For a functional version check out https://makerworld.com/en/models/189440
If you printed this version already and aren't a fan, you can use the balls for this https://makerworld.com/en/models/166267

Note 2/13/2024: At this time please consider this a decorative item. The scales used in this model make the newton's cradle fairly inefficient, and a recently discovered mistake in the sizing of the ball pegs only made the issue worse (at some point between testing and polishing the 3mf). Due to contest rules I can't take this one down yet, but if you want a functioning version I will be releasing one in the near future so check back!

The perfect desktop accessory to add a little class to your office. 

Note that this won't have the same performance as one with metal balls, I generally get 3-4 clicks out of it. Feel free to remix if you have ideas for making one that works better! https://www.tinkercad.com/things/dOpQ7QpihTK-newtons-cradle

Printing

There are 4 plates: 3 for printing the components separately for colorizing and a fourth that prints all components on a single plate (it even fits on the A1-Mini!)

The snaps for the frame will be a tight fit. I printed this using Sunlu PLA+ so it had a little flex but if you have issues with snapping (possible with plain PLA?), bump the x-y hole compensation up .1mm for the frame parts. The other snaps were designed to fit loosely and shouldn't be an issue. USE A SCRAPER TO REMOVE THE STRINGS. Release each connector first then peel up the string. They may break otherwise.

Assembly

Remove the brims

Snap the base together - it helps to lay one side on a table and press into it 1 snap at a time once everything is aligned.

Attach the strings to the balls

Attach the strings to the base

Swing!