Interesting that you used that example. I had lunch with friend today. One of the "practically" smartest people one could meet in life. Engineer by training, brilliant at math but had a knack for communication and realized he could make more money in sales than in engineering. Long story short, he ended up buying one of the companies he worked for. It was a steel fabricator. The company did a lot tension steel.
Much of what you wrote about he was working out quickly in his head.
We started talking about grav forces and then we got talking about the exact horizontal example you mentioned above, "friction" free surface--ice--and restraining one end of the slinky while releasing the other.
You two are so close in what you each said. Since he had such a practical background of manufacturing tension steel, he really keyed in on the tensional relationship between each coil "loop" and force necessary to keep apart from the adjoining loop. This will remain same until it is acted upon.
He also brought up the question, if the slinky is stretched three stories and someone grabs a hold of it at some intermediate point, say somewhere in the second story, nothing should change in the tensional values below, even if the slinky is released above.
To bad science wasn't taught by guys like you in high school.
I was fortunate enough to have had some pretty good teachers, starting with a science teacher in sixth grade.
His hobby was inflating balloons with natural gas from the science room, tying pre-addressed postcards to them, and releasing them to see how far they could travel.
During the demo, we naturally wanted to learn more about the natural gas. The teacher demonstrated how the gas was flammable by holding a match to the bottom of the balloon he was holding.
You should describe this act to your friend and see if he can anticipate the outcome. The balloon failed at the bottom, releasing the flammable gas. The collapsing balloon accelerated the gas past the match, then the teacher's hand, and then the teacher's arm. The sheet of flame burned all the hair off the teacher's arm. We learned a lot that day. Science is fun.