For some it is very easy, for others it is a nightmare. But, after all, pulling off a hula hoop requires more than just hip action: it requires special body geometry and movement patterns that create a unique form of mechanical levitation.
The humble hoop dance may seem like a simple toy, but it turns out there is a fascinating physics behind the way this seemingly magical hoop can defy gravity while rotating around the waist.
And indeed, until now, this popular activity was understood only at the level of basic physics.
In a new study, a team from New York University’s Applied Mathematics Laboratory found that a hula hooping successful requires more than a simple hip action — requires precise body geometry and movement patterns that create a unique form of mechanical levitation.
The results were presented in an article published last week in PNAS.
“We were specifically interested in what types of movements and body shapes could hold the hula hoop successfully and what the requirements and physical restrictions involved,” explains Leif Ristrophassociate professor at NYU and senior author of the study, in a statement cited by .
Such as a helicopter needs specific movements and angless of the blades to stay in the air, a hula hoop needs particular conditions to maintain its mesmerizing orbit around the body.
Using robotic experiments and mathematical modelsresearchers found that two key factors determine whether the hula hoop stays upright or falls over: the body must have “hips” (an inclined surface) and a defined “waist” (an hourglass-shaped curve).
NYU Applied Mathematics Laboratory
Successful hula hoop requires a body type with the correct lean and curvature.
To investigate this dynamic, the team created robotic hula hoopers in miniature at the NYU Applied Mathematics Laboratory. They built their mechanical performers with one tenth of human sizeusing 3D printed bodies with various shapes – cylinders, cones and hyperboloids (hourglass shapes) – to represent different body types.
Estes little dancers they moved with engines that reproduced the movements of human hips, while they were launched 15 cm tree in diameter around it. High-speed cameras captured every wobble and rotation.
When they tried to use a simple cylinder, the rim always fell. A conical shape proved equally fruitless—albeit in a more interesting way. Depending on where they dropped the rim, this climbed the cone until it flew or slid down until it fell.
But when they tested an hourglass-shaped robota magical thing happened: the rim has found a stable sweet spot even below the narrowest point of the waist.
Surprisingly, researchers discovered that exactly of rotary motion or the fact that the cross-section of the body is circular or elliptical it didn’t matter much.
“In all cases, it was possible to make good rotational movements of the rim around the body without any special effort“, notes Ristroph. What really What mattered was having the correct combination of inclines and curves.
The new study’s findings may explain why hula hooping appears to be easy for one person and impossible for another.
“People have many different body types — some have these characteristics of inclination and curvature in the hips and waist and others do not”, observes Ristroph. “Our results may explain why some people are natural jumpers and others seem to have to try harder“.
Some findings validate what hula hoop instructors know intuitively for years. For example, beginners have better luck with larger bows, not because they are easier to see or grab, but because their larger radius helps create more stable forces. Surprisingly, the weight of the bow is not as important as its size.
Another counterintuitive finding involves the direction of rotation. While many people imagine the hoop rotating inward against the body, successful hooping actually involves a “straight out turn”in which the rim maintains contact with the inner side of the body while its center remains positioned outside the axis of rotation.
The mathematics underlying hula hoop levitation can have applications far beyond beyond playground physics.
“As we progressed in the research, we realized that the mathematics and physics involved are very subtle, and that the knowledge gained could inspire engineering innovationsharvest energy from vibrations and improve robotic motors used in industrial processing and manufacturing”, concludes Ristroph.
