Modeling has been shown to correct weaknesses of traditional instruction by engaging students in the design of physical models to describe, explain, and predict phenomena. Although the modeling method can be used without computers, the use of computers allows students to study problems that are difficult and time consuming, to visualize their results, and to communicate their results with others. This combination of computer modeling with theory and experiment can achieve insight and understanding that cannot be achieved with only one approach. This paper describes the Easy Java Simulations (Ejs) modeling and authoring tool and shows how it can be used to teach mechanics concepts with computer modeling.