The concept of equal mass but different distribution of mass will suddenly have meaning for students. Instantly change the distribution of mass in these 4 1/2" diameter discs by inserting up to eight solid steel balls into your choice of compartments inside. Replace the top, screw both halves together, and roll the weighted disc down a slope. Two complete discs are included so you can compare the speed as they roll. You will receive two injection-molded ABS plastic discs, eight solid steel balls. Supplied with manual.