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Aerodynamics, Fluid Dynamics, and Friction

Air Resistance, Clothing, and Drafting

In athletics athletes must pass through air. This interaction with the air causes the body to experience forces while in motion and during windy conditions. The force that causes resistance to the body during performance is called aerodynamic drag. Drag slows the forward motion of the body or causes an increase in energy and force output by the athlete to overcome the force of the air. Drag comes in two forms: friction drag and form drag. Friction drag is caused by the movement of air over the body’s surface. Form drag depends on the shape of the body.1

Athletes take counter measures to overcome friction drag and form drag. Certain types of clothing can cut down on friction drag. Cotton and fleece materials are made of fluffy fibers and loose matrixes. This catches air and increases resistance. Thus athletes employ nylons and polyesters which are comprised of sleek fibers and tightly woven matrixes. This causes the material to be sleek and the air slides off the clothing easily. During form drag athletes might take a certain position or tuck to make the body more streamline. In certain positions, such as a cyclist hunched over the handle bars, the flat chest is not exposed, thus the chest is not acting as a parachute catching the wind. This causes the cyclist to be more streamlined. In addition, athletes will wear tight form fitting clothing in order to make the body less resistant to air. Clothing that is loose and contains wrinkles will act as a parachute. This will slow an athlete down.

Scientist have conducted wind tunnel test on clothing materials, hair, shoes, and body positions. They have determined that it is possible to lower the air resistance of an athlete by 0.5% to over 6% by improving aerodynamics. One effective method of lowering air resistance is for athletes to use the method of drafting. Drafting is a common technique used by cyclist and runners. It reduces the aerodynamic drag on the athlete that is traveling behind another athlete or athletes. The athlete in front takes on the blunt of the air resistance. Thus, drafting produces a wind shield for the athlete in back. This can reduce the total energy output of the athlete in back or middle of the pack by 30% to 40% when compared to the athlete in front. It also must be mentioned that air resistance plays less of a role at high altitudes because the air is less dense.1

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Shaving the Body in Sports

Skin friction drag is the force caused by friction between molecules of air or water and the skin. Reduction of skin friction drag is important in swimming and running. Drag is minimized by reducing the roughness of the skin which comes in contact with the water and air. This may be accomplished by shaving the hair off the body so that it reduces the roughness of the skin surface and provides a sleeker area for the water or wind to move over. Thousands of tiny hairs cover the skin. These hairs catch air and increase the water and air resistance. Many athletes choose to shave in sports today to further reduce the drag affects of hair. Races are won by thousandths of a second. Shaving may make the difference between first and second place.

Note: In swimming, warmer water will produce less drag on the surface of the skin because cooler water is denser than warm water.

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Ball Flight

Ball flight is considered to be the path traveled by a ball. Ball flight can be affected by four main variables. One variable is the surface of the ball. That is, the stitches, groves, and sleekness of the external material. Another variable is the weather. Humidity, wind, altitude, and temperature may affect ball flight. Also ball flight can be affected by the shape of the ball. Some balls are large, small, round, or oval. Lastly rotations per second or revolutions per second of the ball will affect the path in which it travels.

The surface of most athletic balls is made of leather. However, other materials such as pig skin and plastics may be used. The ball is usually made up of multiple pieces of materials and must be stitched together. The stitching process leaves depressed grooves and raised stitches on the surface of the ball. These areas will increase the air resistance at those specific locations, which will alter the path of the ball. Some of the materials are sleek while others are not. Some surfaces such as golf balls are pitted and other surfaces such as footballs are raised. These have different functions such as the raised surface of the football provides gripping by the hands. The shape of the ball may affect its speed capabilities. A golf ball is smaller than a soccer ball and thus produces less air resistance. This allows the golf ball to reach greater speeds. Therefore, ball flight may be altered by the surface of the ball. In addition, it must be noted that the surface of a ball can become rough due to abrasions suffered during ground impact. Sometimes the abrasions can increase air resistance and alter ball flight characteristics.

Weather conditions can affect ball flight. A humid atmosphere is less dense than a dry one at the same temperature and pressure. This provides less air resistance. Also a humid environment may cause the stitches on the ball’s surface to swell, providing more air resistance over the swollen stitch. In addition at high altitudes the air is less dense and provides less air resistance. This provides an optimal environment for maximum speed and distance traveled by the ball. Wind patterns and wind speed can also have an effect on ball flight. In warm weather, the air is less dense than cooler air. This provides for better flight capabilities.

The amount of revolutions per minute greatly affects ball flight. When a ball rotates, one side of the ball experiences an decrease in air resistance because one side of the ball is rotating away from the direction of air flow (air flow and rotations are front to back). The other side of the ball has an increase in air resistance because air is flowing front to back, but the ball is rotating back to front. This increase in air resistance on one side pushes the ball (Magnus Effect). This causes the ball to take a curved path. Also the stitches and grooves of a ball may catch air and increase the air resistance on one side of the ball. This may cause a knuckle ball. In this instance the ball’s flight path may curve, dive, and curve again. Soccer players and baseball players use spin to there advantage when trying to throw a strike or score a goal. Tennis players use topspin to push the ball downward as it passes over the net. Golfers use back spin to keep the ball in flight longer by providing a lifting affect. The ability of athletes to use techniques to manipulate spin and thus produce a wanted affect on the ball will make champions. When athletes learn how to use weather to their advantage and master techniques that produce varying ball flight paths, they will become successful in their sport.

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The Role of Friction in Winter Sports: Skating and Skiing

Winter sports are based on the gliding action of a skate or ski over ice or snow. This gliding action produces friction. Friction can be reduced by a lubricant or a low viscosity fluid. Some scientists believe that when a ski or skate blade travels over the snow or ice it causes heat due to friction. The same principle is used when one rubs their hands together and heat is produced due to the skin rubbing together. When the skate blade or ski becomes hot it melts the snow or ice and produces a thin layer of water between the snow/ice and ski/blade. Thus, the water produces a lubricant. The amount of snow and ice that melts depends on two factors: the temperature of the ski/blade and the distribution of the heat from the ski/blade to the snow/ice. The greater the melting effect the better. Also the weather plays a key role as well. Ice and snow temperatures may very with the weather. Therefore, some athletes may gain an advantage during competition due to shifts in temperature. Heating of blades and skis decreases friction and improves performance. However, in some sports and organizations, heating of blades and skis is forbidden. Equipment temperatures may be taken prior to starting in some competitions.1

In skating, it has been calculated that friction between the blades and ice is 35% higher on the curves than on the straights. Even small changes in friction result in visible differences in performance. A small increase in friction can increase a 500 m sprint time by 0.8 sec and in a 10,000 m race by 23.5 sec. A 0.8 sec difference in the Olympic Games can mean the difference in being sixth place or winning the gold medal.

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The Role of Friction in Traction

Friction is a force that occurs when two surfaces move past one another. In athletics, athletes use friction for traction. Shoe manufactures design shoes to maximize traction so that athletes can stop, turn, and propel themselves forward without losing their footing. This can be both good and bad. Too much traction can cause injuries, while not enough traction can cause injuries. Therefore, there is a fine line between too much and too little traction in athletic performance. The demands for traction vary from one sport to another. Also, varying weather and playing surface conditions pose different demands for traction. For instance, in soccer an athlete’s ability to make a quick turn will depend on the amount of friction between the bottom of the shoe and the field. In wet conditions the athlete may experience a reduction in friction because of the water acting as a lubricant. This predisposes the athlete to injury due to slipping or falling. When choosing shoes, athletes should take into consideration the playing surfaces and the amount of traction needed to effectively play on those surfaces. On the same field of play athletes may need two different types of shoes, one for wet conditions and one for dry conditions. Traction greatly affects performance and athletes should consider it during competition.

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