Butterflying Cups

This experiment is one of my favorites to use when teaching university-level fluid mechanics, because it is quite a complex task to demonstrate and analyze the aerodynamic lift. The easiest explanation is that lift is generated by the rotation of the cups. How and why the vortex generates lift is much more complex, but remember that as the air velocity increases, the pressure decreases. And remember... higher pressure regions always push.

[am4show have='p8;p9;p93;' guest_error='Guest error message' user_error='User error message' ]
Materials: Two paper cups, 5 to 7 rubber bands, and tape.

Here's what you do:
1. Tape two paper cups together, bottom-to-bottom.
2. Chain together six rubber bands.
3. Loop one end of the rubber band chain over your thumb and hold your arm out horizontally straight, palm up. Drape the remainder of the chain along your arm.
4. Place the taped cups at the free end of the rubber band chain near your shoulder and slowly wind the rubber bands around the middle section of the cups. When you wind near the end, stop, stretch the chain back toward your elbow, make sure the rubber band comes from the underside of the cups.
5. Now release the cups. The cups should rotate quickly and take air, then gracefully descend down for a light landing.

You can try further experiments with your butterfly cups: Try reversing the rotation direction, spin the cups in a cloud of fog or smoke while your video-tape their flight, performing the same experiment underwater (add small particles to the water so you can see the lines of flow!), change the size of the cups, and change the number of cups.

[/am4show]