They're relentless....

http://www.youtube.com/watch?v=711bZ_pLusQ

Hogwash. :-))

It's ages since I had a play with PHUN, the free version of the 2D physics sandbox Algadoo http://phun.en.softonic.com/

This: slow mo slinky, is really easy to model. just connect a whole load of discs with springs.

select the top anchored disc

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Press start

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Press delete

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Let it run

I haven't screen captured the images very well I should have always grabbed exactly the same area. But the bottom disc does not move until the bottom springs start bunching, which is exactly what you'd expect when you think about it like that.

Try viewing this on an Android mobile and it's enough to make you wanna drop acid just to see straight.

It's kinda cutely coincidental that the spring rate of upward acceleration equals the gravitational rate of downward acceleration for the bottom-most coils. After some blather about information exchange, the video narrator finally got around (sort of) to the true idea.

So you don't believe, for example,sound speed exists?.In this case you have a second "sound speed" (the only speed media lets travels impulse and energy): the real s.s. which doesn't matter here reffering density-elasticity,and the long.speed referent to mass per unit length-springelasticity.The s.s. exists and is absolute no just for transmitting sound so if you knock a bar at one end the other extreme don't move at all until receive the "info" at l/c seconds after:rigid bodies don't exist.-

It seems to me that the point of the video is that the bottom edge of the slinky does not move at all right away. That's what catches your eye; you don't really focus on the center of mass to see what it's doing. That the bottom edge does not move right away is not something usually noticed due to the speed of this in real time. Only when the action is viewed via slow motion video does this become apparent.

The mechanical force within the slinky requires a finite time to traverse its length. The bottom end can't know that the top end has been released until the 'information' - the mechanical energy, as illustrated in the video below - gets to the bottom. It's similar, in a way, to the notion that if the Sun suddenly went dark we wouldn't know about it for 8 minutes since it would take 8 minutes for the 'information' to reach us due to the finite speed of light and the vast distance involved.

http://livephoto.rit.edu/LPVideos/Slinky/springwave001d.mov

You are right, Del Cat is wrong, he uses the ideal spring without mass with no finite speed for impulse or energy transmission.-

I dont want repeat i said in #7.Watch again the video:nothing moves until is reached by the pulse..I insist you are wrong!

"I won't repeat what I said in #7"
Whew that's a relief, it was gobbledegook.
Here's a simple thought experiment which nullifies the confusion caused by gravity.

Imagine the slinky held outstertched horizontally, then dropped.
The whole thing will simply fall, while each end retracts towards the middle.
I'm not interested in longitudinal oscillations, pulses etc... THAT WASN'T THE QUESTION!
You are exhibiting classic symproms of overthink and overcomplication, you should get a job with Microsoft.
Del

NO NO AND NO,I know my english must be terrible for you but give me a chance: this is not overthink, i don't understand mostly what the video teacher said (because my english) but i'm very sure about his purpose.So I'll go much further than him and you will get the difference of opinions here: just take a bar of any material you want and size at your will too, not special materials or size to show something special, o.k.? you hold it from one end as the spring in video, the bottom have two forces that holds it in that position wright? the stretched part of bar at very small distance of bottom is the updirection force, gravity is the opositive, this situation don't change even if you drop the upper extreme until l/c seconds after so the bottom extreme remains there all that time!!!! Course rigid materials are c≈4km/seg so no easy to see so teacher choose a way to show that and you don't want to see... but whats the meaning of sound speed if this reasoning is wrong?

No need to imagine it. The center does not drop right away. The ends move inward as they fall as the spring contracts.

I think you were expecting the whole thing to fall at once like a rigid body.

http://noschese180.posterous.com/day-170-horizontally-dropped-slinky

What?? You're too bluddy lazy to copy and paste? You're not aware of the virtue of sheer repetition? Don't you know that when you repeat something often enough, it becomes true? As it is, I am merely 1.17% convinced you are correct.

Uh, oops! I got sidetracked by the walk through of Beyond Good and Evil. What were we talking about?

If you'd hold still for a moment you won't end up reading only every other line! :-))

I think Del's explanation is mostly correct. If you watch closely you will see the bottom of the slinky start to rotate long before it starts any downward movement.

Because it starts to move, the information has clearly reached it, yet it still does not move vertically until the center of gravity "catches up to it".

At your request i saw it again trying to see before explain the rot.movement, even when i can't give you my final word about, may be i could get you aware about:the very first info which reachs the bottom is a longitudinal pulse that travels at sound speed of the material and you or me can't see,and more:springs are not just because of tork in its wire,it may have some bending too and this could travel at a different speed i calculated (did you read just my first posts or the others including speed of top calc. too?).So in fact i am not sure if this rotating move is just casual,or from the beginning or after the top release or what, even more i don't know any perfect experience but this one is very conclusive to me, you can't say that the info theory is not only wrong is perverse too as Del thought at first (i think he changed his mind).Of course in my calculation the basic hipo is the center of mass drops with an acceleration=g joining this with perverse theory in few lines you get the speed if i am wright, here i just got insults and "off the topics" and nobody checked my calc.or hypo.When Jorrie made same comment than me refferred to a rod he got a GA.Are these people just suckers or what? will you insult me too now i dont think like you?

There was no argument at all, until your post #6.
If you had just concentrated on your view and your explanation you may have got a more appreciative audience.

It is perfectly reasonable that both our views are 'right' . One is more applicable to the overall view and one more to the micro/macro level.
I think maybe Jorrie was having some fun hinting at that, as if a solid rod does exhibit that effect, but it is pretty irrelevant in the everyday real world.

All physics has it's limitations.
Classical mechanics does just fine in most applications, and my simplification is a reasonable explanation.
The level of detail doesn't make the other answer wrong.
E.G. A detailed subatomic level explanation invoking a unified theory and the effect of gravitons wouldn't make your explanation 'wrong' it would just be another explanation at a greater (and even less appropriate) level of detail.

I have gone to great pains to accept your view and explain my own. Whereas you seem reluctant to concede even the smallest point or answer any of the questions posed.

Because this isn't your first language you sometimes sound pompous
(E.G " i can't give you my final word about " ) This sounds as if you think that only your word has any validity... (maybe you do)

Anyhow, I hope this has explained some of the reaction to your posts.
(Ignore the GAs, they are just a bit of fun and some people mess with them just for the fun of it. One of min dissapeared very early on... I wond who took that off?)
I hope you read this post in the helpful spirit it was written.
Del
(Yeah and why are you bothering to argue with a Cat???)

Okay, somebody's just mucking around now.

There has been nothing 'off topic' about my answers.

I'm not here to pick a fight with Del or anyone. I gently disagree with Del's answer somewhat, and I think I've given a better explanation of what's going on. Clearly the horizontally dropped slinky shows Del's analysis to be flawed. The center does not drop until the 'information' - the mechanical energy - that the ends have been released gets to the center.

Given this, I do not see why Del's answers would rate a GA and mine rate an off topic, unless someone is being spiteful. And I know it's not Del because I know he can't give himself a GA. It's someone else.

And frankly, I don't care if I get a GA or not. But 'off topic' is just a cheesy attempt to insult me.

Complaining for just one? i've got four! but don't bother truth don't comes thru votes...Anyway i think too, Del is a gentleman which to discuss with pleasure."off the topic" should exist only when somebody gets out of subject.Perhaps somebody believes must be used as "i think this answer is wrong" and not bad feeling at all..¿Is that confusing the rate "off the topic"?

If you release both ends at same time in an inertial or not inertial frame both ends finds at the center.The asimetry here is the coils in the upper ends moves and nothing else (not only the botton),so coils in the bottom are separated until the pulse reachs that point..nothing is faster than sound speed in a solid no coil is faster than speed of pulse in a spring as nothing is faster than light in vaccum.

Ferq-san, if you release both ends "in an inertial or not inertial frame" then you simply "release both ends." Period. A difference wot makes no difference *is* no difference.

I don't disagree with your analysis. Your earlier comment implied you thought the whole thing (the horizontally dropped slinky) would drop at once: "The whole thing will simply fall, while each end retracts towards the middle." Clearly, that didn't happen. The 'whole thing' didn't simply fall; the center didn't move until the slinky had completely contracted. But yes, the center of mass did move downward due to gravity. You've made the point and it's accepted.

The question in the original posting was: Why doesn't the bottom fall immediately? Your answer that the center of mass moved was true, but not really to the point of the question.

Your comment that 'The lower edge is is falling, you just don't see it because it is springing back up at the same rate' has a problem too. That statement is true for many seconds before the spring is released and still true for a few moments after the spring is released (though supported is a better choice than falling I think). Since it's true before and after the spring is released, it still doesn't quite answer the original question Why doesn't the bottom fall immediately?.

We are separated by our common language and my turn of phrase.
1. 'The lower edge is falling, you just don't see...' is a sort turn of phrase which deliberately makes a contravercial statement to grab ones attention a bit like an oxymoron.
I mean it is falling relative to the c of g... I know it's stationary in our frame of reference.
The comment is designed to make one think about the FRAME of REFERENCE.

I fully understand your explanation and I hate to admit that yes it too is 'right'.
I simply feel it is largely an overcomplication.
Imagine for a moment the two ends of the slinky are luminous and we see it in a darkened room so you can't see the fine detail of the actual behaviour of individual coils. My explanation would still hold up as sound, whereas yours would seem weird.

Maybe I've confused the issue with the horizontal slinky. What I was trying to prortray is an extended sliky, without sag, being allowed to fall horizontally, as if rolled sideways off the the edge of a table. It will all fall together just like a length of 2x1.
on the subject of 2x1
Following the 'information theory' ...
If you hold a length vertically and drop it you will the bottom remains stationary for some time after release because the bottom doesn't know the top has been released until the information reaches it. This information travels at the speed of sound, or maybe the speed of light?
Reductio ad absurdum.
Del

"If you hold a length vertically and drop it you will the bottom remains stationary for some time after release because the bottom doesn't know the top has been released until the information reaches it. This information travels at the speed of sound, or maybe the speed of light?"

If you hold even a stiff rod at the top and release it, the bottom will indeed start to fall later than the release, according to the speed of sound in the rod. Hold it at the bottom and the top will start to fall later than the release. Hence, if you let a horizontal slinky roll off the edge of the table, the top part should start to fall later, or will it?

Another tricky question is, if you hold both bot top and bottom, with slinky either compressed or stretched and release them simultaneously, which side will fall first?

-J

Not playing anymore, I'm hiding in my secret cat nest
Del

Well, damn, Jorrie. So that's how Wiley Coyote does it, except that the slinky does it backward (more or less): bottom falls, then stops (relative to ground) and waits for remainder of slinky to catch up, then the entire 'bunched' slinky proceeds apace at the speed and acceleration of slinky's CoG (Centre of Gravity) which has been in free-fall all along.

Coyote, OTOH, starts out suspended in mid-air, doesn't realise it and so remains suspended in mid-air in defiance of all known laws of gravity. At this point his CoG is merely his CoC (Centre of Coyote).

Upon realising his plight, Coyote's legs and arse fall first, followed after a short delay by Coyote's torso, followed after a short delay by Coyote's head - a classic example of Observer-Created Reality, except that here Coyote's realisation somehow engages the Higgs field. Sequentially.

Gotta love them Toon Physics! And slinkies!

HA!! GA to Jorrie, and an Assist GA (AGA) to you!

Funny, but insightful

Another tricky question is, if you hold both bot top and bottom, with slinky either compressed or stretched and release them simultaneously, which side will fall first?

Phun again: the top starts very stretched and the bottom fairly compressed

Could this be the beginnings of a new thrill "sport"? Human sized slinky... person hanging from it bungee jump style. Top of slinky is released and person is temporarily suspended, unsupported but not falling. Then eventually falls, but is later caught by normal bungee cord that runs down center of slinky.

Possible? Or does the added weight at the bottom negate the effect that this video demonstrates?

Nice post. Thanks! (It certaintly has got the natives riled)

I made some bills to get upper end speed knowing center mass is moving after y=(1/2)•(L-gt²), and the upper end is "x", y=(-.5/L)x²+x ;so x=L•(1-((g/L)½))•t then speed for that end is dx/dt=(g•L)½ ;being ρ=m/L,and kL=mg you could write too: dx/dt=(kL/ρ)½.So that speed is a constant value.-

Correctness through obfuscation? Dood, there are gaps in your analysis big enough to drive a herd of buffalo through with room to spare. Check it again and take your time. This thread'll still be here when you're done.