A single particle that does not interact with any other particle is a free particle\label{word: free particle}. Ideally, we may have a system of two or more free particles, i.e., particles that form a mechanical system but do not interact with each other, but this would be a very uninteresting system. We may have a system with several particles that interact among themselves but do not interact with other particles or systems, in this case, we call this a closed system\label{word: closed system}, or an isolated system\label{word: isolated system}.