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    Science Of Soccer Balls - Houston, We Have A Drag Crisis
    By Hank Campbell | July 1st 2010 04:41 PM | 1 comment | Print | E-mail | Track Comments
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    It's World Cup time and that means sports fans worldwide are focused on important issues, like complaining about vuvuzelas and this year's soccer ball, the Jabulani, which will push fan and player hatred of the 2006 ball, Teamgeist, into the background.

    Why does this happen (the ball complaints, not the vuvuzelas) every World Cup?  FIFA, governing body of Big Football, loves controversy, that's why. So FIFA has strict regulations on the size and weight of the balls but makes no regulations at all about the outside surface of the balls.  Thus, we go from an inordinately smooth ball one World Cup to one with ridges the next.

    "The Teamgeist was a big departure at the last World Cup. Because it was very smooth – much smoother than a regular soccer ball," says Professor Derek Leinweber, Head of the School of Chemistry&Physics at the University of Adelaide, "it had a tendency to bend more than the conventional ball and drop more suddenly at the end of its trajectory. By comparison, the aerodynamic ridges on the Jabulani are likely to create enough turbulence around the ball to sustain its flight longer, and be a faster, harder ball in play."

    John Eric Goff, associate professor of physics at Lynchburg College in Lynchburg, Virginia, goes into more detail in this month's Physics World, outlining how the Jabulani's surface roughness and asymmetric air forces contribute to its path once it leaves a player's foot.  He also says the reduced air density in games played at higher altitudes, like those in South Africa, can contribute to some of the reality-bending ball trajectories already seen in some of this tournament's matches.

    Gotta love unpredictability.   We have the lowest scoring World Cup in history but some of the goals that have hit home looked like physics miracles rather than science.   Obviously the underlying physics is the same, players are just not adjusting as they had to previous competition balls.

    As we have discussed many times in the past regarding baseball and other sports, the Reynolds number of the air, Re = VD/ν -  where V is the ball’s center-of-mass speed relative to the air and D the ball’s diameter, while the kinematic viscosity ν is the ratio of air’s viscosity to its density ρ - is vital to understanding how a soccer ball will move.  

    Those viscous interactions with the ball give rise to the drag force, which points opposite to the ball’s velocity.   Goff describes that drag force as FD = (ρV2/2)·A·CD ( the extra A you now see being the ball’s cross-sectional area and CD the dimensionless drag coefficient) and when there is a drop in CD there is a drag crisis, which indicates the transition from laminar to turbulent flow - about 12 m/s, he says, comparable to that of a medium-range pass.

    Goff says they used wind tunnels and numerical models to study the traditional 32-panel soccer ball and the Adidas Teamgeist ball from 2006, with its 14 thermally bonded panels, and they failed to find any significant differences in CD over a wide range of Re, but they do suggest that the Magnus force on the Teamgeist ball is slightly larger than on the 32-panel ball.

    When will they get a 2010 Jabulani ball?  Soon, we have to hope.   We are sure the Brazil team will be happy to send him one of they lose in the final match.

    See also: J. E. Goff, M. J. Carré, “Soccer Ball Lift Coefficients via Trajectory Analysis,” Eur. J. Phys. 31, 775 (2010)

    Comments

    Vladimir Kalitvianski
    About Magnus force: I can throw a round stone in water and the stone can jump out of water one, rarely two, and even more rarely three times, like in the figure. Can you believe it? The stone is as round as a tennis ball, not plane. It dives and floats up. No kidding!