Dropping an extended Slinky -- Why does the bottom of the Slinky not fall?

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Discussion Overview

The discussion revolves around the phenomenon observed when a free-hanging extended Slinky is dropped, specifically addressing why the bottom part appears to remain stationary for a moment while the rest falls. Participants explore various explanations, including the role of wave propagation, forces acting on the Slinky, and the dynamics of coupled mass-spring systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest that the bottom of the Slinky does not rise but remains still as the top is released, with the information of the release traveling at the speed of sound in the Slinky.
  • Others argue that the Slinky behaves as a deformable body, with different parts accelerating at different rates due to the forces acting on them.
  • A participant proposes that if the top is held until just after the bottom starts to spring back, the bottom will rise briefly before falling again.
  • Some contributions emphasize that the entire Slinky is in free fall, with the top accelerating more than free fall and the bottom less, leading to an average acceleration at free fall.
  • There are discussions about the differences in wave propagation speeds between a Slinky and a rubber band, suggesting that the slower wave speed in a Slinky affects the observed behavior.
  • A participant expresses discomfort with the wave propagation explanation, proposing instead a model based on coupled mass-spring systems to better understand the dynamics involved.
  • Another participant questions the equilibrium of mass elements in the Slinky as it falls, suggesting that the coupling leads to smaller responses in movement as the mass elements are released.

Areas of Agreement / Disagreement

Participants express a range of views on the explanation of the phenomenon, with no consensus reached. Some agree on the role of wave propagation, while others challenge this perspective and propose alternative models involving coupled systems.

Contextual Notes

Limitations in the discussion include assumptions about the behavior of the Slinky under gravity, the dependence on definitions of equilibrium, and the unresolved nature of the mathematical descriptions of the forces and motions involved.

  • #121
DaTario said:
When we consider the physics of toppling dominoes we clearly see a set of dead times which are basically those intervals between the beginning of the fall and the instant of collision with the next piece.
Yes, for dominoes the finitness of the propagation speed is somewhat simpler to understand, than for a linear mass-spring system. There is no "dead times", here but instead a double integration at each finite element: the position of the element n-1 determines the acceleration of element n (along with two constants).
 
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  • #122
Orodruin said:
Unlike the longitudinal waves, the twist mode speed visibly outruns the shock wave.
So if we filmed the slinky from below from the moment it was released with its lower metal end painted green as in the picture below, we would see this end rotate as it descends, is that it?

slinky seen from below.jpg

fig.: Slinky seen from below.
 
  • #123
DaTario said:
So if we filmed the slinky from below from the moment it was released with its lower metal end painted green as in the picture below, we would see this end rotate as it descends, is that it?

View attachment 343044
fig.: Slinky seen from below.
Not very much. It is a twist wave signal. This signal will also take some time to reach the bottom, but it will be faster than the longitudinal shock wave. As the twist signal reaches the bottom, it will reflect off the end of the spring. This will result in some twisting motion and typically also excite some longitudinal signal from the bottom up. If you look closely you will see the reflected twist signal as well as the lower end actually falling a little bit as the twist signal reaches it.
 
  • #124
Yes, I saw a subtle longitudinal wave rising from the lower end.

It seems to behave as Wilberforce pendulum with almost zero moment of inertia.
 
Last edited:
  • #125
A falling spring, like an ordinary object, is subject to gravity. However, due to its structure and elasticity, the bottom of the spring is pulled upward, following the center of mass. This happens because when the top part of the spring falls, it pulls the bottom part with it, creating a balance between the force of gravity and the elasticity of the spring. The lower part appears to be "hovering" in the air, although in fact it is supported by the elastic force of the spring.
 
  • #126
AlexisBlackwell said:
A falling spring, like an ordinary object, is subject to gravity. However, due to its structure and elasticity, the bottom of the spring is pulled upward, following the center of mass. This happens because when the top part of the spring falls, it pulls the bottom part with it, creating a balance between the force of gravity and the elasticity of the spring. The lower part appears to be "hovering" in the air, although in fact it is supported by the elastic force of the spring.
This has already been discussed extensively in this thread. What is your point?
 
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