Momentum

Linear Momentum of an Isolated System is Conserved:

Momentum = Mass x Velocity (vector quantities with size and direction)

Generally, Momentum describes how much a system resists an external force. An experienced investor managing $100 billion using his proven methods is not going to change his positions based on insignificant fluctuations on the market as he has a big amount of Momentum. In contrast, a small private investor managing his $10.000 life savings is vulnerable to changing his mind about his positions by the most insignificant fluctuations or market news. For a discussion on a system sticking to its previous actions, see Newton’s Laws.