Variable-mass system

A variable-mass system is like a big circle that can be filled with different amounts of weight. You can take out some weight, put in some new weight, or just keep the same weight. It's like if you had a big round box, you could put different things in it, like sand or toys or rocks, and you could also take out some of them if you wanted. Every time you change the weight, it changes the way the system acts and moves.