These 1" (25mm) drilled physics balls are great for doing Newtonian physics experiments such as collisions or for use in mass/volume experiments. A 3mm hole has been exactly drilled in the center of each sphere in such a way as to allow for the recessing of a knot. This allows the set to also be used as pendulums. These crafted balls come in pairs which makes them ideal for comparing how different materials affect your experiment. Set of 14 drilled balls contains 2 each of Steel, Brass, Aluminum, Lead, Copper, Zinc and Wood. Balls come packaged in plastic case.