What Makes A Hula Hoop Stay Up?

And to close the hour, as you know, we strive to bring you the most important science news stories of the week, the stories you absolutely must be looped in on. And there’s one more we just couldn’t drop. A new paper that clears up a long-standing mystery about the hula hoop, specifically, why doesn’t it fall down? Researchers at NYU looked into the head-spinning physics and math of hooping, and as a bonus, they investigated what body types make for the best hoopers.

I’m not going to leave you hanging. Here to tell us more is my next guest, Olivia Pomeranc. She’s a PhD candidate in mathematics at New York University and an author on this new paper. Welcome to Science Friday, Olivia.

OLIVIA POMERANC: Thank you so much for having me. It’s great to be here.

SPEAKER 1: Why hula hoops?

OLIVIA POMERANC: So there’s actually evidence of hula-hooping in the human historical record as far back as 500 BCE. It shows up for the first time in ancient Greece.

SPEAKER 1: Wow.

OLIVIA POMERANC: Yeah. So it shows up again and again throughout history, independently, in a myriad of different cultures. It’s used as a form of recreation, religious ceremony, even exercise. And even today, artists and performers can do some really impressive stuff with hula hoops. And so, we were actually just inspired by just seeing performers in Washington Square Park right outside NYU. And we figured oh, hula hoops must have been studied to death at this point.

SPEAKER 1: That was your assumption?

OLIVIA POMERANC: Yeah, that was the assumption because it’s been around for millennia. But it actually hasn’t been. We dug into the literature just driven by curiosity, and we found that hula hooping isn’t, or wasn’t, I guess, even understood at a basic physics level. And so previous studies have looked at the problem from a top down view, like in two dimensions, and studied the twirling dynamics. But nobody’s answered the really obvious glaring question, which is, how does the hoop even stay up? And that’s the question that we set out to answer.

SPEAKER 1: That does seem like the fundamental mystery of hula-hooping. Why doesn’t it fall?

OLIVIA POMERANC: Exactly.

SPEAKER 1: OK, tell me how you figured it out. I hear there are robots involved.

OLIVIA POMERANC: Yes, there are robots involved. So it seems like all of the forces exerted during hula-hooping should be directed horizontally. It’s not immediately clear what’s counteracting the gravitational force that’s pointing down. So what’s keeping it up? And so we guessed that the shape of the body executing the hula-hooping may be important. And so we 3D-printed a bunch of different body types in order to test, maybe about the size of your hand.

And we made a cylinder, a cone, so pointy part facing upwards, and then we also made basically an hourglass-shaped body. And so we basically gyrated each of these. So that means translating in a circle without rotating it, drove that robotically, and just by hand, tossed plastic hoops around each of the bodies.

SPEAKER 1: Tiny little baby hula hoops.

OLIVIA POMERANC: Tiny little baby hula hoops, exactly. So it was definitely– it’s a funny-looking experimental setup.

SPEAKER 1: OK, so it seems like there’s two questions. There’s like, what’s the best body shape to keep a hula hoop spinning? That’s one. And what is the answer?

OLIVIA POMERANC: So the answer is basically that you need to have a sufficiently sloped and sufficiently curved body. So translating that to humans basically means that you need hips, which provide slope, as well as a waist, which provides curvature. And there’s a sweet spot in there where the hula hoop will naturally want to sit just below the narrowest part of your waist. Then the hoop is sort of resting almost on a tiny little sort of surface.

SPEAKER 1: A ledge, your hip ledge.

OLIVIA POMERANC: Exactly. And so there’s a tiny little force pointing upwards if you have slope.

SPEAKER 1: So that’s it. That’s what’s counteracting gravity. Your hips.

OLIVIA POMERANC: Exactly. Exactly. Your hips. And then the reason you also need a waist and hips aren’t enough is that it’s not enough to just counteract gravity. You also need the hoop to be trapped in place. And so if you have a waist and you have a curved profile, then you actually get the hoop to be trapped in place. So if it goes up a tiny bit, it’ll be pushed down. And if it goes down a tiny bit, it’ll be pushed up.

SPEAKER 1: I love that there’s a physics answer to this. When you defend your dissertation, will you be hula-hooping the whole time?

OLIVIA POMERANC: There is a chance that I bring one along as a prop. Not the whole time. Definitely not the whole time. I don’t know if I have the hula hoops prowess, necessarily, to do that.

SPEAKER 1: Olivia, thank you so much for closing the loop on this mystery.

OLIVIA POMERANC: Yeah. Thank you so much for having me. It’s always fun to talk about this kind of stuff.

SPEAKER 1: Olivia Pomeranc is a PhD candidate in mathematics at New York University.

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