I’ve recently started taking a beginners’ hula-hooping course. I was probably as sceptical about it as you are – but actually, it’s turned out to be really fun, and fantastically good exercise.
It occurred to me the other day, though, that I don’t actually know how hula-hooping really works! So I thought I’d look it up and work it out – after all, the internet being what it is, someone will have already written about it. And I was right – I found this site!
So with some help, here’s my explanation of how it works – and please forgive my bad drawings!
Here we go, hooping away:
So what’s happening to the hoop? Well, gravity’s trying to pull it down to the floor (and, if it’s me hooping, succeeding a fair amount!):
The body’s contact with the hoop creates a normal force, N – a contact force that acts outwards from the body, stopping the hoop from moving any further inwards:
But this doesn’t explain why the hoop stays up, because there’s still nothing balancing the hoop’s weight. The normal force created by the contact between body and hoop also produces another, familiar force – friction. If a car tyre has contact with a road, the friction acts at right angles to the contact (i.e. the normal force points upwards from the road, while the friction acts in the opposite direction to the car’s movement). In the same way, the friction created by the normal force acts at right angles, i.e., upwards! So we’ve got balance:
And this is how hula-hooping works! The normal force keeps acting directly outwards, all around your waist as the hoop goes round, which keeps the hoop’s momentum up.
I have to say, I’m not sure that knowing how it works has made my hula-hooping skills improve – but it’s still interesting, and a different (and maybe more fun) way to think about weight and friction. Grab a hoop yourself and have a go!