Posted by
Bruce Sherwood on
URL: http://friam.383.s1.nabble.com/Droping-a-Slinky-Q-A-tp6851732p6851876.html
The "information" way of describing this is a powerful and appropriate
way to describe this phenomenon. Another word that comes in
(especially in the case of electromagnetic phenomena) is
"retardation": When you flip the light switch in a room, the electric
field in the bulb filament cannot change for an amount of time equal
to the distance from switch to filament divided by the speed of light
("retardation"), and then the field does change and starts moving
electrons in the bulb filament, and the bulb starts glowing.
Incidentally, the mobile electrons in the copper wires leading to the
bulb move with extremely slow drift speeds. On average, a mobile
electron near the light switch takes about 30 minutes to reach the
bulb. So it really is information (change of electric field) that is
transmitted rapidly, not the moving objects.
Until the ("sound" or "mechanical") wave in the slinky propagates down
to lower parts of the hanging slinky, there is no reason/cause for the
lower parts of the slinky to change position, and they don't. And as
long as the lower parts of the slinky are stretched, they will
continue to support the load below them just as they had been doing
before the top of the slinky was released. Hence the bottom of the
slinky will not move until the wave has propagated down far enough
that the slinky just above the bottom is no longer stretched.
Here's an article about the slinky phenomenon, in which the physicist
Rhett Allain, blogging for Wired, models the motion using VPython
(vpython.org):
http://www.wired.com/wiredscience/2011/09/modeling-a-falling-slinky/Bruce
On Sat, Oct 1, 2011 at 7:07 PM, ERIC P. CHARLES <
[hidden email]> wrote:
> Below are links to some science videos nifty for two reasons.
>
> 1) They ask and answers a pretty cool question: What happens when you hold a
> slinky out at shoulder height, so it is extended down (the bottom still off
> the ground) and you let go. Think about it for a second. How does the top
> part of the slinky move, how does the bottom part move, how does the center
> of mass move? A good physics thought experiment! (If you are having trouble
> imagining it, here is the question:
>
http://www.youtube.com/watch?v=wGIZKETKKdw)
>
> 2) The answer illustrates the weirdness of trying to apply the term
> 'information' to all possible situations. At about 1:30, the physics prof
> offers an explanation for what happens, and (for just a second) talks as if
> one part of the slinky is transmitting 'information' about its movement to
> another part of the slinky, which is bizarre way. The slinky itself is
> moving, it is not transmitting information about the movement, it IS moving.
> Why would you say that it takes time for 'the information to propagate',
> instead of simply saying that 'it takes time for the slinky to move'. Weird,
> weird, weird.
>
> At any rate, Here is the cool answer:
>
http://www.youtube.com/watch?v=eCMmmEEyOO0> (and it continues here:
http://www.youtube.com/wa! tch?v=oKb2tCtpvNU&NR=1)
>
> This seemed like the type of thing lots of people on the list would get a
> kick out of... so... hope you do.
>
> Eric
>
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