When the soccer video game FIFA 14 went on sale this past fall, it boasted a ball that, at long last, could sail smartly through the air. In earlier versions of the popular game, the ball sometimes became a bit “floaty,” soaring along an unrealistically linear path.
Last year a team of engineers and animators vowed to get to the bottom of the problem. After an intense audit of all the projectile physics code in the game, they found the problem: their drag coefficient was wrong.
Engineers use the drag coefficient to model air resistance, which affects the speed and trajectory of an object in flight. “The ball moves at its fastest velocity when it comes right off the foot, and air resistance immediately slows it down until it reaches its maximum height,” says John Eric Goff, a physicist at Lynchburg College and author of Gold Medal Physics: The Science of Sports. “The ball should then pick up speed on its way down.”
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In previous FIFA versions the ball violated the laws of physics, accelerating and decelerating at a set rate unaffected by its initial velocity. “So if the ball was moving at 30 or 50 miles per hour, it was going to slow at the same rate as if it were moving at five miles per hour,” says Aaron McHardy, a senior gameplay producer at EA Sports, the company that produces the FIFA franchise.
The drag glitch also made for unrealistic spin. As a spinning ball whips air off to one side, the so-called Magnus effect pushes the ball in the opposite direction. A miscalculated Magnus effect meant the ball was not curving with much variability. “Once fixed, the ball would spin appropriately, and we got so much more variety in the curve,” McHardy says. “The ball now finally dips and swerves and does all these things that we see in the real world.
