Newton’s Billiard Cradle

Classic newton’s cradle is a good way to illustrate conservation of energy. We’re not doing too much different here but scaling things up so they can be a bit more interactive, and using some pool balls we already had available. The only tricky part is lining them up- I chose the garage because there are already those exposed ceiling joists that I can use for reference. You’ll want to drill your holes (or however you care to attach them) as close to exactly one diameter apart as you can manage. I cheated an printed up a little guide with reference holes in it, but you’d be almost as well off carefully using a ruler. I was going to also print some ball carriers for consistency but figured I’d make it more accessible using tape and that probably reduces superfluous mass anyway.

The first go-round I used 10lb test monofilament fishing line. I realized this allows a bit more undesirable stretch/bounce than I’d hoped so changed it up to 70lb braided line. I think that’s a big improvement but wire would probably be even better.

I secured these using round toothpicks which are inserted in the holes along with the strings- this allows for easy installation and quick adjustments as needed, and easy removal too.

I tried this one once before using 1″ bearings I had lying around- but as I recall I thought they were hardened bearings and they were pretty much mild steel. I was surprised in that case to see the drastic difference that makes- so it was a worthwhile endeavor just to illustrate that point.

I considered doing a version of this with a literal cradle (a V-groove or semicircle tube bent into an arc- that way you could quickly change ball size, quantity, or whatever else without need of any strings. But i think that extra rolling energy would eat up a good portion of the available momentum.

Here’s some out takes: