|Popper toy, obtained by my student Mark Wu|
at the NASA store at the Smithsonian
Next week, I will be grading another experimental problem on the AP Physics exam. Since 1996, at least one question on each AP exam has been posed in a laboratory setting, asking students to design and/or analyze an experiment. This will be, I think, the twelfth experimental question I've graded over the years.
The 2016 AP Physics 1 exam problem 2 asks students to design an experiment to investigate whether a toy bounces perfectly elastically, at least for low impact speeds. Then, the problem says, the experiment seems to violate a basic physics principle. What the heck happened?
The obvious explanation is that the toy stored some sort of energy internally, through a mechanism such as a wound rubber band or a rotating flywheel. Then that internal energy was converted into mechanical energy in the collision. But how could that happen in practice?
By an utter coincidence, when I was walking through our freshman dorm on duty Sunday night I discovered one of my AP students playing with the toy pictured above. I've seen these popper toys before, but not like this one. It has a small handle, sort of like the grip of a dreidel, that is accessible once the toy is turned inside out.
Turning the toy inside out stores elastic energy. Using the handle to give the toy spin as it falls stabilizes the orientation of the toy, so that when it hits the ground, the restoration of the toy to its original shape converts elastic to mechanical energy. The toy bounces 2-3 times higher than its release height.
My student found his toy in the NASA store at the Smithsonian Institute in Washington, DC -- that's probably why there's a picture of the space shuttle on it. I found the identical toy on "branders.com", via a google search for "popper toy". The intent of this site is for you to order hundreds of these toys with a customized logo for the purposes of distribution at a sales conference or a marketing event. However, the site offers to sell you a couple of samples for $5 each. I ordered the maximum of 3 for my class.
So yet again, an AP question can be set up in the laboratory. I'll give this problem on some test or quiz next year; immediately thereafter, I'll hand out the toys and ask the students to do the experiment they designed.