The atmosphere was abuzz with a kind of excitement and tension not typically associated with a physics classroom. Of course, technically, we were not in a classroom —we were wheeling a trolley full of sports balls into the terrace of the tallest building on the University campus.
But for all purposes that day, the terrace was a laboratory. While one professor scoured the floor to identify the best and safest site for the experiment, another was busy in the opposite building setting up his 1000-frames-per-second (FPS) slow-motion camera.
In the meantime, a small group of students milled about assisting them, fiddling with the balls and musing excitedly about which ball they thought would bounce the highest. After about twenty minutes of preparation and inter-building communication, it was time.
Legend has it that sometime around the year 1590, Italian polymath Galileo Galilei ascended the stairs of the Leaning Tower of Pisa to conduct a simple, but critically important, experiment.
From one of the tower’s balconies, he is said to have dropped two spherical objects of varying masses, going on to prove that all objects fall with the same acceleration irrespective of mass — one of the most fundamental discoveries of physics.
Just like in the case of Archimedes in his bathtub, there is no way to know for sure if Galileo actually performed this experiment and achieved his Eureka moment so simply. Science tells us that the best way to verify the authenticity of a claim is by reproducing the experiment. Students and teachers of physics at Azim Premji University had decided to spend one warm summer morning doing precisely this.
“It all started when we were hanging out in this corridor one afternoon,” recollected Anish Mokashi, an experimental physicist who teaches at the University. “One of us was playing with this rubber ball (sometimes called ‘crazy ball’). We noticed that if we bounce it from a certain height, it bounces to about two-thirds of the [original] height. That’s when we started to wonder — what would happen if we bounced it from the 15th floor of the GR building (the tallest building on campus at the time)? Would it bounce back up to the 10th floor?”
Anish and his students began to plan their own interpretation of Galileo’s famous experiment. They reconvened on the same corridor on the morning of 25 May 2022, along with fellow group members Rema Krishnaswamy and Murthy OVSN, to discuss how they would go about the experiment and what they expected would happen. And most intriguingly, what physics questions could emerge from the fairly simple act of dropping balls from a height.
Calculations based on the coefficient of restitution suggest that the balls should bounce to around two-thirds of the original height, but the group was fairly confident that the balls would not reach all the way to the 10th floor. There are simply too many external factors that would slow it down — the wind or the softness of the ground, for example. There was even the very real-seeming possibility that some of the balls might break!
Nikhil Tiwari, a student, described the rough idea of the plan: A variety of balls, differing not just in mass, but in a variety of physical properties, would be dropped from the top floor of the building. This would include a small bouncy rubber ball, a cricket ball, a volleyball, a basketball, and a football. Teams of people would observe the behaviour of each from multiple locations — the terrace, the ground, and through a slow-motion camera from the opposite building.
The logistics, of course, were not simple, but the group persisted, and eventually, the experiment was executed. Not everything went according to plan, but if the goal was an exploration of physics, then the morning was a success. As it happened, none of the balls came even close to the 10th floor. The one to bounce the highest wasn’t the rubber ball as most expected, but the basketball! Needless to say, the experiment left everyone with enough physics fodder to chew on for weeks at least.
One of the major takeaways for the group was that when it comes to experiments like these, there’s no such thing as being over-prepared. Murthy, who had the daunting task of trying to capture the fall of each ball on his camera, found it incredibly challenging to ensure the ball stayed in the frame. “The ball accelerates so the tilt of the camera has to match the angular acceleration to have the ball in the frame. I couldn’t do it. Maybe with some more practice, we could get there,” he said.
Notwithstanding the hiccups along the way, the participants of the experiment, especially the students, cherished their experience that morning. “I am happy it was not just a conversation. We took things ahead,” reflected Siddharth Gaikwad, a student, during a post-experiment discussion that day. Siddharth was especially curious to understand why his and most of the other’s predictions turned out wrong.
Interestingly, none of the balls bounced higher than the fourth floor. “Clearly there is something else going on here… some interesting science that needs to be explored,” reflected Anish.
Fellow physics faculty member Rema Krishnaswamy emphasised that the story doesn’t end here, “Now is the hard part of really analysing the experiment, trying to figure out what made it bounce to the second or third floor. And that can be equally exciting!”
Watch the team in action
- Murthy, OVSM, Mokashi, A., & Vyasanakere, J. (2022, October 26). Ball-drop from a tall building (à la Galileo). Hands-On Physics. https://physicsapu.in/2022/10/26/ball-drop-from-a-tall-building-a-la-galileo/
- Crease, R.P. (2003, February 4). The legend of the leaning tower. Physics World. https://physicsworld.com/a/the-legend-of-the-leaning-tower/
We have used natural height units in this problem (like the floor number).
About the Author
Nandita Jayaraj is a Science writer and Communications Consultant at Azim Premji University.