Curveball Demonstrator

1. Make the flier by taping the two cups together, base to base, as shown in the photo below.
   
2. Make the launcher by interlocking two to four rubber bands together, attaching them in a row (see photos below). The unstretched assembly should be about 12 inches (30cm) long. If available, an extra-long single rubber band can also be used.
   
3. Put on your safety goggles, and get ready to fly.

To prepare for launch, you’ll be wrapping your rubber band assembly around the flier in a special way.

Gripping your flier at the taped center, use your thumb to pin down one end of the rubber band and start wrapping it around the center of the cups (see photo below).

Stretch your rubber band tightly as you wrap it. Make sure to overlap each wrap so the rubber band stays in place. Keep stretching as you wrap the rubber band around the flier (see photo below).

When you get almost all of the way to the end, pinch the free end of the rubber band between your thumb and index finger. Then, with the rubber band coming off the underside of the flier, pull the flier back with your other hand (see photo below).

Pull tight—as tight as you think the rubber band and cup can stand.

Aim, let go, and watch it fly!

Weird, right? Your flying cups curved upward. This uplifting bit of lift occurred because you put a spin on the cups as they were launched.

During launch, the rubber band not only propels the cups forward, but also gives them a rapid backspin. As the backward-spinning cups fly through the air, a thin layer of air—called the boundary layer—gets dragged along for the ride and flung downward by the backspin. In accordance with Newton’s Third Law of Motion, every action has an equal and opposite reaction: The downward force on the air results in an upward force on the cups.

This trick, known as the Magnus effect*, is well known to baseball pitchers. By varying the direction and rate of spin of a pitched ball, a pitcher can create a variety of wildly veering pitches: curveballs, fastballs, screwballs, and sliders. The stitches on a baseball add to the trickery; not just decorative, they assist in deflecting air sideways, increasing the deflecting effect of the spin. 

Soccer players, too, make use of the Magnus effect to launch balls that seem to have a mind of their own, diving down or swerving sideways into the net.

* Note: Technically, on a cylinder, the Magnus effect is more properly called Kutta-Joukowski lift. But go ahead and call it the Magnus effect.

You can also use this effect to make your flier bend in different directions. For example, turn your flier upright and try launching it sideways: Can you make it curve right or left?

Try other types and sizes of cups. Will larger cups work better? Will cups with different surface textures work the same?

Try adding a design or markings on the cups to better see your flier’s motion. Or use a camera with a high frame rate to slow the action and better see the path and spin of the cups.

This activity lends itself naturally to competition and challenges: Who can make their flier curve the most? Fly the farthest? Stay aloft the longest?

Investigate other examples of the Magnus effect in sports or engineering.