Hold a buoyant sphere like a ping pong ball underwater and let it go, and you’ll find that the ball pops up out of the water. Intuitively, you would think that letting the ball go from a lower depth would make it pop up higher – after all, it has a greater distance to accelerate over, right? But it turns out that the highest jumps comes from balls that rise the shortest distance. When released at greater depths, the buoyant sphere follows a path that swerves from side to side. This oscillating path is the result of vortices being shed off the ball, first on one side and then the other. (Image and research credit: T. Truscott et al.)
Tag: ping pong
Rio 2016: Table Tennis
Many sports use spherical balls, but the small size and weight of a table tennis ball makes it the one where aerodynamics have the strongest effect. Spin also plays a big role in the game by creating asymmetry in the flow around the ball.
Consider a table tennis ball with topspin, meaning that its upper surface is rolling in the direction of travel. That means that air flowing over the top of the ball is moving in the opposite direction as the ball’s surface. This will tend to make the flow separate from the ball at its widest point.
On the other side, the ball’s surface is spinning in the same direction as the air flow. This helps hold the air to the surface so that it follows the curve of the ball longer and doesn’t detach until well after the ball’s widest point. As a result of both these effects, air flowing around the ball experiences a net upward force, which in turn pushes the table tennis ball downward. This is known as the Magnus effect, and it plays a significant role in many sports. (Image credits: GettyImages; AFP)
Previously: The Magnus effect and the reverse Magnus effect in soccer; curveballs and knuckleballs in baseball
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