"Newton's Cradle, named after Sir Isaac Newton, is a device that demonstrates conservation of momentum and energy via a series of swinging spheres." - Wikipedia

As a curiosity, we made Newton's Cradle using ShapeCrete instead of steel spheres. This demonstrates the high compressive strength of ShapeCrete. Although ShapeCrete doesn't transfer energy as efficiently as the steel balls, it was still a success in that even after banging together hundreds of times, the spheres haven't chipped or cracked.

Figuring out how to make the spheres took some thinking. What kind of form could we use to cast a ShapeCrete sphere? A balloon? A tennis ball? A christmas ornament?

The christmas ornaments worked the best. The first ornaments we tried were plastic. They are spherical on the outside, but the inside turned out to be uneven, with a huge part line where the two halves of the ornament are fused together. Still, we tried it and then we came across another problem.

Filling the ornament through such a small opening is tricky. After trying a few things (our fingers, a turkey baster, a funnel), we settled on using a large syringe to force the mix inside.

Needing a way to hang the ornament from the frame, we sunk a piece of basalt rod down in the ornament after it was full. This gave us a place to drill a hole and insert a sleeve so the finished sphere can hang from the string.

The plastic ornaments worked, but it was difficult to remove the plastic without accidentally slicing the sphere inside. This was a month after Christmas, so ornaments were on sale, but the shelves were emptying fast! A local store had glass ornaments for very cheap (below left), and this is what we used.

The glass ornaments left an extremely shiny finish, and it was easy to lightly tap and break the glass away with a hammer.

To hang the spheres, we bent the frame out of #3 rebar. It fits into a groove on the MDF base and is held in place with small brackets. The spheres by nylon line and are secured to the frame with swivel fittings and crimped connections.