Making sense of gravity, mass and weight with Galileo and my second grader
From the moment we realize we have control of our own fingers, we start learning physics. What can we squeeze? What food pours, what food falls, and what food do our parents keep putting back on our plate? At some point, our brains establish a baseline set of expectations about how the world works so that we don’t need to test it each and every time. Those so-called bits of muscle memory that enable us to catch something without thinking, or being able to estimate where a thrown ball will land, are all thanks to the many observations we’ve made about how things move here on Earth. Of course, as Galileo famously demonstrated in the 16th century, those casual observations don’t always help build the clearest picture of what really happens, even for something as mundane as two balls falling to the ground. That said, it does all make sense when you put the pieces together, as my second-grade daughter found out.
We started talking about the idea of dropping a feather and a ball, which apparently added too many variables to the thought experiment. When asked to predict which would hit the ground first, my daughter was still, muttering things like “oh, but then there’s…” and “but that…” and “that should….” As amusing as it was to watch her invent new ways to furrow her brow in contemplation, we tried moving out theoretical feather and ball to the moon, cutting the Earth’s air out of the equation. This seemed to nudge her in the right direction, and she knew that the air was going to affect the feather, but putting all the pieces together into a coherent sentence was still tough, especially as a kid who would often avoids answering rather than risk being wrong.
There is a history of dropping a feather on the Moon, but that wasn’t something Galileo was even considering. He was looking to test the idea that heavy things fall faster, which was just sort of the assumed truth in Western thought since Aristotle. A feather on Earth faces a lot of resistance from air pressure, which is what was tripping up my daughter. So we reframed the discussion to more closely match the experiment Galileo conducted at the Leaning Tower of Pisa (or rather, the Nieuwe Kerk in Delft). If two balls of the same volume, but different weights, are dropped, which hits the ground first?
Testing gravity, instead of guessing
This trips people up, because many people instinctively assume that a heavier object will fall faster. At this point, my daughter ran to the toy bin and did the test herself. The balls landed together with a single, synchronized “thunk.” Her eyes popped, but she wanted another test, and with better documentation. She grabbed our old digital camera and had me take a video of her dropping the balls, then played it back in slow motion (which I didn’t even know that camera could do.) We know Galileo was somewhat surprised when he saw this result, but we can only guess if he looked quite as elated in his moment of discovery as my second grader was.
The gears in her head were still kind of grinding on how this happened though, which is fine, as plenty of adults struggle with this too. Part of the confusion is that our expectations about heavy objects falling quickly isn’t exactly wrong. Weight is the effect of gravity on mass, and objects with more mass do have more weight, meaning more attraction to the Earth in this case. The part that trips us all up is that that mass also build inertia, and so the heavier ball actually needs more force from gravity to start moving in the first place. To put it another way, when you let go of the ball, the heavier ball resists gravity’s pull more than the lighter one. Galileo apparently added an unintentional twist to his experiment, where his hand’s grip added a small delay in the release of the heavier ball, but in the end gravity gravity still got both balls to the ground at the same time, and helped get our modern understanding of motion underway.
Source: Galileo's Experiment by John H. H. Lienhard, Engines of Our Ingenuity