The solid “thwack” of foot to soccer ball, such as when a goalkeeper like D.C. United's Bill Hamid sends the ball on a goal kick, tells you that the ball is being sent with considerable force. Sir Isaac Newton’s laws of motion allow you to determine reasonably accurately how much force is exerted at the time of impact.
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A regulation adult soccer ball weighs right around 1 pound The Laws of the Game promulgated by FIFA, the international governing body, specify that the ball weigh between 14 and 16 ounces at the start of a match. This translates to 0.45 kg, valuable as metric measures are typically used to measure the force of soccer kicks. The ball is pumped with air to between 8.5 to 15.6 pounds per square inch so that it presents a rigid spherical shape that can be kicked.
The time of contact between a player’s shoe and the ball is 0.05 seconds, according to James Watkins, a professor of Sports Science at Swansea University in the United Kingdom, in “An Introduction to the Biomechanics of Sport and Exercise.” The velocity of the kicked ball is 30 meters per second, or 67 mph, he notes. The force of the ball can be determined by using the impulse-momentum form of Newton’s second law of motion, which holds that the force equals the weight of the ball times its velocity divided by the time of foot contact.
In the example of a kick traveling 30 meters per second, the force is 270 Newtons. This figure is derived from 0.45 kg, the weight of the ball, times 30 meters per second, divided by 0.05 seconds. To convert Newtons, a metric measure of force, to a second measure called kilogram-force, which measures force in units based on the force gravity exerts on 1 kg, you apply a formula that 1 kgf equals 9.81 Newtons. This yields a result of 27.52 kgf for the force of an average soccer player kicking the ball. The calculation of 270 Newtons can also be converted to a standard-measure force of about 1,200 pound-force, by dividing each Newton by 0.22481. A pound-force, also called simply a pound, also equals the gravitational force on a mass of 1 pound.
In “Biomechanical Characteristics and Determinants of Instep Soccer Kick,” Eleftherios Kellis of the sports sciences department at Aristotle University in Greece compiled ball speeds from kicks reported in 27 research studies. Ball speeds ranged from a low of 14.9 meters per second in youths aged 10 to 17 to 30 meters per second for professional soccer players, with many adult amateurs able to send the ball at 25 meters per second. The pounds of force can be calculated knowing that the ball weight is constant for all the studies, with the added assumption that the shoe contact time is also fairly constant at about 0.05 seconds. Thus while professional players send the ball at 30 meters per second and 1,200 pounds of force, an average adult player sends the ball at around 25 meters per second based on a kick of 1,000 pounds of force, while average youth players may only muster a ball speed of 14.9 meters per second, indicating just 600 pounds of force.