A solid sphere is raced against a hollow sphere. They have the same diameter and similar mass.
Race the steel sphere and the hollow steel sphere. Race the aluminum sphere and the steel sphere. Return all four spheres to the box when done.
Yellow Plastic Ball (10 g) Solid Steel Ball (66 g) Hollow Steel Ball (21 g) Solivd Aluminum Ball (24 g)(balls) 1 m track (under stairs)