We see it all the time in soccer. A physics-defying, gravity-altering kick that curves mid-air and finds the back of the net. Often, it happens over a defensive wall during a free kick.
Perhaps the best such example came from this Women's World Cup tournament last month, when Norway scored a free-kick goal on a ball that curved in flight and got buried in the top corner of the net against Germany. (Watch the goal from the 1:05 mark).
Ever wonder how a soccer ball can curve in the air like that? Well, the Massachusetts Institute of Technology (MIT) sought out to find the answer with a comprehensive researched report.
One of its conclusions was that smoother soccer balls are harder to control and curve in-flight. With longer seams, a rougher ball offers more predictable mid-air steering.
"The details of the flow of air around the ball are complicated, and in particular they depend on how rough the ball is," MIT professor of applied mathematics, John Bush, says in the report. "If the ball is perfectly smooth, it bends the wrong way."
Bush adds that in soccer, players getting a spin or curve on the ball mid-air comes from right-footed players brushing toward the outside of the ball, creating a right-to-left hook, and left-footed players, creating a left-to-right curl.
Soccer players often achieve this spin during free kicks, corner kicks and crosses due to their positioning on the field. That top spin or curve in mid-air can be latched onto the "Magnus Effect," a physics law, which can be used to describe the phenomena of a ball spinning through the air and the turning ball dragging some of the air around with it.
Players may not achieve the same curve, kicking a smooth ball as they do kicking a rougher one. Bush explains that the surface of the ball creates motion at the boundary layer between the spinning ball and the air. He adds that the rougher the ball, the easier it is to achieve the Magnus Effect and curve the ball in the desired and expected direction.
"The peculiar motion of a fluttering free kick arises because the points of boundary-layer transition are different on opposite sides of the ball," he explains.
Bush cites former Brazil full-back Roberto Carlos's free kick in a 1997 match against France as a prime example of a positive Magnus Effect.
"That was by far the best free kick ever taken," Bush says. "I think it's important to encourage people to try to understand everything. Even in the most commonplace things, there is subtle and interesting physics."
