The Setbacks and Surprises of a Human Newton’s Cradle

Last year our chapter attempted the daunting task of creating a Newton’s cradle on a large scale—using suspended humans instead of spheres. We reached out to Highpoint Climbing and Fitness, a local rock-climbing gym, and asked to use their space as a venue since its rigging is designed to support the weight of humans. To our surprise, the gym manager was excited and agreed—as long as we signed a waiver and suspended people only about a foot off the ground.

The activity was entirely student run. Preparation and testing took place in the evenings to avoid disturbing others during busy gym hours. We made five trips to Highpoint over seven weeks to check the rigging and materials and develop a procedure. As with any physics project, we had to overcome several problems.

In our initial plan, the human spheres would wear inflatable bubble suits and standard climbing harnesses. Each person would have a rope tied to a single anchor point at the waist. However, having just that one contact point didn’t work well with the bubble suits, which ended up hanging at a tilt, with the rope placing stress on the bubble. To overcome this, we purchased full body harnesses with connection points on each shoulder. They performed much better.

Another issue: where in the gym to rig our system. Climbing-gym walls aren’t straight; they change angles to simulate varied terrain. This posed a problem since our human spheres needed to hang in a perfectly straight line. We needed two parallel walls that faced each other, but there were only two such areas in the gym, and neither was ideal. One was highly trafficked and lacked padding. The other had a height difference of about 15 feet between the walls, which meant the bubbles would swing in an elliptical path, and the walls weren’t quite parallel. Still, it was our best option. We purchased ropes to reduce the height discrepancy, and testing showed that the path of the bubbles appeared to be straight on target.

We needed a lot of volunteers to make this happen—three per bubble (one suspended and two belayers) and two all-purpose assistants on the ground to make sure everything was lined up and to initiate the collision. With five balls we needed 17 volunteers. Since it was important that they be skilled, we reached out to the Rhodes climbing club and local climbers.

The day of the event was hectic. Setup was much more involved than we had expected. We struggled to line up the balls perfectly given the not-quite-parallel walls. Eventually we lowered the volunteers and reassessed the situation. After brainstorming we tried again, focusing less on a perfect outcome and more on volunteer safety and comfort, and switching from five balls to three. We were pleasantly surprised by the results. The impact of the unaligned walls was less pronounced with a three-ball system, meaning the trajectories were more similar. Setup also took significantly less time, so volunteers were happy.

The desktop Newton’s cradle is an excellent demonstration of the conservation of momentum and energy. At this large scale we had significant—and unavoidable—energy loss in our system, but we did see cradlelike behavior for one or two cycles, demonstrating the conservation of momentum.

Curious to see what our human Newton’s cradle looked like? The Rhodes College communications team recorded the event, which you can see on our SPS chapter’s YouTube channel at https://bit.ly/3VTZCnA . We plan to use the video for future outreach events to get younger students excited about physics, as well as to help recruit new members.