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back to article Falling slinky displays slow-motion causality

Researchers from the University of Sydney have explained why a spring dropped from a height - in this case the toy “slinky” – appear to ignore the force of gravity for a time. The very odd thing is that “if a slinky is hanging vertically under gravity from its top (at rest) and then released, the bottom of the slinky does not …

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Anonymous Coward

I never could make my slinky work

I was sending it the wrong signal

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Anonymous Coward

Re: I never could make my slinky work

Slinkys used to be made form steel, now they are cheap plastic imports from china.

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Anonymous Coward

Re: Slinkys used to be made form steel

Weren't they made from steel because they evolved from piston ring technology? You don't think the Chinese auto industry uses plastic piston rings, do you?

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Re:Slinkys used to be made form steel, now they are cheap plastic imports from china.

Blame the globalists.

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Re: I never could make my slinky work

Well the plastic ones are probably not real slinkys (slinkies?)

I bought a new "original" slinky ~10 years ago that says on the box something to the effect that all official slinkys ever made have been made on the same machine. I assume they're still doing the same.

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Silver badge

Re: I never could make my slinky work

I wonder how many miles long that Slinky would be, if they hadn't chopped it into lots of toy-sized pieces as it came out of the Slinky-making machine?

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Re: I never could make my slinky work

Note mine. I only got it a while ago and it's metal.

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Re: Slinkys used to be made form steel

It would explain why all the taxi engines there sound like all 3 pistons are going to shoot through the hood.

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I get it. You're posting this video from six months ago because your ipad is attached to a slinky and it took six months for the news to catch up with you.

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Seems overcomplicated

The bottom doesn't move because it's being held in place by spring tension, which is only released when the adjacent coils collapse.

Nice slo-mo though.

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xyz
Happy

Re: Seems overcomplicated

No what they're talking about is similar to Wile E Coyote running off the edge of a cliff and only falling when he realises he's not on the ground anymore. Then instead of his head staring at the camera as his bottom half goes shooting down, the opposite happens. Thinking about it again, you might be right, the bottom end of the slinky isn't just sitting in the air, it's trying to collapse up towards its other end as the other end is collapsing towards it. I need a coffee.

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Thumb Up

Re: Seems overcomplicated

Upvote for mentioning Wile E. Coyote!

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Re: Seems overcomplicated

Does this mean they are getting close to missing the ground? That's a vital area or research that is.

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Pint

Re: Seems overcomplicated

So Wile E Coyote has a very low spring constant?

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@John Latham / xyz Re: Seems overcomplicated

"Thinking about it again, you might be right, the bottom end of the slinky isn't just sitting in the air, it's trying to collapse up towards its other end as the other end is collapsing towards it. "

I must confess to agreeing with you two - that is precisely what I always thought was happening. Running a thought experiment of a spring lying on a table, held at both ends - release the ends and the spring contracts towards the centre of the spring (due to the tension in the spring). Make the force used to hold the spring equal to the force experienced by the end of the slinky when hanging in equilibrium, and the same thing happens obviously.

Back to the hanging slinky, if we could arrange to be in a free-falling environment, observing the centre of the slinky in front of us and starting from rest when the top of the slinky is released (say in a tiny lift with a port-hole) - surely we should see the same as the spring release on the table above ? (modulo friction, air resistance etc).

Or would we ? The paper seems to say no - but i've not read it yet - but i'd be intrigued to know more, and to have someone pick holes in the analogy above.

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Re: Seems overcomplicated

Bingo. The energy it takes to hold lower end up is stored when we stretch it.

This might be run in reverse horizontally. One end doesn't move until the other end pulls it. In that case,however, we'll need to move faster, or attach a mass to the stationary end, as a Slinky(tm) has much less inertia than is needed reach maximum elongation at ordinary speeds.

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Re: Seems overcomplicated

"getting close to missing the ground?"

yes, but only if distracted at the right moment.

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Re: @John Latham / xyz Seems overcomplicated

"have someone pick holes in the analogy above."

Answering my own question a bit, but of course the strain distribution would be different. Hmm - thinks....

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Happy

Re: Seems overcomplicated

"yes, but only if distracted at the right moment."

A can of Greek olive oil and a bottle of retsina!

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Re: @John Latham / xyz Seems overcomplicated

It's actually relatively simple if you calculate stuff from the center of mass.

In a slinky, anything below the center of mass will suddely retract as that part of it becomes essentially weightless as the spring starts to fall, giving the illusion of the bottom having "hang time".

The center of mass still drops down according to our old pal Newton.

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Re: Centre of mass

I always thought about it in terms of centre of mass as well.

I think what the boffins are getting at is a bit more detail as to teh actual molecular mechanism of how a centre of mass works through communicating molecular bindings. While the slinky is attached at the top, it's part of a combined structure that has a completely different centre of mass somewhere else. The slinky only starts acting independently with it's own centre of mass once it's released. And since nothing can travel faster than light, the information that the top of the slinky has been released cannot be instantaneously transmitted to teh rest of the slinky, this 'information' is passed down the slinky as a release of tension between the molecular bindings making up the spring.

Or something like that.

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Boffin

Re: Seems overcomplicated

The way to understand what is going on is to think about it from the point of view of the bottom of the slinky. Initially it's at rest: when the top of the slinky is released what tells it that it should start moving? The answer is: nothing does, so it stays still, until the signal moving down through the slinky reaches it.

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Gav
Holmes

Re: @John Latham / xyz Seems overcomplicated

"The center of mass still drops down according to our old pal Newton."

QFT.

The slinky is both falling and contracting at the same time. It has two forces acting on it; gravity and the tension within the spring. The speed it contracts up at the bottom end exactly equals the speed the centre of slinky mass is dropping down. Hence the far end does not move until the centre of mass reaches it.

The force *up* stored within the spring is exactly that obtain from gravity when the spring is first stretched out. So it is no surprise that it precisely balances the gravity force *down* when the spring is dropped. At least for the time it lasts and the spring is fully contracted.

If someone was to stretch the spring out further at the bottom end, and let go at the same time as it was dropped, the force up would be greater for a while, and we would see the bottom end *rise* at first. No-one would think this unusual. So why all this nonsense when the forces are balanced?

All this talk about "messages" and "waves" over complicates what is quite simple. And I'm not a physicist.

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Re: The slinky is both falling and contracting at the same time.

That was my thinking, but, watching the video, the gaps between the rings towards the bottom don't seem to be contracting while the top rings are and the top falls. It does seem the point to which the top and bottom would collapse is moving from top to bottom as well, some other lower point is also moving in the same direction; above that point it is a contracting spring, below that it remains non-collapsing spring.

No matter how it actually works; the spring explanation seems far more reasonable to me than 'hangs around defying gravity until it receives a signal not to'. However; superb videos, so an overall thumbs-up.

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Re: The slinky is both falling and contracting at the same time.

Why would it contract at the bottom? The downwards force pulling it down is constant and until there is a change in the stretch at that particular point the upwards force will not change either. So only when contraction above has occurred will any change happen (which will be downwards acceleration as the contracting spring provides a reduced upwards force.

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Paris Hilton

Re: Seems overcomplicated

Exactly, which means the top part should be falling with the acceleration generated by a force equivalent to 2g compared with a "non springy" item being dropped. This state may only be true in the very beginning or until the whole spring has collapsed.

I would guess the initial acceleration is comparable to being pulled with 2g and it will decrease towards 1g

In that state the bottom part will have an acceleration of equivalent to being pulled with 0g.

While collapsing. It would be interesting to see a item being dropped at the same time.

The information is however instant/(or travels with speed of c) over the whole physical object. When released it got released. Remember it's 1 g that pulls bottom end of the spring in it's first place.

What's new in this? All newton physics. Expect that you cannot calculate with centre of mass. Or you can if you wan't to know when it reaches the ground. Centre of the length of the spring should give you the calculation point for when it will reach the ground.

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Stop

Re: @John Latham / xyz Seems overcomplicated

It's not the same, no.

if it's sat on a table horizontally and stretched, then it will spring towards its centre again when released.

If it's hanging, then the bottom rings have round equilibrium between the "up" pull and gravity. If you turned off gravity, it would spring up from the bottom, much like when sat on the table.

But they're not switching off gravity, they're releasing the top. Therefore, you would expect that on the instant that the top is released, the bottom (which now has the same gravity pull down, but less force pulling it UP therefore balancing the gravity), should start to fall immediately.

I'm not pretending I understand this fully, but It intuitively makes sense to me that that the "top" rings, which were always being pulled by gravity and have now lost their balancing force imbued by being tethered, would "spring" down at a faster rate than the bottom rings which fall under gravity alone ....

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Re: Centre of mass

Has it been proved that information is constrained by the speed of c.

What I've read there are theories of that information is instant independent of c. I guess this is quantum physics and I have very little understanding of them.

But I do not believe that this is applicable on this experiment, it should be enough with newton physics. Just break it down to molecular level with their own centre of masses.

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@Kris

"If it's hanging, then the bottom rings have round equilibrium between the "up" pull and gravity. If you turned off gravity, it would spring up from the bottom, much like when sat on the table.

But they're not switching off gravity, they're releasing the top."

Indeed - hence the description of viewing it from a free-falling reference frame (same as the spring itself).

"Therefore, you would expect that on the instant that the top is released, the bottom (which now has the same gravity pull down, but less force pulling it UP therefore balancing the gravity), should start to fall immediately."

The bottom of the spring is being pulled up by the spring tension and down by gravity (along with all the rest of the spring), so it should start to fall but slowly initially (in the static reference frame). In the free-fall reference frame of the centre of the spring it will appear to contract toward the centre - or at least that is what I was pondering ! I've been talking myself around a few - mutually contradictory - scenarios ever since :)

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Re: Seems overcomplicated

"when the top of the slinky is released what tells [the bottom] that it should start moving?"

For that matter, what "tells" the top that it should start moving? Gravity right? I don't understand why gravity can't tell the top and bottom to start moving at the same time @_@

Hence, my upvotes of the tension/contraction theories promulgated elsewhere.

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Boffin

Re: Centre of mass

Information is indeed constrained by the speed of light, or rather: if information can be transmitted faster than c then it is relatively easy to build a time machine, and that's assumed not to be possible.

However as you rightly say, the speed of light has nothing to do with this experiment at all. The information that tells the bottom of the slinky to start moving is transmitted by a longitudinal wave travelling down the slinky, which is essentially travelling at the speed of sound in the slinky, which is remarkably slow. This is, of course, the same for any object: if you drop a vertical metal rod then the bottom will not start falling until the information has reached it, by the same mechanism. In this case you don't see it because the speed of sound in the rod it rather high. You might *hear* it: as the signal is reflected from the end of the rod it may audibly ring.

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tfb

Re: Seems overcomplicated

What tells the top it should start moving is that nothing is now holding it up. The question to ask is: how does each bit of the slinky realise that nothing is now holding it up?

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Gav
Holmes

Re: Seems overcomplicated

"why gravity can't tell the top and bottom to start moving at the same time"

It doesn't even have to do this at the same time. Gravity is telling all of it *all* the time to start moving. It's just that until the top is released there is nothing counteracting that.

What the slinky is doing is probably no different from what a solid iron rod would do, it's just that the spring tension stored in the iron rod, and the time the bottom of it would spend "hanging" is infinitesimally small in comparison.

I'm not a physicists. I may not have a clue what I'm talking about. But it makes sense to me.

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@Tim Parker

In free-fall, the spring collapses towards the centre. This does not contradict the article: if you arranged this in a free-falling environment, you would have one fundamental difference - you would have to hold both sides of the slinky. Therefore, the "release signal" starts from both ends at the same time.

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Bronze badge

nicely pointed out

You do understand that the center of mass varies? It's just the summation (integral) of point-mass positions (here altitudes above ground) of the body's (slinky) divided by the total mass of slinky. What simply happens is that both 2nd and 3d Newton's Laws apply to point masses result in technically infinitely many equations. Those equations being added together result in one equation where you have \bar x\times M (total mass times center of mass) as a (new) variable, all interior forces of the slinky's particles will cancel out. The only exterior force will matter, that's gravity. So here we go

d^2(M\bar x )\over(d^2t)=-Mg, or

d(M\bar v)=-Mg dt (in the differential form, Sir Isaac was actually using)

QED

PS So the original commenter was right. Things do get more interesting indeed with springs and slinkies is that center of mass is clear to vary w/r to the slinky itself. Mechanics is more complex, or rather cannot be turned to simpler model . Different slinky's particles will have different velocities and accelerations at different times. What is more interesting, that not only the lower end will have a near zero acceleration, the upper one will have more than g at some point (2g for a spring, as I believe).

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Go

Re: Seems overcomplicated

This seems to be a perfect "school science project" they could try in their free time on the ISS.

Now we need a suitable K-12 volunteer to submit the idea for us :D

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here's R. Hooke again

Sir Isaac only (while D'Alembert, Leibnitz, Lagrange, Laplace, the Bernoulli's and other titans to be mentioned as well) I forgot to say about another great rival of Sir. Isaac's, Robert Hooke. That law named after him is what makes springs and slinkies appear out of the ordinary in this other examples, those interior forces (the tension) to be canceled make the particles move within the system in such a bazaar way.

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Re: Seems overcomplicated

What tells the top it should start moving is that nothing is now holding it up.

Mechanics (Physics), Math (the three in one) do :) To simplify, apply 2nd, 3d Newtons' Laws to every particle of the slinky, say spring. Apply Hooke's Law. When you hold the upper end of the spring and the spring doesn't vibrate, the system is at rest, the sum all forces is zero. When the upper end is being released, you find all particles to endure two forces, the gravity and the Hooke's tension of the spring. So, at some point the acceleration of top is about 2g (due to gravity + tension) the bottom part has (gravity-tension) hence near zero.

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Re: Seems overcomplicated

"The bottom doesn't move because it's being held in place by spring tension, which is only released when the adjacent coils collapse."

In part. Guesses.

Loss in stored Gravitational Energy, GE, from the bits at the top and in between plus stored Spring Energy, SE, from the bits in the middle act to compensate for the bit at the bottom 'thinking' about gaining Kinetic Energy, KE, from the above.

I note the misleading shiftyware demonstration was done on an Apple.

ARGHHHH ARGHHHHH ARGHHHHHH

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Coat

Re: Centre of mass

"Has it been proved that information is constrained by the speed of c."

Or, in my case, the speed of C++.

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It is an overcomplicated explanation.

If the top of the spring is accelerating at the speed of gravity (just keeping the term simple, it is in free fall as the attraction of the spring to itself (its collapse) is equal and opposite) the reaction to that (tension / intention) is equal and opposite acceleration up towards the top of the spring (with an equal and opposite force.)

TC/DU:

The bottom of the spring is thus accelerating upwards at the speed of gravity.

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Re: Seems overcomplicated

You nailed it. My explanation of that simplicity was overcomplex. And wrong.

Well done.

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Headmaster

Re: It is an overcomplicated explanation.

Accelerated at the speed of gravity

Acceleration is the instantaneous rate of change (derivative) of the velocity w/r to time, the second derivative of the position w/r to time

You're just trying to illustrate 2nd and 3d Newton's Laws

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Re: Seems overcomplicated

Hang the slinky from a hook, then start pushing up the bottom with your hand.. Notice that now it is the top that doesn't seem to react to the force being applied to the bottom, and the tension has to be removed from the slinky before the top starts to move up. The paper is saying, I believe, that the release of tension itself acts as a signal.

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Re: @Kris

Yes .... I don't doubt that the centre of gravity still falls as expected (since the top can fall faster than gravity, and the bottom of the spring slower than gravity...)

However,

I'm starting to think that (on the instant after release) each "ring" is still holding up the ring below. i.e. that each small component in the chain is effectively still in equilibrium ....... each bundle of springy atoms is still holding up the next bundle of springy atoms .... and that the really slow "propagation" inherent to the slinky means that the "release" of tension takes ages to propagate down ....

After all, if you're sky diving, you could still push and pull on objects, regardless of the fact that you're not anchored to any thing.

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Anonymous Coward

Is he running Unix?

On that Apple machine? Or is that Gnome/Unity UI stuff is so similar that one cannot easily tell them apart?

Apple lawyers, take note.

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Linux

Re: Is he running Unix?

No - just got a Terminal session behind the main window with the video.

Sorry about the icon - I couldn't find an 'OS to the minor-deities' one...

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Interesting little detail

At about 1:12 to 1:23, you can see small disturbances near the bottom of the slinky being propagated towards the bottom. These seem to be mechanical vibrations being propagated along the length of the slinky material (i.e. high speed travel through a metal) as the coil above bash into each other. As might be expected, these disturbances travel faster than the compression wave of the slinky structure itself.

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Anonymous Coward

derogatory terms explained

She/He's a slinky,

Meaning the face has dropped but the arse is still holding up.

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Happy

Re: derogatory terms explained

I thought that describing someone as a 'slinky' meant that they served no useful purpose other than to make people smile when you push them downstairs?

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