Kind: captions
Language: en
[Music]
the
so this is the modeling that I've been
doing and and so this this was done with
the purpose of trying to explain the the
the data that was extracted from one of
the movies of real falling linky what
you see in this one is that the turns at
the top are are snapping together um
behind a front that propagates down so
the blue section at the top is the part
that has collapsed so initially
nothing's collapsed and then more and
more of the slinky is uh is collapsed as
that front runs down the slinky and how
close is that to what actually happens
well I think that if you watch the
movies you can see that the turns don't
collapse instantly so how did you
improve on that model well I assume that
there's essentially a fixed number of
turns over which that collapse occurs
behind the front those turns they're not
collapsing they're not hammering
together at the top they gradually
relaxing and in fact I've colored blue
the section that is uh under collapse or
has collapsed and that's that's far more
obvious if you look at the other video
that but this to me looks much more
realistic much you know more true do
that's why I did it so if someone asked
you why when you let go of the slinky
does the bottom not fall what do you say
I'd say that there is you know what
you're doing you're changing something
at the top and then there's a finite
time for that information about the
change to get to the bottom of the
slinky I mean that happens even with a
rigid bar with a steel bar it's just
that the time is very very sure but a
lot of people on the internet get
uncomfortable with the term information
I mean what are we saying by information
in a physic it's a signal so it's
something uh you know whenever you do
something physically to to affect A
Change causality Is you know you do
something and there's a um a cause and
an effect and that's and between the two
uh information has to propagate a signal
has to propagate if they're not at the
same location physically at the same
location so how long does it take for
the compression wave to get from the top
to the bottom about a third of a second
is the collapse time is there is there
any way to extend that time because you
know if you decrease the the um the
spring constant make it uh a softer
sprinking yes then that takes longer to
collapse which is sort of makes sense
the wave propagates more slowly if you
make it more if you increase the mass of
the slinky it gets longer as well
there's more inertia in that collapse
process in the wave that that yeah you
need you need kind of a heavy Slinky
that is very loose yeah it's odd isn't
it what like a you reckon like a lead
Slinky or
well
I if you have extended systems then to
consider the the motion of the center of
mass of an extended system you only need
to consider the the external force that
acts on the Cent yeah on the center of
mass and that's gravity and that starts
in acting you know instantly that this
is released it's there to begin with but
it's suspended it's held up and then
once you take away that suspension that
that Center of mass has to start
accelerating downward it's instantly so
if you watch the movie you see that the
Red Dot indeed it's a good test for the
modeling the Red Dot does start to
accelerate and you didn't build that
into the model you basically allowed
that to after the fact I calculate you
know once you've got the model at each
time step I calculate where that Center
of mass is and it you know it does
indeed start to fall immediately I think
we were we were talking about this
earlier and you actually see the bottom
of this this thing start to rotate about
now so there's some kind of torsional
mode some twisting
mode signal that gets down to the bottom
of the slinky first you know rushes
ahead but it doesn't actually rela it
doesn't really relase any tension
clearly cuz the bottom just stays
sitting there it's only when those all
those other turns come down that the um
that tension is relaxed I think this
one's kind of neat so in this one you
don't let go of the top of the slinky
but you hold the slinky collapse to the
top and you release the bottom and you
hold the you keep the top fixed and and
so what it does is it oscillates back
and forth that's a basic mode in which
that whole thing can oscillate and of
course that mode just depends in a
simple way on the parameters Slinky so
the period of oscillation of that Mode's
a good test for the parameters that we
got out of the other modeling out of the
falling the falling L this is a very
basic mode this is what I I think of
this as a kind of breathing mode it's
this in and out you know every turn
moves in a very simple way in and out so
this breathing mode or or fundamental
mode of
oscillation