Have you seen the Veritasium video about what happens when you drop a Slinky? If you haven't, you must go there now. (And make a guess, watch the answer video, and make sure you watch the amazing slo-mo video that's linked to the answer video.) It's a great little piece of physics.
You can do force diagrams of both the top of the Slinky and the bottom of the Slinky and the top of the Slinky accelerates at greater than 9.8 m/s². So we talked about that, and then I got out my foam apple.
Where do I have to hold the foam apple so when I drop the Slinky and the foam apple at the same time, they hit the ground at the same time?
As soon as I asked the question, it was pandemonium. Kids started grabbing whiteboards. Heated discussions broke out everywhere. One student started counted the number of turns in the Slinky. There was much arguing.
But what they all knew, what they all seemed to get, is that the center of mass fall at 9.8 m/s². (I think this lesson really helped.) Here were their whiteboards after 25 minutes of discussion:
They wanted to keep going. It was such an interesting lesson. Their answers weren't that bad; we only had to tweak their answers a little bit to get the great trial you see above. (Don't expect your students to get it perfect; it takes some pretty hard-core calculus to derive the theoretical value.)