EAA AirVenture, Oshkosh WI
Sponsored by Build-a-Plane and GAMA
“Make and Take” Project provided by EAA chapter 1158
West Bend, WI (eaa1158.org), courtesy of Paul Lupton
The Flying Cups Demonstration
- 2 Styrofoam cups, 12oz. size are fine but others are OK.
- 2 feet of common string.
- 1 rubber band about 2 inches in length.
- About 1 foot ¾ inch masking tape.
Construction: Join the bottoms of the cups with the tape. Attach the rubber band to one end of the string.
Flying Instructions: Wrap the loose end of the string around the joined area. Hold the cups around the joined area with the axis of the cups horizontal. Hold the rubber band on the top for downward flight or on the bottom for upward flight. Extend the rubber band away from you till stretched. Release the cups.
Results: If you chose the upward flight the cups will rapidly move away from you while spinning. The flight path will briefly be upward but will quickly drift downward as the forward motion decreases. The downward choice will not only be downward but may even reverse itself and partially return toward you. As the speed decays the cups remain spinning but will drift downward.
Discussion: Ah, the important part for you and your students. Take the opportunity to practice a bit before you “go on stage”. Show the simple nature of the experiment. No hidden parts, no motor, etc. Describe what you will do and ask them to predict the results. Do the demo for both flight directions and then let a few students (more if time permits) try it.
Encourage their ideas to explain the demo.
The scoop! Engineers and scientists could discuss this for hours. Keep it simple for the kids. For you, look up the Magnus Effect or the Bernoulli Effect in Wiki or a textbook. The simple explanation is that the same forces that allow a pitcher to throw a curve ball, or allow a table tennis player to put “top spin” on a ball are at work with the cups. For older students you can get into the idea of air pressure and how the speed of air flowing around an object and thus being forced to speed up will actually lower the pressure. Airplane wings use this principle by creating a curved top to the wing, compared to the bottom, thus forcing air to speed up and thus reduce the air pressure on the top of the wing allowing the “normal” pressure on the bottom of the wing to create a lifting force.
The reason the motion of the cups deteriorates into a downward drift is that the dramatic part occurs when the cups have the highest forward velocity. Aerodynamic friction, called drag, is quite effective in stealing the little bit of energy that the rubber band imparted to them. In other words if the cups were made of a heavier material such as metal, the air pressure effect would still be there but the weight (mass) of the cups would “dilute” the effect. Thus a golfer who slices or hooks a ball will observe a curved path too but it will be far less dramatic than the lightweight cups. Given the distance the ball travels, however, the ball may end up far off course.
A fun part of the experiment is to ask the students to predict what will happen if you launch the cups with the axis in a vertical position. This will cause an effect similar to the hook or slice of the golf ball.
The last interesting thing to note is that the cups drift downward in a very stable manner. This is caused by the gyroscopic forces of the rapid spin. If you have a small model gyroscope available you can show the amazing stability of the wheel when turning rapidly. Although aerodynamic drag slows the forward progress of the light weight cups the rotational speed is relatively constant because the spinning surfaces disturb much less air, than forward motion, and thus cause much less drag!
Last fun fact: Historically, scientists in the days of Newton and Bernoulli would never have had the opportunity to witness this demonstration because Styrofoam had not been invented! They would have understood the forces but would have been amazed at how dramatic the forces’ effect would be when acting upon a lightweight material.
For more info on Air Camp go to: eaa1158.org, click on Air Camp.