It’s the middle of summer, which means it’s baseball season! For you sports fans, it’s always a fun time to relax on a warm night and catch your favorite team in action. As you’ve watched a game, have you ever wondered to yourself how pitchers can make baseballs move and dance the way they do?

The stitches on the baseball, as well as how the pitcher holds that ball and uses those stitches, account for a lot of the movement–but there’s quite a bit of science behind why fastballs rise and sink, curveballs curve, sliders slide, and knuckleballs…umm…knuckle.

Keep reading to learn about the science and how to re-create the curving phenomenon yourself with just a few basic household supplies, courtesy of Exploratorium! [Note: this experiment won’t involve a baseball being thrown indoors…in fact, it won’t involve a baseball at all, so don’t worry!]

What is the Magnus effect?

Back in 1852, a German physicist named Gustav Magnus was attempting to understand why spinning artillery shells sometimes curved in strange, unpredictable ways. By studying this, he discovered that a sphere or cylinder spinning in a moving airstream develops a force at a right angle to the direction of the moving air and curves away from its principal flight path.

Got all that? That’s the Magnus effect.

So, how does this relate to sports? The Magnus effect is why pitchers can throw breaking balls and why soccer players can bend a soccer ball into the net around a wall of people. The spinning object in motion (in this case, a ball) exerts a net force on the air which, according to Newton’s Third Law of Motion, exerts an equal and opposite force back on the moving/spinning object (the ball)–this alters the object’s trajectory. And that would be the “curve” or the “break” of the ball that’s being thrown, kicked, or struck.

Let’s look at curveball dynamics before you start experimenting. If you’re familiar with baseball, you’ve probably heard the term “12-to 6-curveball”. The numbers represent the face of a clock (12 at the top, 6 at the bottom) and refer to the movement of the curveball. This type of curveball, when thrown properly, creates a high-pressure zone of air on top of the ball as it makes its way to home plate. This high-pressure zone helps deflect the ball downward in flight.

Per Diamond Kinetics (a very cool sports science/technology website), this explains why, “instead of counteracting gravity, the curveball actually adds additional downward force, thereby giving the ball an exaggerated drop in flight. The unsteady top-to-bottom pressure difference on the ball aids gravity in forcing the ball toward the ground. The injected particles follow the instantaneous flow velocity and thus trace out the unsteady flow pattern behind the rotating baseball.”

A fastball, on the other hand, is thrown through the air with backspin, which creates a higher pressure zone of air ahead of, and under, the baseball–which is the opposite of the curveball and explains why it doesn’t drop like a curveball does. The upward Magnus effect opposes gravity and keeps the ball in the air longer.

Now let’s see the Magnus effect in action!

The Curveball Demonstrator: What You Need & How to Build It

To create this experiment, all you need are a few things that are probably already in your house!

  • Two disposable cups
  • A bag of rubber bands
  • Masking tape
  • Safety goggles
  1. Tape the two cups together at their bases.
  1. Create the launcher by interlocking two to four rubber bands, attaching them in a row. The upstretched rubber bands should be about 12 inches long. You can also use an extra-long single rubber band.
  1. Put on your safety goggles!
  1. Prepare to launch by gripping your flier at the taped center and using your thumb to pin down one end of the rubber band; then start wrapping it around the center of the cups.
  1. Stretch the rubber band tight as you wrap it, and make sure to overlap each wrap so that the rubber band stays in place. Keep stretching as you wrap the rubber band around the flier.
  1. When you’re almost at the end, pinch the free end of the rubber band between your thumb and index finger. With the rubber band coming off of the under-side of the flier, pull the flier back with your other hand.
  1. Pull it as tight as you think the rubber band and cup will allow you to pull. Aim and let go!

What Happened?

Did your cups fly in an upward direction? If so, you performed the experiment correctly! Do you know why they flew up? Because of the spin you put on the cups as you launched them. Like the fastball we talked about earlier, you created backspin on the cups as they propelled forward.

When the cups were spinning backward through the air, a thin layer of air called a boundary layer was created and sent downward by the backspin. Going back to Newton’s Third Law of Motion (every action has an equal and opposite reaction), the downward force on the air created an upward force on the cups.

You can try to get more of a curveball action on the flier by launching it the opposite way, or go even further and see what happens if you launch it sideways. Play around with it and record the results to see how different positions affect the flight of the cups.

You just witnessed the Magnus effect in action! And now you know how pitchers get their pitches to move all over the place–by varying the spin rate and direction of the baseball, the pitcher can achieve different movements with their curveballs, fastballs, sliders, and other breaking pitches. The stitches on the ball also help deflect the air sideways, which helps increase the spin effect even more!

So, next time you’re watching the Dodgers’ Clayton Kershaw drop his 12-to-6 curveball on some helpless batter or you see the Mets’ Jacob deGrom make hitters look silly with his nasty slider, you’ll understand why the ball moves the way it does!