Extruded Plastic Dingus

(Title from a favorite movie of mine, The Hudsucker Proxy)

While I was visiting my brother Nate, we went to the Baltimore Farmers' Market, where a local performance group had brought a large selection of hula hoops.  He felt the need to show up our mother, who had also come across a group lending out hula hoops back here in Northampton.  I've never been any good with them, but I thought I'd explain a bit about how they work.  I had a basic understanding, but I figured I should try to read up a bit on the details.  Since I'm between schools, I no longer have access to the pricey journal databases, but I did manage to find a paper on the free physics article site, arXiv.  As it turns out, the dynamics involved are rather complex, so I thought I'd try to give a basic explanation and highlight some interesting aspects.

The article presents the situation with this diagram:

The large gray circle is the hula hoop, and the smaller filled circle is your waist.  The dotted ellipse represents the hip movements that keep the hoop going.  The important factors that determine the movement of the hula hoop are: the friction between you and the hoop, the shape that you move your hips in, the distance that you move your hips, and the size of you and the hoop.  Rather than get into the details, I'll just summarize some of the article's more interesting conclusions:

• More friction or a large hoop requires you to move your hips more
• Low friction allows you to twirl the hoop in the opposite direction that you swing your hips
• Theoretically, there are unstable solutions to the hoop's movement, meaning if you were able to get everything perfectly lined up, it would work, but in reality, you'd never manage it

I'm not sure any of this has helped my ability to actually hula hoop, but at least I know how it works now, and to a theorist, that's all that really matters.  As always, you're welcome to ask for more details about anything that piques your curiosity in the comments.