Users of social media have seen the video: a man twice drops a basketball off of a dam.
The first time, the ball plummets straight down the dizzying drop. The second time, he gives the ball a small twist before dropping it. The ball plummets, and then partway down makes what looks like a magical horizontal turn away from the wall, eventually landing in a lake some distance from the base of the dam. All of that from a small rotational twist? Meet the Magnus effect.
The Magnus effect, aka the Robins effect, was discovered by 18th century British mathematician Benjamin Robins while researching accuracy in gunfire. Robins wanted to know why musket and cannon balls tended to drift from their targets, and predicted that air resistance was responsible. After years of work, eventually Robins realized that air resistance deflects a spinning sphere in the same direction as the spin. A spherical body, e.g. a musket ball, has greater airflow on the side in the direction of rotation. For example, a musket ball spinning to the right has more air pushing on its right side, forcing the ball in that direction— that is the Magnus effect (Magnus was a 19th century German physicist who did a more thorough follow-up study of Robins’ work). Robins proved his hypothesis by bending musket barrels to the left, forcing the ball to spin to the right; skeptical observers then watched in shock as the musket ball deflected in the opposite direction to the curve.
Robins’ experiment solved a longstanding ballistic conundrum: why rifles were so much more accurate than smooth tube muskets. The greater spin on a rifled bullet “bores” through the air, reducing air resistance and limiting the Magnus effect. His work led to the modern aerodynamic shape of bullets.
Most of us, however, are probably familiar with a less lethal application of the Magnus effect. Baseball fans, you’ve seen it—the curveball. The Magnus effect applies to all pitched baseballs, which, like the basketball falling from the dam or musket balls, are spinning spheres subjected to air resistance. In a curveball, the pitcher adds additional downward rotation to the ball as it is released, resulting in the telltale dropping movement of the curveball. Magnus effect 1, frustrated batter 0. (If it’s the bottom of the 9th, Magnus effect 1,000,000, batter 0.) The Magnus effect plays a role in football, baseball, golf, basically any sport involving a ball. It has always been right in front of our nose, but it took the internet and some good showmanship to make us notice.