Slotted masses are used in student lab classes, to teach physics and other sciences. The slots allow them to be placed on weight hangers, which are lightweight platforms attached to a thin rod with a hook at the top. Various masses are added to the hanger to create the desired amount of total mass (standard masses are 100, 50, 20, 10, 5, 2, and 1 gram), then the combination is hung by the hooked end from a string or other support point.
 
The slotted mass/weight hanger combination allows a student to quickly create any desired amount of mass, to use in experiments involving force, acceleration, and mass. There are also many other experiments in other fields where a variable hanging mass comes in handy.