Why Is the Banana Flank Crooked?

Why is the banana flank crooked?

Soccer balls can move along a curved path if they are only properly cut. Behind it is the Magnus effect and a lot of physics.

With the banana flank, footballer Manni Kaltz from HSV tricked the opposing defense in the 1980s: By putting the ball into rotation by cutting it, he elegantly moved around the opposing wall towards the goal. Also in tennis, table tennis and many other ball sports players use a physical effect that makes balls fly around corners: the Magnus effect.

The Magnus effect

Named is the effect that leads to the Bananenflanke, after its discoverer, the German physicist and chemist Heinrich Gustav Magnus (1802-1870). The phenomenon always occurs when a sphere simultaneously rotates around its own axis and moves through a medium such as air or water.

The rotation around its own axis creates a pressure imbalance, which deflects the ball to the side on which the ball surface moves with the air. The pressure imbalance arises because the rough surface of the ball carries air with it. The one side of the ball rotates with the air flow, causing the air to flow faster near the ball. On the other hand, the entrained air slows down the oncoming traffic. If you now know that the pressure decreases with increasing airspeed (an effect that makes planes take off), pressure imbalance and web deflection are already clear.

What looks so simple at the ball is in reality then much more complicated. Rarely can the currents around complex objects be calculated with pencil and paper; in research and in the construction of ships and airplanes, models and high-performance computers are part of everyday life.

Ming Dao of the Massachusetts Institute of Technology at Cambridge and his colleagues bred the tiny nanotubes by growing crystals in a hot carbon atmosphere and then structuring them in a subsequent etching process. Using a scanning electron microscope, the scientists realized that the 300-nanometer and conical diamond needles were arranged side by side. To test the strength of the tiny diamond structures, the team around Dao pressed the diamond nanopiples from above with a larger diamond tip. Actually, the filigree needles should break under a pressure load of about 100 gigapascals. But instead, they flexed by a few hundred nanometers and even compressed elastically. Without pressure load, the nanopipes resumed their original shape. "We were very surprised at how much diamond can be elastically deformed on the nanoscale," says Dao.

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