The broom challenge is back! This fun little trick returns every few years on social media. It’s supposed to show a gravitational alignment (whatever that is) among the planets that allows a broom to stand up by itself. It’s super special and rare. At least that’s what I hear from Cal Tech high-energy physicists and NASA engineers (to quote My Cousin Vinny).
In my ranking of social media “challenges,” this one is better than the ice bucket challenge and the cinnamon challenge, but not as good as the invisible box challenge. And by the way, I wrote about it back in 2012. To quote myself, “All of this has happened before, and all of it will happen again.” It’s sort of silly, but hey, it’s harmless, and it makes people happy. Go ahead and try it!
Will it work tomorrow? What about in 6 months, when Earth is on the other side of the sun? (Remember it takes 12 months to make a complete orbit.) See, the broom challenge is an invitation to do some great home science experiments. Don’t just believe whatever “NASA” tells you on TikTok–challenge the challenge. That’s what science is all about.
It Works! … But Why?
What you’ll find out is that it doesn’t matter when you try this, because it has nothing to do with the planets and everything to do with some down-to-earth truths about brooms: First, the bristles flex a little bit and act like a spring. So if you get it almost, but not quite perfectly, balanced, the bent bristles will push it back toward an equilibrium point.
But the more important thing has to do with the broom’s shape and center of mass. Think about things that stand up, like a four-legged table. It will stay upright so long as its center of mass is positioned between its support points. Even a two-legged human: When you stand up from your desk, your center of mass is between your feet, so you don’t tip over.
The broom has a bunch of support points–all those bristles. If you position its center of mass within the footprint of the brush, it can remain upright. And crucially, the broom’s center of mass is low, probably just a few centimeters above the brush. Because of that, the handle can lean quite a bit without moving the center of mass very much. It’s very forgiving. Try this with a billiard cue and you’ll see why it’s not a billiard cue challenge.
OK, but what about gravity? Does it have anything to do with the broom challenge? Yes, the broom is interacting gravitationally with the Earth. But that’s the only gravitational interaction of any significance. To see why, let’s do a quick review of gravity.
Basically, there is a gravitational interaction between any objects that have mass–so, that’s just about everything. This gravitational force depends on the product of the two masses and the distance between them. We can model this interaction with the following equation: