Abstract: A system is an artificial defined well-organized collection of related objects or components. The model is a simplified system after processing, but it can reflect the essential characteristics of the original system, and it is an effective tool to describe and understand the system. The real problem is complex, but can described by a simplified model. Some problems are introduced in this paper involved the particle dynamics, variable mass and interdisciplinary field of a broader “force” and “movement”, such as string vibration, raindrops falling and biological competition. These problems show how to simplify the process of practical problems and establish the model, the conclusion based on the model can well explain the phenomenon of concern, even with the actual situation is very close to physical truth, and it is helpful for students to understand a way of dealing with problems and thinking mode.