Playground Swings Violate Physical Laws?

[Simon Bridge]

You'll like this...

Ever sat on a swing?
Of course you have!

Have you figured out how to go from stationary to swinging an high as you like without pushing on the ground?
Of course you have.

Dosn't this violate the conservation of energy?

You start off with 0 energy, and, by rocking back and forth, you end up storing mgh in the motion. The rocking is cyclic, so there is no net work done in a closed cycle right? Or is there?

Perhaps we could put a small rocker on a pendulum and enter it for the challenge?

...
What's wrong with this picture?
How good are you guys?

(And could the college proffessors sit out a bit and let the others have a go, thanks.)

<Dons aspestos underpants>

[HeyLeroy]

For every action there is an equal but opposite reaction. And I only squeaked through high-school physics.

Put on roller-skates and chest-pass a basket-ball. Same effect.

[Art Vandelay]

Conservation of energy isn't an issue; your body is a source of energy. The only issue is momentum. On that issue, you're attached to the frame through the chains, and you exchange momentum through them.

Oh, and it's spelled "asbestos".

[Dogdoctor]

I am not sure what you are saying since I am not so familiar with physics. To me you rock and change that energy into forward or backward motion. Energy is stored as you pass perpendicular to the ground which sends you in the opposite direction once gravity equals the energy of the motion plus the energy from your rocking. It all seems to make sense to me.

[Simon Bridge]

HeyLeroy: For every action there is an equal but opposite reaction. And I only squeaked through high-school physics. Put on roller-skates and chest-pass a basket-ball. Same effect.

A good try - but a little too flip there. Also a bit quick - it dosn't really address my post.

Consider: I can sit in a stationary swing and chest-pass a basketball. I'd swing back from the reaction rule - after all, I pushed off the basketball. Then the swing gradually slows to a stop.

Of course, I could receive a succession of basket-balls (dropped down to me say) and toss them out one at a time. Then I'd be able to keep swinging.

But eventually I'd need a continual stream of them to push against - I may as well use the ground right :)

Dogdoctor:To me you rock and change that energy into forward or backward motion. Energy is stored as you pass perpendicular to the ground which sends you in the opposite direction once gravity equals the energy of the motion plus the energy from your rocking. It all seems to make sense to me.OK Cool - so how does rocking transfer energy to the swing and where does the energy come from? (OK - the energy comes from me, see next.)

You're right - are you sure, however, about that transfer point?

Art vandelay: Conservation of energy isn't an issue; your body is a source of energy. The only issue is momentum. On that issue, you're attached to the frame through the chains, and you exchange momentum through them.

I do notice that I get tired :) This is a better answer. You also spotted that moving your legs isn't really the main thing that helps you swing. Most people don't get that. However: while you correctly point out that my body is the source of the energy, you really need to provide evidence of this. The fact that swinging tires me out is what you needed to add there.

You have explained where the energy comes from -

So I swing-up by transferring momentum to the swing by pulling on the chains? How can this happen, when I'm sitting on the swing? Isn't this like pulling myself up by my own bootstraps?

In general: when addressing a claim like this, it is a good idea to make the answer reffer to the question as much as possible. It is also a good idea to exaplain why such confusion can arise in the first place.

[Dogdoctor]

The energy is created by changing the force on the chain from one of just your weight pulled by gravity down on the seat to one of angular motion due to shifting your weight and force applied to the chain.

[Art Vandelay]

[color=green]However: while you correctly point out that my body is the source of the energy, you really need to provide evidence of this. The fact that swinging tires me out is what you needed to add there.It seems rather self-evident to me. If you believe that there is some violation of conservation of energy, then it seems to me that the burden of proof it on you to show that the person isn't supplying any energy.

So I swing-up by transferring momentum to the swing by pulling on the chains? How can this happen, when I'm sitting on the swing? Isn't this like pulling myself up by my own bootstraps?If your bootstraps are connected to a horizontal bar several feet above your head, then yes.

Suppose you're hanging from a rope which attached to a bar, and you let go, pushing the rope to the north. You'll go the other way, right? Now, if you immediately grab the rope again, the only thing connecting you to the bar is the rope, which is slightly to the north, right? So you'll exert a northward force on the bar, and the bar will exert an southward force on you. Do this enough times, and you'll work up quite a bit of movement.

[HeyLeroy]

(sigh)

For it to be a closed system, you'd have to strap a mummified corpse into the swing in a windless room.

It's not a 'closed' system, as you ate at some point before mounting the swing. You start 'swinging' and you'll sweat. Measure the rise in temp at the bearings where the chains meet the bar, more energy being given off.

And if you swing high enough you'll poop your pants. Not a direct transfer of energy per se, I realize, but I wanted to get that in there.

It's not a question of thermodynamics. I stand by my first answer.

(ETA)thppbt.

[69dodge]

[...] moving your legs isn't really the main thing that helps you swing. Most people don't get that.Like me, for instance. :D

Can you elaborate?

So I swing-up by transferring momentum to the swing by pulling on the chains? How can this happen, when I'm sitting on the swing? Isn't this like pulling myself up by my own bootstraps?You're not trying to pull yourself up, exactly; you're trying to push yourself backwards or forwards. If you push your legs forward, say, that pushes the rest of you backwards. Of course, the problem is that now your legs are moving forward, which they can't continue doing forever since they're attached to the rest of you, and when they inevitably slow down they will pull the rest of you forward again, so seemingly you've gotten nowhere. Luckily, however, to increase the amplitude of the swinging motion, sometimes you want to be pulled forward, namely, anytime you're already moving forward. So, you time the pushing forward of your legs to occur slightly before you reach the rearmost position of the swing's motion, when you're still moving backward and want to be pushed backward; then, by the time your legs slow down and pull you forward, you'll be moving forward and want to be pulled forward.

That's how I see it, anyway. It seems to make sense.

[Zep]

I'd tend to say that it is a continually changing centre of gravity that is being used.

At rest and sitting still, the combination body-plus-chain, ie. "the system", has a COG pretty much straight down.

However if the weight of the system is redistributed (e.g. by leaning back, i.e. doing work), the COG moves backwards, so the whole system moves forwards towards equilibrium again (i.e. straight down). However it has to move to do so - it is now in motion, and thus has momentum - in effect you now have an oscillating pendulum. And if you stay in that laidback position unmoved, eventually the pendulum will indeed stop after a few swings and the energy is dissipated (through friction).

However, at the far end of one of the oscillations, by doing some work again and redistributing the COG to make it even further from equilibrium point than it is at that time (i.e. leaning back and forth at the end of each swing), you are adding potential energy each time which converts into higher momentum. Done at the right time, the added momentum exceeds the loss of energy expended in a single oscillation - it will swing higher each time. But by adding just enough momentum each swing to equal the loss in energy of one oscillation (usually just a little friction), the swing can be kept in steady oscillating motion indefinitely.

This explains why it takes much work to bring a swing up to speed, but relatively little to keep it going at a set speed.

(Also I think I have misused the words "momentum" and "energy" a few times. I've had a hard day - you know what I mean, so sod the critics!)

[HeyLeroy]

Yuh-huh, Zep,

This explains why it takes much work to bring a swing up to speed, but relatively little to keep it going at a set speed.


you got it, it's called inertia. A body inn motion tends to stay in motion, and all that.

[Simon Bridge]

Zep: well done.

When I lean back, the COG goes back with me and the swing moves forward. Sit up again at the forward apex and the swing moves back - through the equilibrium point.

I lean back again, at the backwards apex, and the process is reversed.

That is the basic part.

Legs swinging don't actually do much because you're not shifting much mass around compared with your trunk. Note: compared with. It does have an effect but if you don't lean, you'll take a long time.

stuff you havn't mentioned.

When you lean, you also shift the cog up and down.
At the forward apex, you sit up - using your muiscled to pull yourself an extra few feet. When you swing back, you stay sitting up so the swing wants to return you to the same height you last had - which was higher than the previous swing. And so you gain height.

Lastly: resonance.
You have to do the sitting up and lying back in phase with the swing. Well, the description already said that. But, it follows that if you want to slow down - you do the sitting and lying in anti-phase, setting up destructive interference.

[Simon Bridge]

Postscript:
This was a request as to why a particular thing did not violate known physical laws - the kind of thing that may be feilded quite frequently. Generall, it is put in confused terms, after all the querant will be confused.

A responce along the lines of: "It's up to you to prove it" would inappropriate here. If there is a known physical reason for something, then giving the reason is the responce. Then it is up to the instigator to demonstrate that this reason does not apply.

Well done, this actually took less time than it normally does. but then, I don't normally put this particualr puzzle to folk who are used to thinking about this stuff.

[Zep]

Simon, I wanted to keep it simple as possible, initially. ;)

You might also consider another posibilty for moving the COG even further each time - standing on the swing actually allows the legs to become an integral part of the shifted mass (the pivot point is then the foot, not the hips). You might also like to consider the ramifications of swinging the moving mass from below the swing, as happens with trapeze-artists.

[NeilC]

So the friction of the chain assembly and air drag don't come into it then?

[Nick Bogaerts]

When you lean, you also shift the cog up and down.
At the forward apex, you sit up - using your muiscled to pull yourself an extra few feet. When you swing back, you stay sitting up so the swing wants to return you to the same height you last had - which was higher than the previous swing. And so you gain height.

Lastly: resonance.
You have to do the sitting up and lying back in phase with the swing. Well, the description already said that. But, it follows that if you want to slow down - you do the sitting and lying in anti-phase, setting up destructive interference.

The vertical motion, once you start swinging, is enough.The traditional example is the Botafumeiro of Santiago de Compostella. Here's a nice flash animation (http://www.sciences.univ-nantes.fr/physique/perso/gtulloue/Meca/Oscillateurs/botafumeiro.html) of the phenomenon.

[Bone_Vulture]

So the friction of the chain assembly and air drag don't come into it then?

Those are the elements that prevent a person from swinging back and forth infinitely without any additional effort. ;)

[Art Vandelay]

Postscript:A responce along the lines of: "It's up to you to prove it" would inappropriate here. If there is a known physical reason for something, then giving the reason is the responce. Then it is up to the instigator to demonstrate that this reason does not apply.But simply asking someone to explain why something does not violate physical laws is asking them to prove a negative. It is a known fact that human being can supply energy to a system. If you think that there is a reason why this does not apply in this case, it's up to you to explain why.

[Goshawk]

Simon, FYI:

The Physics of Playground Swings. (http://www.intuitor.com/resonance/swings.html)

The Physics of Playground Swings: Part 2. (http://teacher.scholastic.com/products/instructor/Aug04_playground.htm)

Son of The Physics of Playground Swings (http://web.stclair.k12.il.us/splashd/swingexp.htm)

The Physics of Playground Swings: A New Hope (http://www.edwardwillett.com/Columns/playgroundphysics.htm)

The Physics of Playground Swings: The Final Chapter (http://www.hk-phy.org/articles/swing/swing_e.html)

The Physics of Playground Swings: Jason vs. Freddie (http://www.bsharp.org/physics/stuff/swings.html)

Enjoy. :D

[Simon Bridge]

GoshHawk: Nice one.

But simply asking someone to explain why something does not violate physical laws is asking them to prove a negative. It is a known fact that human being can supply energy to a system. If you think that there is a reason why this does not apply in this case, it's up to you to explain why.

1. It is not asking to prove a negative, but to demonstrate how known laws account for a thing. In this case, how the human supplies the energy.

2. I did state why someone may not realise how the energy arrives in the system. I stuck it in my first post. It represents a partial misunderstanding - a phenominon familiar to many skeptics.

You know - physics teachers have been explaining to students for generations how existing physical laws can account for common phenomina without feeling they need to prove a negative.

A proposition phrased as a negative in language is not the same as a proposition being negative in the philosophical sence. In this case, proving a system violates no known laws is the same as proving that it does conform with them - so it is a positive proposition.

When someone sits for the JREF challenge, they first demonstrate that they are doing something worthy of the investigation. A psychic would do this, for instance, by diagnosing medical complaints by using their x-ray vision.

If the initial demonstration offers a more commonplace explaination, the JREF gives that explaination. Otherwise, more controlled conditions are needed to isolate the phenomina.

A tight test can indicate a simple success or failure. No she dosn't see through things and nor can she otherwise detect ailments. OK. Even so, this is not a very satisfying responce scientifically. We also need to account for why the initial demonstration seemed so convincing.

In application of scientific method, we must be careful not only to overturn the previous proposition, but that any new proposition explains the old phenomina. (... if I remember my Popper right.)

Um... and we should avoid getting preachy too, I guess :)

[Art Vandelay]

You know - physics teachers have been explaining to students for generations how existing physical laws can account for common phenomina without feeling they need to prove a negative.But here, there is already an explanation: the human is capable of providing energy. All that is left is to prove that this is what is happening.

A proposition phrased as a negative in language is not the same as a proposition being negative in the philosophical sence. In this case, proving a system violates no known laws is the same as proving that it does conform with them - so it is a positive proposition.If you assert that a proposition phrased as a negative is not necessarily a negative, then you must accept that a statement phrased as a positive is not necessarily a positive. The essential nature of the question is whether some nonnatural processes are at work, and so what you are asking for is a negative.

When someone sits for the JREF challenge, they first demonstrate that they are doing something worthy of the investigation. A psychic would do this, for instance, by diagnosing medical complaints by using their x-ray vision.Note that the JREF never tries to prove that natural laws aren't being violated, as that would be proving a negative. Instead, they ask for people to prove that they are violating natural laws.

If the initial demonstration offers a more commonplace explaination, the JREF gives that explaination. Again, note that that protocol assumes that the burden of proof is on the person denying natural laws. If the applicant has a supernatural explanation, and the JREF has a natural one, the JREF will consider its explanation to take precedence, even if they are completely unable to prove that their explanation is correct.

[Simon Bridge]

But here, there is already an explanation: the human is capable of providing energy. All that is left is to prove that this is what is happening.Exactly.

note that that protocol assumes that the burden of proof is on the person denying natural laws. If the applicant has a supernatural explanation, and the JREF has a natural one, the JREF will consider its explanation to take precedence, even if they are completely unable to prove that their explanation is correct ... and the natural explaination in JREFs possession can be made available.

The question I asked required the natural explaination, as well as some indication fo where the confusion lies. Participants in the challenge would be directed to this forum for such discussion would they not? And here I am... :)

Me: I demonstrate my supernatural powers by apporting JREF to this very room. <concentrates> PING <JREF appears>

JREF: I have a natural explaination for this.

Me: OK. What is it then smarty-pants?

JREF: Er... I'm not telling you. The protocols of the challenge clearly state that you have to prove that you have a supernatural one.

Or.

Me: Swings violate natural laws

JREF: <crosses arms> Prove it

Me: well, I suspect I'm just confused, but it seems that swings go from a standstill to high amplitude without anyone pushing or anything being thrown away. I see there is a person there - but that persons motion is cyclic and I read in physics text books that cyclic motion does no net work. So we seem to have a system which current physical laws says can do no work, yet this same system is clearly doing work. Either I've misunderstood this or there is something supernatural happening.

JREF: The person provides the energy. You have misunderstood things.

Me: OK - but how? The person's motion is cyclic isn't it? Cyclic motion does no work does it not?

JREF: You have failed the challenge - I refer you to the JREF forum if you want to discuss this further.

Me: OK

Me: (later) but but <sniff> Art Vandelay says he won't tell me...

If you see D Simanek's Museum, as I'm sure you have, you'll see where I'm coming from. The proof that I'm providing the energy, as I said, is that I am getting tired. After that, the request is to understand the phenominon. If you don't want to tell me or you don't know, jus say so. Nobody else is having a problem with the request.

[69dodge]

That's how I see it, anyway. It seems to make sense.Hmm...the more I think about it, the less sense it's making...

I think it's essentially correct for someone oscillating in one dimension on the end of a spring. But oscillating in a circle on a swing under the influence of gravity seems more complicated.

When I lean back, the COG goes back with me [...]How can you move your center of gravity backward, without pushing forward on something external? The swing's chain can only pull parallel to itself (i.e., up), not perpendicular to itself (i.e., forward or backward).

[Art Vandelay]

JREF: I have a natural explaination for this.

Me: OK. What is it then smarty-pants?

JREF: Er... I'm not telling you. The protocols of the challenge clearly state that you have to prove that you have a supernatural one.I guess I thought we had gotten past that part, and both agreed what the alleged explanation was, and were only discussing whether that explanation is true. Sorry if I missed what you were asking.

I see there is a person there - but that persons motion is cyclic and I read in physics text books that cyclic motion does no net work.Well, you have establish what you mean by "cyclic", then show that the swing follows that definition. If there is, at any time, transfer of work from the person, then, for the total net work to be zero, there must be transfer of work to the person, right? But how is that possible?

69dodgeHow can you move your center of gravity backward, without pushing forward on something external? The swing's chain can only pull parallel to itself (i.e., up), not perpendicular to itself (i.e., forward or backward).The COG doesn't move, at least not at first. Remember, the person is connected to the bar above by the chains. If the person leans back, something has to move forward to compensate. That something is the swing, which is connected to the chains.

I think that's the key. Thinking about it, I now think that it's not the location of the COG that matters, but the location of the chain. If the chain is pushed forwards, gravity will pull it backwards.

[69dodge]

The COG doesn't move, at least not at first. Remember, the person is connected to the bar above by the chains. If the person leans back, something has to move forward to compensate. That something is the swing, which is connected to the chains.

I think that's the key. Thinking about it, I now think that it's not the location of the COG that matters, but the location of the chain. If the chain is pushed forwards, gravity will pull it backwards.You're saying that the weight of the chain matters? Suppose the chain is weightless; then, why should gravity care whether it's vertical?

Gravity pulls down (effectively) at the center of gravity. If the COG is directly below the chain's upper attachment point even after the person leans back, won't it stay there?

A person sitting on a swing holds onto the chain with his hands. I can see that the part of the chain below his hands will move forward with the seat when he leans back, but not necessarily the part of the chain above his hands.

[Zep]

No, you need to consider the person-plus-chains as a unit, a single system pivoting about one point and affected by a number of forces (gravity, friction, air-resistance, inertia, etc). The issue is that the distribution of mass within that system is not in one place over time (if it were, the swing would never move without external forces being applied). And given that the system is actually a pendulum once it is moving, the forces acting on that mass will change the way the system performs as a pendulum-type system.

From here we go to non-linear systems...what fun! As stated above, it is possible for the system to settle into a number of different phase-change attractors, depending on the timing and size of the changes in the mass location.

[Simon Bridge]

Well, you have establish what you mean by "cyclic", then show that the swing follows that definition. If there is, at any time, transfer of work from the person, then, for the total net work to be zero, there must be transfer of work to the person, right? But how is that possible?Now this is more like it... you are quite correct, you don't have to spoon-feed things. You could, as easily, Socratic Method them. This way questions the underlying assumptions in a way that encourages the me :) to think through my ideas. Personally I will only do that with people I know won't try to lace my drinks with hemlock. Like students.

69 Dodge: You'll have to give it a go.

When you lean back, the swing seat goes forward. The chains bend where you hold them. The balance is held between the tension in the section of chain between your hands and the top of the swing, the tension in the chain between your hands and the bottom of the swing, and the tension in your arms. (In my childhood, the physics was simpler because swings didn't use chains, they used solid steel bars - and steel reinforced seats - you could get killed if you got hit by one!)

If that's all you do, then that's all that happens. But if you then, sit up sharply, the swing will arc back and pass through the equilibrium position. (If you sit up slowly, the swing returns to it's equilibrium position - but it has to be very very slowly.)

Now you are swinging.

If you repeat the motion, timed carefully with the motion of the swing, you can easily increase your amplitude.

Once moving, the height of your center of mass becomes more important than the forward-backward position. When you sit up, you raise your com (hmmm... not much of an improvement on "cog" is it?) while doing work with your muscles. You pull on the chain (but it dosn't flush ha ha - <sigh>) but radially, to sit up. Chemical energy in your arm-muscles converts to additional gravitational potential energy. You notice that your arms get tired.

But don't trust me. Bear this description in mind the go out and do it!

[Simon Bridge]

Just for fun: there is a second dimention - if you try to get a swing to rotate like a circular pendulum (where the seat goes around in the horizontal plain) it just won't do it. Instead it describes a scribble like a diagram of how to tie a bad knot. Every half-cycle, it reverses itself.

This dosn't violate any laws either - it's just that everyone I ask about this predicts the swing will go on a circle (or ellipse) if some sideways component is added to it's motion. Despite, I might add, having done this thousands of time while young.

[Zep]

That's to do with the "Left Hand Rule" for rotational force vectors!

[Art Vandelay]

You're saying that the weight of the chain matters? Suppose the chain is weightless; then, why should gravity care whether it's vertical?You misunderstand. The location of the chain matters because of the weight that it supports.

Gravity pulls down (effectively) at the center of gravity. But it is then projected onto the displacement vector between the person and the bar.

If the COG is directly below the chain's upper attachment point even after the person leans back, won't it stay there?Think about this. Let's try an extreme example. You have half of a wheel (the top half is missing) attached to the axle by one horizontal spoke. If we assume that the spoke has negligent mass, then the COG is directly below the axle, right? Will the wheel remain motionless?

A person sitting on a swing holds onto the chain with his hands. I can see that the part of the chain below his hands will move forward with the seat when he leans back, but not necessarily the part of the chain above his hands.Perhaps we should consider this system to be made up of four separate bodies: the person, the seat, the chain, and the bar (the earth is included in the bar). If the COG of the person moves backwards, then the COG of the seat moves forwards to compensate, right? And the chain will follow, since it's attached to the seat.