Here's an idea for a contraption that involves a slingshot. It could also be used to create a YouTube video that would get a lot of interest. It would be a follow-up to the "Bullet Block" experiment on the Veritasium YouTube channel. https://www.youtube.com/watch?v=vWVZ6APXM4w Go watch that video if you're not aware of the outcome. It's very non-intuitive, and maybe even more so if you have a lot of science knowledge. I suspect Rhett Allain is on the right track with his explanation. It can be tested by replacing the blocks of wood with steel rectangles and the bullets and rifles with steel balls and slingshots. https://www.youtube.com/watch?v=qaIqanZ00uo By switching to mostly elastic collisions, you should be able to eliminate the "internal energy" terms, and you'd get a dramatically different outcome. I think that would make for a great YouTube video.