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

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    Graviity Slinky
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The discussion centers on the behavior of a Slinky when dropped from an extended position. Participants clarify that while the entire Slinky is in free fall, the bottom does not rise but remains stationary momentarily due to the propagation speed of tension changes within the Slinky. Key insights include the role of gravitational forces and internal tension, which cause different parts of the Slinky to accelerate at varying rates. The explanation emphasizes that the bottom of the Slinky does not move until the entire structure collapses, demonstrating principles of classical mechanics.

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  • Basic grasp of the Slinky as a coupled mass-spring system.
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  • #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.
 
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  • #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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