How does a hula hoop stay up against gravity?
As you gyrate your hips, why does a hula hoop rise and stay suspended around your waist instead of falling to the ground, and what shape of body is required for this to work at all?
History and references
Settled in 2024 by Zhu, Pomerenk & Ristroph (NYU).
Kapitza's problem #47 asks what motions are needed to keep a hoop rotating around the body. The everyday toy physics went essentially unanalyzed mathematically until Leif Ristroph's group at NYU combined robotic hoop-twirling experiments with a contact-force model, showing hula-hoop levitation is a special case of a general class of rolling-contact levitation that requires a body with 'hips' (a waist whose curvature exceeds a critical value) to generate the right time-varying normal forces. A companion 2025 mathematical paper further formalized the shape criterion for stability.
A children's toy turns out to be a working demonstration of a previously unrecognized general mechanism for defying gravity using only rolling contact forces and body shape.
Explanations that circulated
- The hoop is simply 'thrown' upward by the hips and coasts on momentum alone (ignores the continuous rolling-contact mechanism that actively pumps the hoop upward every cycle)
- Any body shape can hula-hoop equally well if spun fast enough (the 2024 work shows a critical hip/waist curvature is required for stable levitation, not just speed)
References
- Geometrically modulated contact forces enable hula hoop levitation, Xintong Zhu, Olivia Pomerenk, Leif Ristroph, PNAS, 2024
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