon, and curling shoes consist of one shoe with a larger friction coefficient than the ice and another shoe which has a friction coefficient smaller than that of the ice, thus providing players with a “gripping” shoe on one foot and a “sliding shoe” on the other. A friction coefficient is the measure of how much a substance resists sliding on another substance. The granite stones are highly polished, very hydrophobic, and have a small hollow on their underside so that only a small ring on the bottom of the stone actually touches the ice at any one time.

Each of the pieces of equipment needed to play curling is designed to maximize the impact it has on friction coefficients. Style of play also influences friction coefficients. For example, some sweepers use a conventional style of sweeping, which consists of the sweeper standing to the side of the stone while they sweep. The sweepers employ more force closest to their feet, so there is a greater rise in temperature on the side of the stone closest to the sweeper, which causes asymmetric friction acting on the stone. This matters because the sweeping causes the uppermost layer of ice to melt, thus providing a lubrication layer for the stone to glide on. On the other hand, a high-angle style of sweeping consists of the sweepers being behind the stone while it travels, giving a more even distribution of heated ice, and so lessening the curl of the stone (Marmo 2006). Both can be useful, depending on where the stone is desired to go.

There is much scientific debate about the role friction plays in causing the stone to curl. The friction acting on the stone is undoubtedly asymmetric, but how this results in the stone’s curling trajectory is not yet fully understood. Some believe that as the stone twists, it pushes the water layer to the side, creating a lubrication film for the rock to slide on (Bradley 2009). Others say that the rotating stone not only pools water one its one side, but also chipped ice and other debris (Denny 2002). As curling continues to gain popularity, it can be expected that further scientific inquiry will continue to be directed at exploring the role of friction in the curl of the stone.

Learn more about the technical aspects of friction and curling.

By: Jessica Egan, University of Utah
Jessica Egan began her love of chemistry under her mom’s direction with homeschool experiments in middle school and then up into high school through her high school chemistry teacher. She pursued a chemistry and art double major at Hillsdale College and decided to continue her chemical education by attending graduate school at the U. She has recently graduated with a M.S. from the U in Analytical Chemistry and is looking for a career in industry.

References

Bradley, J. L. 2009. The sports science of curling: a practical review. Journal of Sports Science and Medicine 8:495-500.

Clark, D. 2008. The roaring game: a sweeping saga of curling. Key Porter Books, Toronto, Ontario, Canada.
Denny, M. Curling rock dynamics: towards a realistic model. 2002. Canadian Journal of Physics 80:1005-1014.

Esser, L. 2011. Swept away: exploring the physics of curling. Science Scope 35:36-39.

Marmo, A. A., I. S. Farrow, M-P Buckingham, and J. R. Blackford. 2006. Frictional heat generated by sweeping in curling and its effects on ice friction. Proceedings of the Institution of Mechanical Engineers, Part L.: Journal of materials: Design and Applications 220:189-197.

Marmo, B. A., M-P Buckingham, and J. R. Blackford. 2006. Optimising sweeping techniques for Olympic curlers. The Engineering of Sport 6 3:249-254.

Shegelski, M. R. A. 2001. Maximizing the lateral motion of a curling rock. Canadian Journal of Physics 79:1117-1120.

Willoughby, K. A., and K. J. Kostuk. 2005. An analysis of strategic decision in the sport of curling. Decision Analysis 2: 58-63.

Curling: The Friction Sport (Technical)

The modern game of curling consists of a 41 pound granite rock sliding across some 42 m of ice to a target called a house. The rock, technically termed a “stone,” is typically preceded on the ice by two players with “brooms,” who vigorously sweep the ice immediately in front of the stone to influence its trajectory (Willoughby et al. 2005, Bradley 2009, Esser 2011). This unique winter sport, affectionately termed the “roaring game” because of the sound the stone makes as it slides on the ice, has its roots in 16th century Scotland, where river bottom rocks were the stones which slid across ice-covered lochs to a target (Clark 2008). Since its revival in the mid-1800s in Scotland, curling has somewhat switched continents with Canada now being the world’s main curling powerhouse. Curling has grown to an Olympic sport and enjoys a unique niche of fans and followers (Holt 1989, Tranter 1989).

From the view of science, curling is a game about friction. The friction of the stone on the ice, the brooms on the ice rink, and even the footwear of the curlers on the ice, makes the game possible. Curlers wear a sliding shoe, which has a lower friction coefficient than the ice, and a non-sliding shoe, which has a friction coefficient larger than that of ice, giving the players the ability to step and slide (Dassler 1986). The sweeping of the brooms varies the friction coefficient of the ice, making it possible to influence the trajectory of the stone once it has already been pushed toward the house. A covering for the handle of the brooms has even been proposed, to lessen the friction (and thus wear) of the broom on the curlers’ clothing, a long recognized problem for curlers (Robertson 1987). The entire game centers on creating and decreasing friction to one’s advantage.

In order to understand the science behind the sport, a more thorough description of the physical characteristics of the rink and the stone is needed. First of all, the ice on the curling rink is not polished and smooth. Rather, it has a pebbled surface made by spraying the rink with water and allowing the tiny droplets to freeze on its surface. The pebbled surface is critical to the game. As for the stone, which is made of a very hydrophobic granite traditionally from northern Wales or the island of Ailsa Craig in Scotland, its under-surface is not flat either. Instead, it is slightly hollowed out, making room for a small air pocket and causing only a thin annulus around the bottom to make contact with the rink. Coupled with the fact that the ice rink is pebbled, the annulus actually only touches a few of the little pebble protrusions on the ice, making its true contact with the ice very low indeed (Shegelski 2001). The pebbled surface and the stone’s small hollow also do away with the potential problem of liquid getting trapped under the stone and suctioning it to the rink surface.

There has been much debate among scientists about how the different sources of friction influence the game. Of particular interest is the three-way interaction between the stone, the ice, and the sweeping of the brooms and how the interaction of all three causes the stone to “curl,” i.e. have a trajectory which curves, giving the sport its name. This mysterious curling motion has received a lot of attention, although a complete understanding of the motion is still lacking. To begin to understand the curling motion, one must first understand the physics of how the stone glides across the ice. One thought is that as the stone slides along the ice, the vigorous broom sweeping directly in front of the stone causes the ice temperature to raise just enough to create a thin lubrication layer of water from the melted ice, reducing the coefficient of friction that the stone is traveling over (Marmo et al. 2006). Another viewpoint, however, points out that there cannot possibly be a lubrication layer of water supporting the stone because the stone rests on a pebbled, not a flat, surface (Denny 2002). The lubrication layer would simply never accumulate because the liquid would spread out between the small pebbled ice protrusions that the stone rests on. On the other hand, since the stone rests on a pebbled surface, each individual protrusion receives more pressure from the stone, thus melting the ice on the surface of the protrusions and creating a lubricating layer even more easily (Shegelski et al. 1999). Regardless of the mechanism, it is clear that the friction coefficient of the ice does change with sweeping, and the pebbled surface also influences curling. Studies have shown that on a smooth ice surface, little to no curling occurs. On a dry surface, the friction coefficient is constant, but the effects of a stone on such a surface cause it to behave in the opposite way than on ice (Jensen et al. 2004).

 

Friction


So why does the stone actually curl? It has been suggested that it may be because of an asymmetrical buildup of friction. If, for example, a stone is sent forward with a slight clockwise twist, any debris on the ice will be pushed to the right side of the stone. Any liquid layer that forms from the pressure and friction will also push the water to that right side of the stone, slightly pooling the water on one side. This means that on the right and front side of the stone, there is more lubrication from the melted ice, so the stone as a whole experiences an asymmetric coefficient of friction (Bradley 2009). Additionally, if a sweeper stands to the side of the advancing stone to sweep, more heat is generated in the part of the sweeping stroke that is closest to the sweeper, so the ice will have a larger decrease in the friction coefficient on that one side of the stone (Marmo 2006). Additionally, if the stone moves too slowly, it may actually increase the coefficient of friction because as the heavy stone slides across the pebbled surface, its pressure and heat of movement causes the small ice pebbles to break off, leaving small pebbles to grate underneath the advancing stone (Denny 2002).

Despite these theories, one model concludes that no matter how the coefficient of friction is changed and distributed across the different sides of the stone, the real motion of a curling stone remains entirely unaccounted for compared to the models (Nyberg et al. 2012).

How one sweeps also affects the way the ice melts. Sweeping plays a large role in the game by simply raising the temperature of the ice so that the friction coefficient decreases and with the raised temperature, it will be all the easier to melt the ice once the stone hits where the brooms have just swept. As the ice is brought closer to its melting temperature, the energy it takes to completely melt it decreases (Marmo et al. 2006). Next comes a question as to whether one ought to sweep harder or faster, with a low angle with respect to the ice or with a high angle. And does sweeping play a larger role in altering the friction coefficient by polishing the ice, or is its chief role to melt the ice? Since the ice is orders of magnitude smoother than the stone, any polishing done to the ice has a negligible effect, although sweeping aside any debris is useful, and we see that the key use of sweeping is how it raises the temperature (Marmo 2006). We thus see that the key use of sweeping is how it raises the temperature of the ice.

Even the styles of sweeping also make a difference in the amount of friction present in the game  – sweepers may use the conventional style, which consists of sweeping in front of the stone while standing to the side, or a high-angle style in which the sweeper standing behind the stone reaches over the stone to sweep in front of it. The high-angle style produces a stroke that overlaps each preceding stroke, giving a thermal history of ice at a more elevated temperature. The friction coefficient is thus lowered since each patch of ice is swept (and heated) several times. This is a clear advantage of this style – however, the drawback is that the high-angle style is farther away from the stone than the conventional style, meaning that although the ice experiences a greater increase in temperature when utilizing the high-angle style, the ice has a longer time to cool back to freezing before the stone reaches it, unlike the conventional style (Marmo 2006). There is also the question of whether it is better to sweep faster or harder. Sweeping twice as hard produces twice as much heat (Bradley 2009) which is desired in some instances, but in others, it is more advantageous to sweep faster so that the strokes can overlap each other and warm the ice more in that way. Regardless, when sweeping is done at the proper moment with the right technique, the power of friction can be leveraged to add 20-30 feet to the distance a stone can travel (MacDonald 1996).

The competing sources of friction and more particularly, the various means to alter friction coefficients during the game, give curling its unique qualities and intriguing scientific questions. As curling continues to gain popularity, it can be expected that further scientific inquiry will continue to be directed at exploring the role of friction in the curl of the stone, until it can be answered with certainty how the asymmetric friction coefficients acting on the stone cause it to curl.

 

By: Jessica Egan, University of Utah
Jessica Egan began her love of chemistry under her mom’s direction with homeschool experiments in middle school and then up into high school through her high school chemistry teacher. She pursued a chemistry and art double major at Hillsdale College and decided to continue her chemical education by attending graduate school at the U. She has recently graduated with a M.S. from the U in Analytical Chemistry and is looking for a career in industry.

 

References 

Bradley, J. L. 2009. The sports science of curling: a practical review. Journal of Sports Science and Medicine 8:495-500.

Clark, D. 2008. The roaring game: a sweeping saga of curling. Key Porter Books, Toronto, Ontario, Canada.

Dassler, A. A. 1986. Pair of shoes for the sport of curling. Patent Number 4,578,883.

Denny, M. 2002. Curling rock dynamics: towards a realistic model. Canadian Journal of Physics 80:1005-1014.

Esser, L. 2011. Swept away: exploring the physics of curling. Science Scope 35:36-39.

Holt, R. 1989. Sport and the British: a modern history. Oxford University Press, Oxford, U.K.

Jensen, E. T., and M. R. A. Shegelski. 2004. The motion of curling rocks: experimental investigation and semi-phenomenological description. Canadian Journal of Physics 82:791-809.

MacDonald, J. A. 1996. Artificial curling rink. Patent Number 5,566,938.

Marmo, A. A., I. S. Farrow, M-P Buckingham, and J. R. Blackford. 2006. Frictional heat generated by sweeping in curling and its effects on ice friction. Proceedings of the Institution of Mechanical Engineers, Part L.: Journal of materials: Design and Applications 220:189-197.

Marmo, B. A., M-P Buckingham, and J. R. Blackford. 2006. Optimising sweeping techniques for Olympic curlers. The Engineering of Sport 6 3:249-254.

Nyberg, H., S. Hogmark, and S. Jacobson. 2012. Calculated trajectories of curling stones sliding under asymmetrical friction. Conference Paper from the 16th Nordic Symposium on Tribology 12-15.

Robertson, C. M. Curling brooms. 1987. Patent Number 4,638, 522.

Shegelski, M. R. A. 2001. Maximizing the lateral motion of a curling rock. Canadian Journal of Physics 79:1117-1120.

Shegelski, M. R. A., R. Niebergall, and M. A. Walton. 1996. The motion of a curling rock. Canadian Journal of Physics 74:663-670.

Shegelski, M. R. A., M. Reid, and R. Niebergall. 1999. The motion of rotating cylinders sliding on pebbled ice. Canadian Journal of Physics 77:847-862.

Tranter, N. L. 1989. The Patronage of organised sport in central Scotland, 1820-1900. Journal of Sport History 16:227-247.

Willoughby, K. A. and K. J. Kostuk. 2005. An analysis of strategic decision in the sport of curling. Decision Analysis

 

Gloves and Geckos: How Football Gloves Echo the Gecko (Basic)

In sports, one of the most important forces is friction.  In football, players have started to use gloves as a means to help control the ball better, particularly with making catches.  Companies such as Nike, Cutter, Reebok, and Adidas all market gloves that have higher and higher levels of ‘tack’, a term used for how well the gloves can grip things.  In order to develop better gloves, an understanding of how things actually adhere (stick) to each other is required.  This is a concept that scientists have been studying for a long time, making better and better glues.  Now, scientists and companies are focused on making better ‘dry adhesives’ that can go on gloves, tapes, or anything where an adhesive might find use.  Scientists examine the molecular basis of adhesion using various methods, including Atomic Force Microscopy, to explore new substances and identify possible adhesive materials for use with sports applications.

Looking for inspiration, scientists have turned to nature and examined the gecko to see how it can stick to glass walls and run up rock faces with such ease. In this research, scientists have determined that geckos use uniquely structured keratin setae pads to stick to and release from slippery surfaces (Qu 2008).  This research has prompted chemists to develop a non-biological material that has a very similar structure to the gecko’s own pad.

Football gloves have come a long way, from the use and ban of the axle grease-like substance Stickum, to the purported use of neoprene scuba gloves, and to the gloves players use today.  In time, it may be possible that some of the newest discoveries in adhesive materials – such as the material inspired by the gecko – will find their way into the sports arena.

By: Bret Van Ausdall, University of Utah

Learn more about the technical aspects of adhesion and polymers.

References:

Qu, L., L. Dai, M. Stone, Z. Xia, & Z.L.Wang. 2008. Carbon nanotube arrays with strong shear binding-on and easy normal lifting-off.  Science 322:238

Gloves and Geckos: How Football Gloves Echo the Gecko (Technical)

Figure 1

How can a gecko affect a football game? “By selling car insurance on commercials during halftime?” would be the wrong answer.  Rather, instead of watching a gecko sell insurance, one would have to look carefully at a gecko hanging off the glass walls of its aquarium, much like a reptilian Spider-man, to see that football pros could learn a lot from geckos.  This is exactly what researchers at the University of Dayton and Georgia Technical Institute have done (Qu et al. 2008). Researchers garnered inspiration from a paper published in Nature (Autumn et al. 2000) which examined the pad of a gecko to determine exactly how it is able to stick so effortlessly to something as slippery as glass. They then applied the sticky gecko pad concept as they created a carbon nanotube array on a small piece of silica. What researchers over the years have been able to determine is that a gecko’s foot can have upwards of 500,000 seta, which are small fibers that are composed of keratin with spatula-shaped portions at the end (see figures 1 and 2)

Keratin, a polymeric protein comprised of several units of various forms of keratin monomers, is the same material found in our hair and skin, though gecko’s cells align the keratin in a different fashion than our bodies.  Mammals produce a form of keratin called alpha-keratin that has within it lots of alpha-helices (Huggins 1957), basically columns that stack together (Huggins 1977), that are found in our skin, fingernails, hair, and elsewhere in our bodies (Moll et al. 2008).  Geckos, and reptiles in general, produce and assemble keratin together in a form called beta-keratin, which contains within it repeating  beta-pleated sheets instead of alpha helices (Maderson 1964).  The geckos with their beta-keratin also have the extra feature of these strong, rigid setae on their pads, which enable them to perform their miraculous wall-climbing feats.  Researchers have determined the pad of a gecko can support up to 10 Newtons per cm2 (N cm-2) (Irschick et al. 1996); however, the carbon nanotube silica pad produced by the Dayton and Georgia Tech researchers produced 100 N cm-2!  In other words, if the same gecko weighed 10 times as much, it would still be able to hang from the glass wall safely if it had the silica and carbon nanotubes on its pad!

Figure 2

So, one asks, how can this relate to football?  How can the pad of a gecko provide an advantage to improve my favorite high school/college/NFL team’s chances at winning? Notice the gloves some of the players wear. These gloves, as one would find out quickly after wearing a pair, amplify the grip of the athlete on the football, allowing them to make even more amazing and athletic catches.  The underlying concept of these gloves revolves around the material covering the fingers of the glove, which has a higher coefficient of friction to the material of the football than skin does.  Basically, this means that the material on the glove ‘sticks’ to the football better than your skin.  Cutters, a leading manufacturer of football receivers’ gloves, has data showing that their gloves have some of the most ‘tack’ in the field (Basic Guide to van der Waals forces 2012).

How do any of the materials, be it silica and carbon nanotubes, or material found on the football players’ gloves, work?  It’s a similar question one might ask when playing with super glue: how does glue stick two different materials together? This is a question that has been posed for some time and was even addressed at the First International Congress on Adhesion Science and Technology in 1996 (Somorjai 1998).  Generally speaking, adhesion can be divided into two classes: Strong adhesion and Weak adhesion.  Strong adhesion deals with the strong chemical bonds of an adhesive to a material, where the adhesive’s interactions are ordering on the magnitude of, or at times stronger than, the bonds within the material the adhesive is sticking to.  Weak adhesion deals primarily with van der Waals force, and it is thought that the setae on the gecko’s pads operate using these forces. Van der Waals forces are a form of weak intermolecular interactions with which molecules and atoms can interact with each other (Basic Guide to van der Waals forces 2012).  Though van der Waal forces may be weaker than actual chemical bonds between atoms, if there are enough molecules present, for example on the pad of a gecko, the interactions generated between the pad of a gecko and a glass wall can be quite large.

The various strengths of intermolecular forces and interactions at the molecular level lead to the concepts of friction, lubrication, and adhesion.   The concept of adhesion has industrial applications (such as motor oil as a lubricant in an engine and ‘tacky’ gloves for athletes), but in order to test the ‘adhesiveness’ of a substance, scientists must have methods to test a substance’s properties.  One method used to test the ‘adhesiveness’ of a substance is atomic force microscopy (“AFM”).

AFM is conducted by using an extremely small pin, approximately one atom wide, attached to a lever.  As the pin is lowered towards a surface to be tested and moved around the surface of the material, the tip starts to move in response to the molecules at the surface of the substance. The movements of the lever are recorded.  When the interactions are compared to other materials, it provides insight into how the properties of different materials compare.  The types of forces a pin tip can experience are chemical bonding, magnetic forces, electrostatic forces, and van der Waals forces, among others (Lang et al).  A similar technique, scanning force microscopy, was invented by Gerd Binnig and Heinrich Rohrer in the 1980’s, but led to the discovery of AFM.  The extreme utility of these techniques led to Binnig and Rohrer winning a Nobel Prize in Physics in 1986.  Researchers all over the world have used AFM and variations of it to examine various types of materials and test them for their adhesive forces (Thomas et al. 1995) as well as to examine molecular origins of friction.

When companies start to design products like receivers’ gloves, they are not just going out and using AFM to find the stickiest substance around.  A material known as Stickum, produced by Mueller Sports Medicine, was used by Lester Hayes in the NFL in the 1970’s. His liberal use of the material prompted a ban on the substance. Stickum itself has been redesigned from the original gooey, stick-to-everything substance to a new powder that is sprayable.  Companies and researchers are still perfecting the ‘dry’ adhesive technologies, going from leather gloves, leather gloves treated with tackifying resins, and leather gloves treated with a silicon resin to enhance frictional forces (Kang 2002). Regardless of how, science has helped make football more competitive and exciting by providing highly advanced polymer coated gloves that, when coupled with some of the most athletic people in the world, enable breath-taking, one-handed, catches.

 

By: Bret Van Ausdall, University of Utah

References:

Autumn, K., Y.A. Liang, S.T. Hsieh, W. Zesch, W.P. Chan, T. W. Kenny, R. Fearing, and R.J. Full. 2000. Adhesive force of a single gecko foot-hair.  Nature 405:681.

Basic Guide to van der Waals forces. 2012. Retrieved from http://www.chemguide.co.uk/atoms/bonding/vdw.html#top

Cutter Fall Catalogue. 2012. Retrieved from http://www.cuttersgloves.com/catalog_fall12/

Huggins, M. L. 1957. The structure of alpha keratin. Proceedings of the National Academy of Sciences of the United States of America 43:204.

Huggins, M. L. 1977. The Structure of α-Keratin. Macromolecules 10:893.

Irschick, D. J., C.C. Austin, K. Petren, R.N. Fisher, J.B. Losos, and O. Ellers. 1996. A comparative analysis of clinging ability among pad-bearing lizards.  Biological Journal of the Linnean Society 59:21.

Kang, H. S. “Athletic glove having silicone-printed surface for consistent gripping ability in various moisture conditions.” Patent 6,408,442. 25 June 2002.

Lang, K. M., D. Hite, R.W. Simmonds, R. McDermott, D.P. Pappas, and J. Martinis. 2004. Conducting atomic force microscopy for nanoscale tunnel barrier characterization.  Review of Scientific Instruments. 75:2726.

Maderson, P. 1964. Keratinized epidermal derivatives as an aid to climbing in gekkonid lizards. Nature 203:780.

Moll, R., M. Divo, and L. Langbein. 2008.  The human keratins: biology and pathology. Histochemistry and Cell Biology 129:705.

Qu, L., L. Dai, M. Stone, Z. Xia, and Z.L.Wang. 2008. Carbon nanotube arrays with strong shear binding-on and easy normal lifting-off.  Science 322:238

Somorjai, G. A. 1998. The molecular surface science of adhesion. Pages 3-15 in W.J. van Ooij and H.R. Anderson, Jr., editors. First International Congress on Adhesion Science and Technology. Ridderprint bv, Ridderkerk, The Netherlands.

Thomas, R. C., J.E. Houston, R.M. Crooks, T. Kim, and T.A. Michalske. 1995. Probing adhesion forces at the molecular scale.  Journal of the American Chemical Society 117:3830.

 

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

From Tee to Fairway: How Physics Affects the Drive, the Club, and the Golf Ball

Golf Ball Velocity

Golf Ball Velocity

The average golfer drives the golf ball with an initial velocity of over 100 miles per hour.  If the player uses a club with a flexible shaft, the act of swinging adds an additional measure of torque as the head of the club also propels forward to connect with the ball.  The head of the club has grooves that increase the friction between the club and the ball, allowing the club to more effectively focus the area of contact.

The optimal angle to hit the ball ranges from about 12 to 20 degrees.  Putting a backspin on the ball increases lift and can add significant distance to the drive.  The dimples on the golf ball itself help reduce drag from the air stream by reducing turbulent air pressure around and behind the ball, shifting the wake further behind the ball, thus allowing for smoother, less resistant flight.   Any combination of these variables contributes to how well the ball overcomes the forces of gravity and air resistance.

Learn the basics of how physics affects golf or read the more technical details here.

Articles by Trevor Stoddard

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

Tennis Courts and Equipment: How Physics Affects the Speed of Play (Basic)

UTAH MEN'S TENNIS Ben TasevacModern day tennis is a game of power, speed and spin.  Improvements in tennis rackets allow players to hit the ball harder and faster than ever before.  This faster pace has made the tennis serve more dominant in tennis matches, which means that tennis points can happen quickly.  There is some interest in slowing down the game to make points last longer. One approach is to engineer tennis balls differently, so that the balls themselves can be selected to adjust the speed of play. Another approach is to consider the properties of the court surface.

There are three main types of court surfaces used in tennis: grass, clay, and acrylic.  Each court is considered to have its own “speed.” Grass courts are firm and have a slippery surface.  When the ball hits the surface, it tends to slide and will have a low bounce.  This means that the player has little time to react and move, making grass a “fast” paced surface.  Clay courts, made of small pieces of crushed rock, are considered to be a “slow” surface. This is because the rough surface prevents the ball from sliding when it hits the court. This causes the ball to bounce much higher, giving the player more time to move and to choose how and where to hit the ball.  Acrylic courts are “medium” paced surfaces.  They have an asphalt or concrete base with a playing surface that is made of acrylic paint mixed with sand.  These courts are the most commonly used and require the least amount of maintenance when compared to either grass or clay surfaces (Lees 2003).

Three types of balls have been developed for use on the different types of court surfaces.  The balls vary in size and firmness. The standard ball is the type 2 ball.  It is intended for use on medium paced surfaces.  The type 1 ball is the same size as the type 2 ball, but it is firmer.  This means that it will not change shape as much when it hits the court, so it is a “fast” ball.  Type 1 balls are designed for use on “slower” surfaces such as clay.  Type 3 balls are larger than type 2 balls.  This means that they have a harder time moving through the air, so they will travel slower.  “Slow” type 3 balls are designed for use on “fast” surfaces such as grass.  With a variety of tennis balls to choose from, players can adjust the speed of play to complement court conditions.

Learn the technical details of how physics affects the speed of play.

By: Lindsay Sanford, University of Utah
Lindsay received her B.S. in Mechanical Engineering from Washington State University and is currently pursuing a PhD degree in Bioengineering. In her spare time, she likes to travel, hike, read, and play with her two year old son.  She is also an avid runner and tennis player.

 

References

Lees, A. 2003. Science and the major rackets sports: a review. Journal of Sports Sciences. 21(9): 707-32.