Researchers at Yamagata University in Japan are using finite-element analysis to simulate how people kick footballs. This illustration shows the deformation on a leg and ball, ranging from pink (the lowest) through blue, green and yellow to red (the highest). These studies have confirmed what most footballers know. If you kick the ball slightly off-centre with the front of your foot – and with your ankle bent into the shape of an “L” – the ball will curve in flight. This causes the applied force to act as a torque, which gives the ball a spin, enabling the Magnus effect to come into play.
The drag coefficient of a ball plotted against Reynold’s number – a non-dimensional parameter that takes into account both the velocity and diameter of the ball. The drag coefficient drops suddenly when the airflow at the surface of the sphere changes from laminar to turbulent.
The position of the discontinuity depends on the roughness of the surface of the ball. Footballs are relatively smooth and so need to be kicked relatively hard to gain enough speed to move into the turbulent phase.First published in Physics World magazine, June 1998 pp25–27. https://physicsworld.com/a/the-physics-of-football/
A bird’s-eye view of a football spinning about an axis perpendicular to the flow of air across it. The air travels faster relative to the center of the ball where the periphery of the ball is moving in the same direction as the airflow (left). This reduces the pressure, according to Bernoulli’s principle.
The pressure increases on the other side of the ball, where the air travels slower relative to the center of the ball (right). There is therefore an imbalance in the forces, and the ball deflects in the same sense as the spin – from bottom right to top left. This lift force is also known as the “Magnus force”, after the 19th-century German physicist Gustav Magnus.First published in Physics World magazine, June 1998 pp25–27. https://physicsworld.com/a/the-physics-of-football/
How does the amount of air in a soccer ball affect how far it travels when struck by the same force? The amount of air or air pressure in a soccer ball effects how far the ball will travel when struck by the same force. The higher air pressure that is put into a soccer ball improves the ball’s rebound off the foot of a player. More energy is transferred to a “stiff” ball in an elastic collision. In other words, the ball deforms less during the impact, so there’s less energy lost to deformation.
Does the atmospheric air pressure effect how far a soccer ball travels when struck by the same force? The atmospheric air pressure (the air surrounding the ball) also plays a role in how far a ball travels. At lower pressure, there’s less air friction. You can compare it to kicking the ball in a tank of water to kicking the ball on the moon. Balls go farther at high altitude because of the reduced drag from the air, which is thinner as you go higher up. So there’s a case where “reduced” air pressure makes the ball go farther.
Also, the materials that the soccer ball is made out of effects how far the ball will travel…but that is another question and experiment.
How much air pressure should I put into a soccer ball? Use Proper Air PressureDo not over or under pressurize a ball. Use the manufactures recommended air pressure that is printed on most balls. Most soccer balls have a pressure rating of 6 to 8 lbs. or 0.6 or 0.8 BAR. It is recommended that you use a pressure gauge to measure the exact amount of pressure in a ball after inflating and before use. BAR or PSI or LBS?Some soccer balls have recommended pressure values indicated in BAR while others have the values indicated in PSI or LBS. To convert the pressure values, use the following formulas:To convert BAR (KGS) to PSI (Lbs.):Answer = 14.5037 X The amount of BAR(KGS)For example: A soccer ball has a recommended pressure of 0.6 BAR labeled on it. To convert BAR in Pounds Per Square Inch (PSI), multiply 0.6 times 14.5037. The answer is 8.7 PSI or Lbs.To convert PSI (Lbs.) to BAR(KGS):Answer = .068948 X The amount of PSI(Lbs.)For example: A soccer ball has a recommended pressure of 7.9 Lbs. (PSI) labeled on it. To convert Pounds Per Square Inch (PSI) into BAR, multiply 7.9 times .068948. The answer is 0.545 BAR.
How Do I inflate my soccer balls? Soccer balls lose air pressure over time. Sometimes over a few days (soccer balls that use butyl bladders keep air pressure longer than balls that use latex bladders). Be sure to check the pressure frequently to make sure the ball is properly inflated. Therefore, invest in a good ball pump, have a supply of inflation needles and use a low pressure gauge to measure for proper inflation. Before you first inflate a soccer ball, place a couple drops of silicone oil or silicone lubricant spray or glycerin oil into the valve. You can purchase one of the oils or spray at your local hardware store. Using one of the lubricants will improve the life of the valve and lubricate the valve for easy insertion of the inflation needle. Always moisten the inflation needle before you insert it into the valve. Preferably, use some silicon oil, silicon spray or glycerin oil to moisten the needle. However; most people use spit…yuk, but that is not recommended. Manufacturers recommend that you reduce the air pressure in your match balls after a game to reduce the amount of stress on the ball seams or stitching. Be sure to inflate the ball back to proper pressure before the match.
Why do I always have to pump up even expensive balls? Many balls use bladders made out of latex. Natural Latex Rubber bladders offer the softest feel and response, but do not provide the best air retention. Micro pores slowly let air escape. Balls with natural rubber bladders need to be re-inflated more often than balls with butyl bladders. Even after one or two days, the latex bladder will leak enough air so that you will have to inflate the ball back to recommended pressure. Some balls use carbon-latex bladders in which the carbon powder helps to close the micro pores. Soccer balls with carbon latex bladders usually increase air retention to approximately one week. Of course, check the ball for punctures that may cause the air to leak out.Soccer Balls with Butyl bladders or PU bladders offer an excellent combination of feel and air retention and can be found in most middle to upper priced balls. Air retention is significantly increased to weeks and months instead of days compared to balls with latex bladders.
Why do some soccer balls get bigger over time? Many soccer balls do tend to get larger over time. This is due to the pressure of the air in the bladder against the linings and cover. Over time the material and stitching may stretch out causing the ball to become larger. Also, soccer ball abuse may cause the stitching to loosen and the ball to expand.
Questions about Soccer Ball Material Physics
I’m still working on this part…check back soon.
Questions about Curving a Soccer Ball
How does a ball curve when you kick it? For the answer to this question and others relating to the physics of a curving soccer ball, click here.
Since the 1999-2000 school year, soccer balls used in interscholastic competition in sports for which the National Federation of State High School Associations (NFHS) writes playing rules are required to have the NFHS authenticating mark.
Ball Testing Requirements:
Made of leather or other suitable material that is weather resistant
Of a circumference of not more than 70 cm (28 ins) and not less than 68 cm (27 ins)
Not more than 453 g (16 oz) in weight and not less than 396 g (14 oz) at the start of the match
The ball shall be inflated to manufacturer’s recommended pressure
For additional information on NFHS, you can visit their web site by clicking here.
The above logo’s are printed on all soccer balls that are either FIFA Approved or FIFA INSPECTED. Only the highest of quality balls can pass the testing requirements.
FIFA Denominations Programme
The testing procedures and designations offer many benefits to those who buy soccer balls (footballs) including guaranteed quality, value for money and better playing performance. Since January 1, 1996 only those outdoor footballs which have been tested and meet the demanding quality criteria, bearing either of the official markings ‘FIFA Approved‘, ‘FIFA Inspected‘ or ‘International Matchball Standard (IMS),’ are allowed to be used in FIFA (Fédération Internationale de Football Association www.fifa.com ) competition matches and competition matches under the auspices of the six continental Confederations.
As of January 1, 2000 the quality testing and certification has also become compulsory for indoor footballs used for international matches under the auspices of FIFA and the Confederations.
FIFA has set out to ensure that the balls used in top matches meet the most exacting standards. It has meant a general upgrading of standards of footballs (soccer balls) throughout the world.
Testing procedures for the balls submitted for these designations are designed to simulate match conditions. Manufacturers have to submit seven balls if they are applying for “FIFA Inspected” status, and ten samples if they seek the “FIFA Approved” label. All these are put through their paces at EMPA, the Swiss Federal Laboratories for Materials Testing and Research in St.Gallen.
There is another qualification level, too, less demanding than the others. This is the designation “International Matchball Standard”, and for this the applicant footballs (soccer balls) can be sent to any of seven European test institutes which have been selected by FIFA to test and certify the balls for this category, which is free of any royalty fees.
There are a total of seven tests. All footballs are submitted for the first six and only potential “FIFA Approved” candidates for the seventh test, which is a shooting test for shape and size retention, including change of pressure.
However, the criteria for “FIFA Approved” footballs are higher throughout. For example, 25% loss of pressure is acceptable for a “FIFA Inspected” applicant, but only 20% for a “FIFA Approved” ball.
Testing The Euro Ball
Circumference: The first test – for circumference – requires the first three footballs to be conditioned for at least 24 hours. They are then inflated to a defined pressure and the diameter of the ball is measured in ten exactly defined points. The test is designed to ensure a well-balanced response in play.
Sphericity: The second test is for sphericity, which ensures the ball’s in-flight stability. The apparatus used is the same as that used for determining the circumference. The conditioned balls are inflated to a test pressure of 1.0 bar and the diameter of the ball is measured at the middle of the 16 panels, with a high degree of accuracy. The difference between the highest and lowest diameter is determined as a percentage of the mean diameter. The mean difference in the highest and lowest diameter of the three footballs in recorded in the test report.
Rebound: This is important too, to make sure that the ball bounces in a predictable manner, vital in top class matches. The football is dropped, in guided free fall and with a defined velocity, onto a steel surface. With the help of a video camera, the height of rebound at the lower side of the ball can be determined.
Water absorption: It is important that a football does not become too heavy when wet. The conditioned footballs are placed in a receptacle filled with 2 cm of water. After being compressed a number of times, with a pneumatic position to simulate playing conditions, the ball is removed, wiped dry and reweighed. Its increase in weight is expressed as a percent of the original weight of the ball, defining precisely the amount of water that has been absorbed.
Weight: Weight is vital, because it ensures a consistent playing response when the ball is struck. Football samples are inflated and weighed in a standard atmosphere, with a wind protected electronic balance. The mean weight as well as the single values of the three fallouts are recorded in the report.
Loss of Pressure: The football should not lose pressure over time, thereby remaining playable. The balls are inflated to a defined pressure, then left in a standard atmosphere for a certain period of time, after which the pressure is measured again.
Shape and Size Retention (“FIFA Approved” only): This test is designed to ensure that the footballs last, even in the most challenging situations. The footballs are inflated to a defined pressure. Two rotating cylinders accelerate the balls to a specific velocity, so that they hit a steel panel at a consistent speed and angle. The footballs are examined after 2000 shots and the increase in circumference and deviations on sphericity are measured. The testing procedures and designations offer many benefits to those who buy footballs including guaranteed quality, value for money and better playing performance.
Referees of FIFA and Confederation matches are among the beneficiaries. They simply have to check that the ball carries one of the three marks and that the pressure is correct, rather than having to check weight, circumference and other details before matches.
The testing criteria are indeed tough, but they have been set by the industry itself. Only the best products pass the test, which means that the new standards are worthwhile. Licensees include most of the major leading brands and also manufacturers from India, China, Thailand and Japan, showing that the value put on the FIFA mark is as universal as football itself.
What Players think of the Modern Ball We look at the effect the Programme has had on the game itself. Has it improved skill levels? Has the job of match officials been made any easier – and have there been any medical benefits or all-round improvements? 10-Dec-1998
Information and articles graciously provided by FIFA. www.fifa.com
MSC.Patran Used By Researchers to Model Complex Physics of Soccer Free Kicks
SANTA ANA, Calif. — May 31, 2001 — MSC.Software Corp. (NYSE: MNS), the leading global provider of simulation software, services and systems, today announced that MSC.Patran was used by researchers to help solve the mystery of ‘trick’ or ‘bending’ free kicks in soccer. Through the collaboration of three research groups, researchers at the University of Sheffield’s Sports Engineering Research Group, Yamagata University’s Sports Science Laboratory and Fluent were able to simulate the complex physics acting upon the ball and determine how world-class soccer players fool defenders and goaltenders with kicks that change trajectory in mid-flight. Dr. Takeshi Asai of the Yamagata University’s Sports Science Laboratory used MSC.Patran to model the stress and deformation of the foot and ball as the player strikes the ball. This simulation, combined with the computational fluid dynamics research done by Fluent and wind tunnel and trajectory modeling done by the Dr. Matt Carré of the University of Sheffield Sports Engineering Research Group, allowed the research teams to determine the actual physics governing free kicks in soccer. “The use of simulation software is continuing to grow around the world, especially in industries like sports and recreation who traditionally haven’ t used advanced simulation to understand how their products really function,” said Frank Perna, chairman and chief executive officer of MSC.Software. “We are proud to be a small part of this research into soccer ball flight and are looking forward to seeing these complex physical interactions on the field during the World Cup.”Soccer Ball WindtunnelSmoke Test Soccer Ball Wake Pathlines When a soccer ball is traveling through the air, its trajectory is influenced by a number of factors, including wind flow, air speed and pressure. As the player strikes the ball, the drag and force experienced by the ball strongly influences its trajectory, especially if the ball is spinning. When a player strikes the ball attempting to induce a shot that bends, a reaction known as Magnus Force causes an imbalance of pressure to occur. This imbalance can be so pronounced at the end of a ball’s flight that the sideways ‘spin’ force and ‘drag’ force causes the ball to alter its trajectory considerably near the goal.“The computer modeling techniques my group has developed with MSC.Patran will help us design better soccer boots in the near term and explain how a soccer player’s foot deforms as it interacts with the ball,” said Dr. Takeshi Asai of Yamagata University’s Sports Science Laboratory. “This has important implications for kicking techniques and preventing injuries to the foot and improve the overall understanding of the science of soccer.” CFD prediction of flow separation pattern behind a non-spinning soccer ball “We believe that our research into the underlying physics of soccer balls is crucial to helping us explain more about soccer free kicks than ever before,” said Dr. Matt Carré from the University of Sheffield Sports Engineering Research Group. “The work we are doing will lead to insights that can be applied to making better soccer balls and in improving the techniques of young soccer players.”Information graciously provided by MSC.Software Corporation of Santa Ana, CA, Fluent, Inc., Dr. Takeshi Asai of the Yamagata University’s Sports Science Laboratory, Japan and to Dr. Matt Carré from the University of Sheffield Sports Engineering Research Group.
Yokohama/Herzogenaurach, December 13th, 2007 –Today, adidas and Cairos Technologies presented the new Goal Line Technology and the adidas intelligent football tested at the FIFA Club World Cup™ in Japan from December 7 through 16, 2007. The official match ball of the tournament, adidas Teamgeist II, features a new intelligent technology designed to assist the referee’s decision in determining when and if the ball has crossed the goal line, making it the most accurate football ever produced.
“The purpose of the adidas intelligent ball and Goal Line Technology is to provide greater transparency during a match and to assist the referee in making quick decisions that can impact the outcome and quality of the game” said Hans-Peter Nuernberg, Senior Development Engineer, adidas Innovation Team. “We expect the system to perform very well during the FIFA Club World Cup™ in Japan and we will continue to refine the system so that it is 100% accurate.”
The intelligent technology implemented in the Teamgeist II uses a magnetic field to provide real-time feedback to a central computer, which tracks the location of the ball on the field and sends the data directly to the referee. By using a magnetic field and more stabilized and robust components within the ball, the new system is more precise and is not influenced by in-game factors, adverse weather or nearby technical systems.
“With the complexities and precision needed for Goal Line Technology, it is imperative that the system is tested in a variety of competitive in-game situations,” said Christian Holzer, COO of Cairos technologies. “The opportunity to test the new technology during such a competitive tournament will supply us with the valuable feedback needed in order to continue refining the system.”
Since 2003, adidas and Cairos in cooperation with FIFA, have developed the Goal Line Technology, which was first publicly tested in 2005 during the U-17 FIFA World Cup™ in Peru. The first system used radio transmissions to correspond with a central computer and a microchip suspended in the ball to determine its location on the field. The new Goal Line Technology and adidas intelligent ball have been redeveloped since 2005 to address the critical situations in which better accuracy is needed.
Following the testing during the FIFA Club World Cup™ in Japan, the results will be evaluated and next steps will be determined by Cairos technologies and adidas as to when the system will be ready to test again publicly. The new system currently meets all International Football Association Board (IFAB) requirements and the ball has been approved by FIFA for competitive international play. adidas’ experience in football manufacturing is unsurpassed anywhere in the world and leads the industry in the production of the most innovative footballs since 1963. The Goal Line technology is a testament to the ongoing commitment adidas has to innovation and the development of new technologies in football and across all sports categories.