magnus-effect

Like many sports, the gameplay in football can be strongly affected by the ball’s spin. Corner kicks and free kicks can curve in non-intuitive ways, making the job of the goalie much harder. These seemingly impossible changes in trajectory are due to airflow around the spinning ball and what’s known as the Magnus effect. In the animation above, flow is moving from right to left around a football. As the ball starts spinning, the symmetry of the flow around the ball is broken. On top, the ball is spinning toward the incoming flow, and the green dye pulls away from the surface. This is flow separation and creates a high-pressure, low-velocity area along the top of the ball. In contrast, the bottom edge of the ball pulls dye along with it, keeping flow attached to the ball for longer and creating low pressure. Just as a wing has lift due to the pressure difference on either side of the wing, the pressure imbalance on the football creates a force acting from high-to-low pressure. In this case, that is a downward force relative to the ball’s rightward motion. In a freely moving football, this force would curve its trajectory to the side. (GIF credit: SkunkBear/NPR; original video: NASA Ames; via skunkbear)

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Spinning an object in motion through a fluid produces a lift force perpendicular to the spin axis. Known as the Magnus effect, this physics is behind the non-intuitive behavior of football’s corner kick, volleyball’s spike, golf’s slice, and baseball’s curveball. The simulation above shows a curveball during flight, with pressure distributions across the ball’s surface shown with colors. Red corresponds to high pressure and blue to low pressure. Because the ball is spinning forward, pressure forces are unequal between the top and bottom of the ball, with the bottom part of the baseball experiencing lower pressure. As with a wing in flight, this pressure difference between surfaces creates a force — for the curveball, downward. (Video credit: Tetra Research)

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Almost everyone has tried skipping rocks across the surface of a pond or lake. Here Professor Tadd Truscott gives a primer on the physics of rock skipping, including some high-speed video of the impact and rebound. In a conventional side-arm-launched skip, the rock’s impact creates a cavity, whose edge the rock rides. This pitches the rock upward, creating a lifting force that launches the rock back up for another skip. Alternatively, you can launch a rock overhand with a strong backspin. The rock will go under the surface, but if there’s enough spin on it, there will be sufficient circulation to create lift that brings the rock back up. This is the same Magnus effect used in many sports to control the behavior of a ball—whether it’s a corner or free kick in soccer or a spike in volleyball or tennis. (Video credit: BYU Splash Lab/Brigham Young University)

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A remote control plane that gives up rigid wings, and instead, rotary wings that take advantage of the Magnus effect are utilized… Truly unique, and an incredible sight.

The physics of pitching a baseball

Recently, I stumbled on a story about New York Yankees pitcher Freddy García. In april of 2011, García was pitching against the Toronto Blue Jays. In the top of the 5th, Garcia threw this pitch to batter Juan Rivera. Now to the casual baseball observer, you might think this was just was an exception throw, which it was, but then move on to the rest of the game. However, to the folks who watch these events carefully, there was something unusual about this pitch. So unusual in fact, that there existed at the time no physical explanation for that observed movement of a baseball.

There are a number of forces acting on a pitch. The most basic ones we can initially think of are 1) the forward force given to the ball by the pitcher and 2) the force of gravity. The less force a pitches gives to the ball, the longer it will take the ball to arrive to the catcher and the more time gravity will have to accelerate the ball toward the ground.

Typically, to get movement on a baseball that deviates from this gravity dependant sinking, a pitcher will alter his grip to deliver the pitch with different kinds on spin it. This spin alters the way air flows around the ball as it heads toward the catcher. This 3rd force, caused by the the Magnus Effect, tells us that there will be a force on the ball perpendicular to the axis of rotation. If a pitcher throws a 4-seam fastball with enough backspin, the magnus force be greater then the force of gravity (at least over the 60 feet 6 inches between the pitchers mound and the catcher) and cause the ball to rises up on the batter. This is due to how air flows around a spinning ball. Much like how the wing of an airplane alters airflow, the spinning ball causes differences in air pressure around in, resulting in a new force component.

These three forces are all well and good for explaining most movement on a baseball, but they dont explain García’s pitch. There is only a small amount of backspin on a relatively slowly thrown ball, which explains why gravity has time to pull it down. However, the ball cuts to the pitchers left, while the Magnus effect would predict rightward motion due to its spin. So what’s going on??

Australian physicist (and apparent baseball and cricket enthusiast) Rod Cross discovered an explanation for this effect by testing polystyrene balls, which are lighter and show exaggerated movements. In his paper published in American Journal of Physics earlier this year, Rod demonstrated how the seam of a cricket ball, along with surface difference that accumulate during a game, affect the movement of the ball when bowled.

But a cricket ball has its seam straight down the middle, while a baseball has its seam in a figure eight pattern. So how are the two connected? If you watch the original video of García’s pitch closely, you’ll notice that the axis of rotation of the ball is such that the smooth face of the ball is always toward the axis of rotation (top left of ball). Most of the time this isn’t the case, and the ball rotates in a way to average out its smoothed and seamed faces. Because of that, this effect is almost never observed.

Check out the man himself explaining it:

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Physics students are often taught to ignore the effects of air on a projectile, but such effects are not always negligible. This video features several great examples of the Magnus effect, which occurs when a spinning object moves through a fluid. The Magnus force acts perpendicular to the spin axis and is generated by pressure imbalances in the fluid near the object’s surface. On one side of the spinning object, fluid is dragged with the spin, staying attached to the object for longer than if it weren’t spinning.  On the other side, however, the fluid is quickly stopped by the spin acting in the direction opposite to the fluid motion. The pressure will be higher on the side where the fluid stagnates and lower on the side where the flow stays attached, thereby generating a force acting from high-to-low, just like with lift on an airfoil. Sports players use this effect all the time: pitchers throw curveballs, volleyball and tennis players use topspin to drive a ball downward past the net, and golfers use backspin to keep a golf ball flying farther. (Video credit: Veritasium)