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Can someone explain this results?

https://www.youtube.com/watch?v=eCMmmEEyOO0

If I grab one of the ends of the slinky, then the slinky will extend because of the gravity. Assume that the other end of the slinky does not touch the ground, and when we let go the slinky, (with inital speed $0$), I saw that the other end of the slinky does not move until the upper part comes really close.

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Nothing happens at the bottom until the longitudinal wave arrives from the top. The speed of propagation on a slinky is quite low.

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  • $\begingroup$ This is a good point Pieter and definitely correct! I wonder, however, whether this speed is so slow that it takes 3 to 4 s to reach the other end of the spring as shown in this newly added amazing video. The lower end really doesn't move at all which would be best explained by the wave consideration! $\endgroup$ – freecharly Mar 11 '18 at 18:38
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    $\begingroup$ @freecharly This is a slow-motion video. Reality is faster. $\endgroup$ – user137289 Mar 11 '18 at 18:40
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    $\begingroup$ Thanks Pieter, I didn't listen to the audio. This makes your wave hypothesis definitely the best explanation! I tried to model it unsuccessfully on a real back of an envelope which turned out to be too small. $\endgroup$ – freecharly Mar 11 '18 at 18:43
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    $\begingroup$ @freecharly Here is a close-up of what happens at the bottom. It seems that there is also a torsional wave that is faster. But also here: no vertical motion until the longitudinal wave arrives. arenan.yle.fi/1-4150110 $\endgroup$ – user137289 Mar 11 '18 at 19:12
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    $\begingroup$ Thank you Pieter for the link, this is really impressive! It is the first time I see these delightful slinky-experiments. $\endgroup$ – freecharly Mar 11 '18 at 19:18
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Initially you hold the helical spring at one end and it is extended by the distributed gravitational force pulling it down and the reactive force of your hand holding it back on the upper end. The extension of the spring is asymmetric, because near the hand the weight causing the extension is larger. When you let the spring go, the center of gravity of it will fall with the gravitational acceleration, but the spring will contract towards its center of gravity. The initial extension is smaller below the center of gravity than above. Thus the movement of the upper end towards the center of gravity is faster than the movement of the lower end. The contraction movement of the lower end is, however, against the gravitational acceleration. Therefore, you have the impression that the upper end falls down very fast whereas the lower end stands still for a while.

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