A small (pea-sized) ball falls from a height of about 30 cm onto a slightly concave steel surface and bounces hundreds of times. The hypnotic ball bouncing is phenomenal and fascinating.
The Dropping Ball is one of the exhibits from Exploratorium, the first science centre in the world. When designing it, we used the so-called Exploratorium Cook Book, i.e. a book containing general recipes without any details. We built it to meet our expectations. While designing the exhibit, we were looking for a solution to make the ball bounce as long as possible. We wanted everyone to be able to see how the height and frequency of the bounces change and discern the sounds that accompany the bouncing and its volume. We started our work by looking for the manufacturers of the hardest steel. We chose a specific type of hardened steel which was very strong. We also tested balls of different sizes and materials. We developed hypotheses about which ball would keep bouncing for the longest time. As a result, the ball loses only little energy and little height each time it bounces off the board.
We performed measurements by looking and listening as well as applying more advanced measuring methods. We wanted to find out how much energy our ball would lose in percentage terms on each next bounce. We wondered how much time it takes for the ball to reach half the height. We recorded the ball using a high-speed camera and wrote an automatic ball positioning algorithm. We found out that the ball lost about 5% of its height during its first few bounces and less and less with each subsequent bounce. After it has bounced approx. 100 times, it will lose about 2-3% with each bounce. The measuring data shows that the ball loses half of its height after every 7 seconds of bouncing. This means that it is at height of 34 cm at the beginning, at a height of 17 cm after 7 seconds, at a height of 8.5 cm after another 7 seconds, at a height of 4.2 cm after another 7 seconds and so on. Isn’t that fascinating?