Does a Falling Slinky Defy Gravity?

Key Takeaways

[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