Slinky = Sorcery



Reality is weird! Most noticeably so, when it comes to how gravity works on a slinky.

If you keep your eye on the bottom of the Slinky, on the last curl at the very end, you will notice that as the top of the Slinky starts to fall, the bottom doesn’t drop. It just hangs in the air, levitating, as if it had its own magic carpet. It will stay there, hovering quietly, until a wave, or signal, passing through the Slinky finally reaches it. Apparently, the bottom doesn’t know it’s supposed to fall, so it sits there, seeming to defy gravity, until the very end.


Watch the video, have your understanding of gravity tweaked:
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  • Umlud

    have your understanding of gravity tweaked

    Nah, not really. You have to recognize that a slinky is a system (albeit not a hugely complex system, but a system nonetheless). Therefore, the bottom of the slinky should not and cannot be thought of as somehow “separate” from the rest of the system. Looking at the slinky as a whole, and you’ll see the mundane effect of gravity.

    To me, it’s another example of how mundane effects can – themselves – be fun, which can lead to it being thought-provoking. (This latter part being the more important thing, since it then opens the possibility that someone is open to an explanation.)

    • OverlappingMagisteria

      Correct. The center of mass of the system falls normally at 9.8 m/s/s. It’s by focusing on one segment of the system that makes the effect look strange. Similarly, if you throw a battle axe and just watch the end of the handle, it will travel in a curly motion as the axe spins around.

      However, it IS valid to look at only one segment of the slinky instead of the full system. It’s all a matter of perspective. From this point of view, the bottom segment of the slinky stays still because it is still experiencing a force from the parts of the slinky above it. And that force must match the force of gravity since the slinky was at rest initially.

      What I find interesting is that the slinky doesnt contract uniformly, but instead contracts from the top down: the top contracts all the way while the bottom and middle stay stretched until the wave reaches them. That part is cool!

      • John Horstman

        Ah, good, your conclusion was the same as mine (it’s been years since I’ve done any physics or calculus, for which we frequently used problems with springs as examples). I agree that the effect of the balanced forces is very cool. :-)

  • John Horstman

    So, it’s just the spring exerting force equal to that of gravity, but opposite, no? The held slinky is at an equilibrium state, where the upward force on the bottom part of the spring is balanced against gravity (else the spring would extend further or contract to reach equilibrium), and the force is still exerted as long as the rest of the spring is above the original point at which the bottom started, hence no movement. Or is there something trippier going on?

    • eric

      Seems that way to me.
      Every part of the slinky will fall at a rate determned by (local spring force) + (gravitational force). For the bottom, the direction of the two forces is opposite. For the top, they are in the same direction (the top will fall faster than gravity, until the spring is compressed). For the middle, spring force= 0.

      If you stretched out a really tight spring, then dropped it, for the first few tenths of second the bottom of the spring would probably go up. And I bet that this would look less wierd to our eyes than the slinky does! Because what’s probably most interesting about the slinky experiment is that the spring force is close enough to gravity that it looks to our eyes like its not moving at all.

    • Drakk

      Clearly the answer is that GAWD pushes up on the bottom of the spring while simultaneously pushing down on the top. This is proof that Intelligent Falling is TRUE since gravity could never work that way because of the second law of thermodynamics.

      • Drakk

        I have no clue why this ended up as a reply to John Horstman, in which context it makes no sense, instead of being a comment all on its own.

  • Durga

    I don’t like the way he was explaining it in the video. I was trying to work it out in my head, as the contraction of the slinky occuring at the same speed as its falling, and there he was talking about how there was a delay in the information or signal to the bottom of the slinky to fall. I’m not sure if that’s indeed an accurate way to describe it or if he was trying to dumb down the concept to the layperson, but all it made me think of was Wile E Coyote running off the cliff and not falling until he realizes he’s supposed to fall, which didn’t help me understand at all.

    • Drakk

      It’s kind-of-sort-of right depending on what you take “signal” to mean. It’s true that there’s no net force acting on the bottom coil of the slinky because tension cancels weight, and in that sense he’s right, there is no “signal” to the bottom coil to fall. I take issue with the use of “signal delay” because it makes it sound as though gravity itself doesn’t act on the lower coil until the spring has collapsed – which is patently untrue.