Magnus effect

The Magnus effect is a physical phenomenon that results in the generation of a sidewise force on a spinning object moving through a fluid when there is relative motion between the object and the fluid. The Magnus effect is most commonly seen in sports when athletes apply spin to a ball that is thrown, kicked, or struck in order to produce a curve in the ball's flight path. This phenomenon plays an especially important role in baseball, in which pitchers use it to produce curveballs and other special pitches. Investigated by and named for nineteenth-century German scientist Heinrich Gustav (H.G.) Magnus, the Magnus effect happens when an object's spin alters the airflow around the object. This results in pressure changes to the air moving around the object that lead to changes in the object's direction of travel. In addition to the curve of baseballs, the Magnus effect can also be seen in the curve of a driven golf ball or the trajectory of a spinning cannonball.

Background

Despite eventually becoming the phenomenon's namesake, H.G. Magnus was not the first scientist to observe the Magnus effect. Renowned English physicist, mathematician, and astronomer Isaac Newton initially described the Magnus effect in 1672 after watching a Cambridge tennis match. Eighteenth-century English scientist and military researcher Benjamin Robins later correctly identified the Magnus effect as the reason for changes in musket ball trajectories. However, it was not until Magnus studied the phenomenon in the 1850s that it came to bear his name.

Born in Berlin in 1802, Magnus grew up as part of a wealthy German family. His father was the founder of a large and successful trading firm. Thanks to his family's wealth, Magnus received a private education in mathematics and natural science. His skill in these subjects eventually led him to enroll at the University of Berlin in 1822. Magnus subsequently took a strong interest in studying chemistry and published his first paper in 1825. Upon earning his doctorate two years later, he moved to Stockholm to work alongside famed Swedish chemist Jöns Jacob Berzelius. During the time he spent studying with Berzelius, Magnus discovered green salt—the first platinum-ammine compound—and potassium salt, among other things. He eventually returned to Belin in 1828 and spent the rest of his life studying in his hometown. Magnus continued to focus on chemistry through the 1830s and 1840s. Some of his most important accomplishments during this period included discovering periodic salts and studying the oxygen and carbon dioxide content of blood. By the 1850s, Magnus began turning his attention to physics.

In 1853, Magnus's quickly growing interest in physics led him to conduct a study quite similar to the one that Robins carried out around a century earlier. While Robins's subject was musket balls, Magnus focused on cannonballs and their tendency to veer off their intended flight path. During the course of his study, Magnus correctly ascertained that this phenomenon was related to the way the cannonballs spun while flying through the air. His work in this area eventually led to the phenomenon being named in his honor.

Overview

The Magnus effect is a product of the interaction between a projectile and the flow of the fluid through which it travels. The mechanics of the effect are relatively simple and easily explained when demonstrated using the example of a ball moving through the air. A spinning ball traveling through the air automatically creates a boundary layer of air that hugs the ball's surface as it moves. On one side of the ball, this boundary layer of air collides with air moving in the opposite direction. At the same time, the boundary layer of air on the other side of the ball meets with air that is moving in the same direction that the ball is spinning. The collision between the boundary layer of air and the air that is moving in the opposite direction causes the air to decelerate and results in the creation of a high-pressure area. Because there is no collision on the other side of the ball, the air on that side moves faster and creates a low-pressure area. The resulting pressure differential between the two sides of the ball subsequently gives rise to a lift force that causes the ball to change direction and curve toward the lower-pressure side. This lift force is the Magnus effect.

The Magnus effect has many practical applications, most of which are associated with sports. Athletes who play ball sports can use the Magnus effect to manipulate the direction in which the ball in play will travel when thrown, kicked, or struck. They do this by applying one of several types of spin. These include topspin, backspin, and sidespin. Topspin causes the ball to spin in the direction of travel, while backspin causes the ball to spin in the opposite direction. Sidespin, which also involves a certain amount of topspin or backspin, causes the ball to spin horizontally as well as vertically. By varying the type of spin they put on the ball, athletes can take advantage of the Magnus effect to achieve a desired result.

Some of the sports where the Magnus effect comes in to play include baseball, golf, tennis, volleyball, and soccer. Baseball pitchers vary the spin they apply on the ball to throw different types of pitches. The more spin they apply, the more the ball will curve when thrown. As a result, applying a great deal of spin will produce a curveball, while applying little to no spin will produce a much less predictable knuckleball. Soccer players take advantage of the Magnus effect to pull off curved banana kicks by applying the right amount of spin. Golfers apply backspin to extend the range of their shots or topspin to shorten that range. Applying sidespin to a golf ball can lead to a fade or a slice. Tennis and volleyball players also use topspin and backspin to alter the trajectory of their serves with the help of the Magnus effect.

Outside of sports, the Magnus effect has some military applications. Most notably, the effect was used during World War II (1939–1945) in the 1943 Dams Raid. In this unique assault on German infrastructure, British forces seeking to disable three hydroelectric dams found in the Ruhr Valley designed a special rotating bomb that would utilize the Magnus effect to skip along the water's surface before detonating. Using this unusual approach, the Dams Raid succeeded in disabling two of the three targeted German dams.

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