Loren Pechtel

We were at a small circus last night. There was this girl (probably early teens) who did a couple of things I can't understand with hula hoops.

First, she had some hoops on the ground that she started spinning and then they quickly zoomed up her body to where she wanted them. I saw how she started them, no problem. What I don't understand is how she made them rise above her ankles. Where does the lifting force come from???

Second, her final trick was to take something like 20 hoops and start them spinning around her entire torso. They were stacked vertically and there was little if any contact betwen them--you could see her clearly through them so there must have been a reasonable amount of space between them. It looked like she was surrounded by a giant spring. Again, where's the lifting force coming from to keep them separate??


Incidently, she had one good one that got quite a reaction from the whole audience. She had her hair done up in a small ponytail on the top of her head. At one point she was spinning a hoop on the base of the ponytail.

theyeti

I think it's from tilting the leg forward and backward. Imagine that a hoop in motion is perfectly perpendicular to your leg, and is touching your leg at the front, meaning that all the extra space in the hoop is protruding behind you. Now you tilt your leg slightly back. As the hoop spins around, it will contact your leg at a higher point. Once the hoop has rotated so that it touches your leg in the back (with the empty space in front of you), do the opposite and tilt your leg forward. As the hoop continues around, it will "climb" your leg because you keep presenting it with a higher point each time it rotates. The energy comes from the back and forth motion of your leg.

You can do something similar with a pencil and a ring of some sort. Just play with it and you'll quicly find a way to get the ring to rise up the pencil until it shoots off the top.

Doing it with a hula hoop though is not easy. A skilled hoopist (is that what they're called?) will make it look effortless, with perfectly timed leg motions. I always sucked at those things myself. I couldn't even keep one from falling, much less get it to climb.

theyeti

Loren Pechtel

I don't see how this is enough to explain the ones she lifted from the floor--one went from the floor to her chest in 3 seconds at most.

It does explain the huge batch of them she had around her. There was a clear rhythm to them that she could have matched her body to.

perfessor

I have no explanation - just applause.

There are physical skills that people have, and they make it look so easy. How does a 150 lb. girl hit a golf ball 300 yds with such an effortless swing?

When I was coaching my daughter's soccer team, the league brought in some pros from England. So this one guy was talking to the girls about this and that, and he casually turned and kicked a nearby ball - it looked just like a casual flick of the leg. Except that the ball went flying farther than I could imagine. We were all agape. How did he do that??? I think that the eye does not always see the crucial motions - or the eye is expecting big motions for big results, and so gets fooled.

Loren Pechtel

The ones that really impress me are the ones whose bodies seem to be made of rubber.

Aria

Ironic, because contortion takes far less talent than keeping all those hoops going.


Anyway, the lift comes from slanting the body and letting the hoop ride up. The same force making the hoop spin can be used to make it climb.

liquid

Coordination modes in the multisegmental dynamics of hula hooping
Ramesh Balasubramaniam, M.T. Turvey
Biol. Cybern. 90, 176–190 (2004)

http://aix1.uottawa.ca/~rbalasub/hulahoop.pdf

It's a paper on the control systems issues of hula hooping, but it does contain a description of the dynamics and the relevant references.

It's pretty much as I suspected - there's a vertical component to all the inputs... imagine a ball on a flat surface, and rapidly sliding a wedge underneath it, for those who can't visualise how such a component is created by horizontal motions.

As far as keeping separated hoops spinning, she is probably just skilful enough to maintain the dynamics for each hoop individually. It would be made slightly easier if all the hoops had the same properties, as then all the body rotations would be at the same frequency.

And in the case of the ankles... one quick small motion of kicking/flicking the hoop stack could lift it, people do that all the time with objects (although not so subtly!). The other option would be some kind of trickery, which wouldn't be inconceivable for a circus.

OdysseusTheInnkeeper

Was she naked?

Loren Pechtel

Oh, come on now. I posted that from *CHINA*.