In his Phys-L post "Re: curvature of buckets of water" of 7/4/99, Bob Sciamanda writes:
"Of course the helium balloon tethered to the floor of an accelerating car (contrasted with a plumb line hanging from the ceiling) is a standard puzzler to put to an intro class; many will not believe the predicted results until they actually see one of the above demos. As already noted, best advantage is taken of these demos by analyzing them both with and without the Equivalence Principle."
The non-equivalence-principle analysis (NEPA) of the He balloon tethered to the floor of an accelerating car (or airplane) may be too obvious to bear comment, but it's something I've never seen in the textbooks. It may suffice to say that the NEPA is analogous to the standard elegant derivation of Archimedes Principle using Newton's Laws.(1)
I used to joke with students that I often made money on airline flights by tethering a He balloon to a seatback and then betting fellow passengers on which way the balloon would move when the plane moved down the runway for takeoff. Of course, most of them would bet that the balloon would move backward, so I collected on the takeoff bets. Then when the plane started to come in for a landing I would make the same bet, the passengers now thinking that, as on takeoff, the balloon would mysteriously move forward. So I would collect again on the landing bets.
In his post, Bob Sciamanda writes:
"I have demo'd this effect with candle flames (covered to eliminate wind effects), helium balloons and floating bobs tethered to the bottom of a bottle of water, all (in turn) fastened to the ends of a horizontal 2x4 spun slowly by hand about a vertical axis through its center."
In a lab one can also do this experiment with the bobs or balloons tethered to the bottoms of bottles of water at the ends of a 12-ft plank merry-go-round.(2,3) The same apparatus can be used for students to enjoy the fun and analyze the physics of playing catch on a merry-go-round,(2,3) and to demonstrate the "bathtub vortex"(4)in the northern and southern hemispheres.
One year one of my students gunned her car down a Bloomington Indiana street at 6 am while watching a He balloon tethered to the car floor. Unfortunately, she was apprehended by a local policeman. When she told him she was doing a physics experiment, he was at first incredulous, but her scholarly sincerity finally won him over and he merely gave her a warning to stop doing physics experiments on city streets. That same evening the student was contemplating some helium balloons tethered to strings in a department-store window (probably considering the force vectors acting on the balloons) when the same policeman walked over to her and said "I knew I'd find you here!"
Richard Hake, Emeritus Professor of Physics, Indiana University
24245 Hatteras Street, Woodland Hills, CA 91367
<rrhake@earthlink.net> PLEASE NOTE NEW ADDRESS! <http://carini.physics.indiana.edu/SDI/phys-ed-new.html>
REFERENCES
1. See, e.g., D. Halliday and R. Resnick "Fundamentals of Physics" (John Wiley, 1988, third edition), pp. 371-372.
2. L. Evans, "The Coriolis Effect and other Spin-Off Demonstrations," The Physics Teacher 20, 102 (1982).
3. R.R. Hake, Socratic Dialogue Inducing Lab #3 "Circular Motion and Frictional Forces: Lab Manual at http://carini.physics.indiana.edu/SDI/; Teacher's Guide (password protected) at the Harvard Galileo site http://galileo.harvard.edu/ under Hands-On Methods/SDI Labs/Resources/Teacher's Guides.
4. R.R. Hake, "More on Coriolis myths and draining bathtubs," Am. J. Phys. 62(12), 1063 (1994).