swinging a swing

[miriam_e]

A child sitting on a swing, legs too short to reach the ground is, to all intents and purposes, a closed system. Every action has an equal and opposite reaction. Forces in equal forces out. But if that's so, how does that kid manage to get the swing going?

On the surface of it, it doesn't look like it should be possible. Whether you lean back or forward your mass is still simply at the end of the pendulum that is the swing.

Yet we all intuitively know how to do it. You lean back then forward at the right times. At first the movements are tiny, but gradually you add to it, using the resonant frequency of the swing, and you can end up taking the swing to the absolute peak of its travel.

So how the hell do we do it?

Comments

[annie-lyne.livejournal.com]

I'm not a physicist, but I would reason that leaning back shifts your center of gravity. That shifts the swing forward. Don't forget you also kick off with your feet as well (though I haven't ridden a swing in ages, so my memory may be dodgy.)

[miriam-e.livejournal.com]

That's why I suggested a kid whose legs are too short to reach the ground. I can remember being intrigued by this when I was little. I remember deliberately starting with a stationary swing, and getting it to move without touching anything else. (Yes, I was a geek even when I was little.) :) It seemed impossible, yet there I was, doing it. I marvelled at that. (I never liked swings -- motion sickness -- but I loved to do this baffling thing and puzzle over it.)

Yep, it is probably done by shifting your center of gravity, but how? When you lean back you are still on the end of the rope. It is like moving weights around on the end of a string. When one part sticks out one way the center stays where it was because the exact same mass sticks out the other side because you don't have anything to push against.

I have a feeling it is something to do with using the fact that the same mass object moving closer or further from the pivot point of a pendulum has different effects. When you move the swing forward you lean back and down. Then when moving back you sit up straight and high. I'm still thinking this out so I don't fully understand it yet... but I'm sure that is tied up with it somehow.

[annie-lyne.livejournal.com]

Bad habit of mine, reading too fast!

[annie-lyne.livejournal.com]

Oh, and I'm adding you by the way. You sound like an awesome and interesting person.

[miriam-e.livejournal.com]

:) Right back atcha. Gotta say I like your list of interests on your userinfo page.