The behavior of elastic spheres caught my attention due to a proposed, but not completed, experiment on ball bounciness turned in by a student from the South Dakota School of Mines and Technology.* I decided to see for myself what would happen.

To start, I went shopping for suitable elastic spheres. As pictured, I found two ball-toys with the same diameter—one of them with an eye-catching Spider-Man graphic.

My grandkids all thought that “Spidey” would bounce higher than the other ball—the one in swirly blue and yellow. Little did they know just by looking that “Swirley” was the one with superpowers, it being made from exceptionally elastic, solid synthetic rubber. Sadly, Spidey turned out to be a hollow airhead. This became immediately obvious when I dropped the two balls side by side from shoulder height. Spidey rebounded only to my knee while Swirley shot all the way back to nearly to the original drop level, which really amazed the children.

My next idea for the bouncy experiment came from Frugal Fun for Boys and Girls, a website that provides many great science projects. Their bouncy ball experiment focuses on the effect of temperature as seen here.

However, I could see one big problem straight away: How can you get an accurate measure of bounce height? That led me an amazing cell-phone app called Phyphox (Physics Phone Experiments) which provided an ingenious way to calculate how high a ball bounces by listening to them hit the floor.** Watch this short video to see how. (If you are a physicist, stay on for how the narrator of the demo, Sebastian Staacks, worked out all his calculations for the Phyphox (In)elastic tool.)

The third factor came easy: Height of drop. To make this obvious but manageable, I chose three versus six feet.

The fourth and final factor occurred to me while washing dishes. We recently purchased a thick rubber mat for easy cleanup and comfortable standing in front of our sink. I realized that this would provide a good contrast to our hardwood floors for bounce height, the softer surface being obviously inferior.

To recap, the four factors and their levels I tested were:

A. Ball type: Hollow or Solid

B. Temperature: Room vs Freezer

C. Drop height: 3 vs 6 feet

D. Floor surface: Hardwood vs Rubber

Using Design-Expert® software (DX) I then laid out a two-level, full factorial of 16 runs in random order. To be sure of temperature being stabilized, I did only one run per day, recording the time the first bounce and its height (calculated by the Phypox boffins as detailed in the videos).

When I completed the experiment and analyzed the results using DX, I was astounded to see that neither the type of ball nor the differing surfaces produced significant main effects. That made no sense based on my initial demonstrations on side-by-side bounce for the two balls on the floor versus the rubber mat.

Keeping in mind that my experiment provided a multifactor test of two other variables, perhaps you can guess what happened. I will give you a hint: Factors often interact to produce surprising results, such as time and temperature suddenly coming together to create a fire (or as I would say as a chemical engineer—an “exothermic reaction”).

Stay tuned for Part 2 of this blog on my elastic spheroid experiment to see how the factors interacted in delightful ways that, once laid out, make perfect sense to even for non-physicists.

*For background on my class and an impressive list of home experiments, see “DOE It Yourself” hits the spot for distance-learning projects.

**I credit Rhett Alain of Wired for alerting me to Phyphox via his 8/16/18 post on Three Science Experiments You Can Do With Your Phone. From there he provides a link to a prior, more detailed, post on Modeling a Bouncing Ball.