Around 1983, I was playing backyard baseball with my friend Kevin. We used a tennis ball and an aluminum bat and a backstop. I threw a pitch with spin and the ball started out looking low and outside. My friend ignored it but the ball rose and curved into the middle of the strike zone. I was intrigued by the swerve I generated and excited about what would happen if I threw with the same spin but adjusted my aim. So on my next pitch, I used the same release on the ball but this time I aimed at the center of the strike zone. The ball appeared to Kevin like a fat easy target. He swung hard and missed because the ball that looked like it was going to hit the center of the strike zone went up and in at the last moment and hit him in the eye.
He wasn’t happy with me. I’m pretty sure his face and ego hurt for a bit, but he was going to be fine. It was only a tennis ball. That was the end of our play that day but it was worth it. What an amazing pitch I conjured up! I can’t believe he swung at a pitch that hit him in the eye! I created a curve that broke in about two feet and up about two feet.
What do you think happened to the tennis ball that made it swerve like that?
Gemini says:
What you experienced was the Magnus effect, magnified significantly by the unique physics of a tennis ball.
When you threw that pitch, you accidentally or intentionally imparted heavy backspin on the ball. As a ball spins with backspin, the top surface moves in the same direction as the oncoming air, while the bottom surface moves against the airflow. This difference creates high velocity and low pressure on top, and low velocity and high pressure underneath. The resulting pressure imbalance generates an upward lift force.
With a standard baseball, backspin simply fights gravity, causing a fastball to sink slower than a batter expects—what players call a rising fastball. A baseball does not actually defy gravity; it just drops less than a standard projectile path.
A tennis ball changes the math completely. A tennis ball is much lighter than a baseball, weighing roughly two ounces compared to a baseball’s five ounces. It also possesses a fuzzy felt surface that creates massive aerodynamic drag and grips the air far more effectively than smooth leather and raised seams.
Because the ball had very little mass but generated tremendous lift from the fuzz and the rapid backspin, the upward force actually exceeded the downward pull of gravity. The ball gained net lift and curved upward in flight.
On your first pitch, the ball started low and outside. The backspin caused it to climb up into the strike zone. On your second pitch, you aimed for the center of the zone with the same release point and spin. Because the ball started higher and closer to his body, the upward curve carried it straight up and in, tracking toward his face.
Your friend swung because his brain calculated a normal trajectory based on the initial release and velocity. By the time the Magnus force broke the ball upward and inward, his swing was already committed, and the light ball drifted right into the path of his eye.
I once got a friend out in cricket with a similar technique. I used a Googly on my third delivery, the ball spun in the opposite direction from what I had previously bowled, and I smashed through his wickets.
