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What makes a swing swing?

eowyn

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eowyn
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Many years ago, I was a bit miffed with a lecturer because my group did not get bonus marks for what i considered to be a more accurate answer than any other group had come up with.

I can't remember the question exactly, but it was about what makes a swing swing - ie a childrens' playground swing.

A number of study groups went and did some experimentation (We did ours with a couple of beers at around 1am in a local playground.)

The consensus amongst the lecturer and other groups was convservation of vertical momentum - ie the person on the swing raises and lowers their centre of gravity at the very top part of the swing.

Our group concluded that it was conservation of angular momentum - as you are swinging forward you rotate your body around the seat, kicking your legs forward, and your body back.

Was our answer more correct?
 
There is an experiment that you can do to confirm if your theory has merit. See if you can get a swing going without any impulse other than the `kicking and swinging back' motion that you described. If you can then the angular momentum argument must be at least part of the solution since from a standing start a purely vertical momentum mechanism would produce no horizontal motion.

I suspect that as with all things in physics the real world answer is more complex than the theory and that there are components of both angular and vertical momentum conservation involved.
 
eowyn said:

The consensus amongst the lecturer and other groups was convservation of vertical momentum - ie the person on the swing raises and lowers their centre of gravity at the very top part of the swing.

Our group concluded that it was conservation of angular momentum - as you are swinging forward you rotate your body around the seat, kicking your legs forward, and your body back.

Was our answer more correct?
Click to expand...

First off, you're taking advantage of angular momentum, so you're right about that, but the ideal "swing" is different from either description. Ideally, what happens is that at the bottom of the swing, you lift your center of mass up. Conservation of angular momentum means that at the shorter radius, you should swing higher. When you're at the top of the swing, you drop your center of gravity back "down", but since you're at an angle, you're actually not lowering yourself as far. Now on the swing back down, you're traveling farther downward than you did on your swing up, and you pick up speed. So the sequence is raise your center of mass at the bottom of the swing, lower your center of mass at the top of the swing. You generally do that by leaning back at the top of the swing and sitting up at the bottom of the swing, the kicking the legs out is mostly for balance. You can test this idea by standing on the swing instead of sitting on it, then all you do is stand up or squat a bit, it's a little harder to get started from completely stationary but you can really get going fast. You'll find that the best method really is to lower your center of mass at the top of the swing and raise it at the bottom of the swing.
 
The way I've always envisioned it is this. Any pedulum has a intrinsic frequency, based solely on the length of the arm and the weight at the end (this is why pendulum clocks work). You just need to kick your legs (the driving force) at the right frequency, and you'll get a resonance behavior in the pendulum (swinging higher). Take a look at the limiting cases of the driven damped simple harmonic oscillator and you'll see what I mean.

Or you can just do an experiment for yourself, swing your legs alot faster or alot slower than you think you should, and you get very small response. This is because you're not near the resonance frequency.
 
I remember being puzzled about this for a long time.

If you're sitting on a horizontal frictionless surface, you can't get anywhere by kicking your legs or anything like that. If you suddenly push your legs forward, that will push the rest of your body backward, but you'll only move backward for a very short time, because as soon as your legs slow down, they will pull your body forward again. Everything cancels out.

Trying to pump yourself up on a swing seemed to me to be similar. How could you ever get anywhere?

But there's a difference. The "natural motion" on a flat surface is movement with constant velocity. To increase your speed permanently, you need to somehow apply a force to yourself which isn't later cancelled out by an opposite force. With nothing to push against, this is impossible. The "natural motion" on a swing, however, is oscillation. You move back and forth. To increase the height of your swing, you can either apply a backward force to yourself when you're moving backward or a forward force to yourself when you're moving forward.

Or both.

And there's the trick.

If you kick your legs forward (thus pushing the rest of your body backward) just as you're reaching the end of your backward swing, by the time your legs need to slow down (thus pulling the rest of your body forward), you'll have started your forward swing already, so although the two forces on your body are in opposite directions, they're both in the right direction to help you swing higher.

I'm not sure how to approach this problem using conservation of linear or angular momentum. Momentum is conserved, of course, but only in a closed system. The earth and the swing exert forces on each other, so where does that leave us? The momentum of the swing when it's moving forward is clearly not the same as its momentum when it's moving backward. So, what is conserved, exactly?
 
Must fight urge...
Cannot resist...

What makes a swing swing?
Click to expand...

Swinging? :teacher:

Sorry, but I couldn't resist.... :hit:

Yes, it is energy that is conserved. Momentum is obviously not.
 
Swing swing, swing the spinning step.
You wear those shoes
and I will wear that dress.
 
Swing swings because the 3rd note in 4/4 is syncopated.
 
OK, perhaps a little experiment. Take a rope or string and a stick, rod, screw driver, anything that is more or less shaped elongated.

Tie one end of the string to the whatever shape around two thirds of its length and the other end to the end of the object. Pick up the string and balance it, so that the object to be suspended horizontally.

Take the free tip of the object and move it so that the object is now suspended vertically, with the centre string tight and the other part tied to the end loose. This is the starting position of the swing.

Let go of the end. The object will fall horizontally and the swing starts to swing a little bit. We just simulated how you start swinging a swing.

How?

Whe the object was falling to the horizontal position, it insisted on having it's centre of gravity falling straight down. Since the COG was in between the two strings, the tight string in the middle moved forward, to make that happen. The moment that the object ended up horizontally reaction forces took over, while the inertia of the forward string and the forward part of the object insisted continuing moving forward for a while.

Just a thought.
 
69dodge said:
To increase your speed permanently, you need to somehow apply a force to yourself which isn't later cancelled out by an opposite force. With nothing to push against, this is impossible.
Click to expand...

When we kick on a swing, aren't we pushing against air? How would swinging action differ in a vacuum?
 
zer0vector said:
Any pedulum has a intrinsic frequency, based solely on the length of the arm and the weight at the end (this is why pendulum clocks work).
Click to expand...
Length, yes. Weight, no.

The force of gravity pulls on all objects the same, regardless of weight, so changing only the weight of the pendulum does not change the speed of the swing. :teacher:
 
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