Vensim-PLE is a software packet for System Dynamics modeling that offers an easy-to-understand and easy-to-use point-and-click graphical user interface (GUI) which allows depicting stocks and their dynamics (flows) with intuitively understandable symbols. The following gives a very concise overview of the basics of Vensim-modeling on the example of a simple model for the simulation of exponential growth as discussed in respect to bank-account dynamics.A slightly more elaborate example can be found here.

Vensim allows to generate symbol-based representations of system models which can be loaded with equations, parameters and some computer commands, like shown in the illustration below.

The rectangle with the inscription Stock

When using Vensim, clicking on the rectangle Stock or on the arrow with subscript Flow opens up dialogs like the one shown on the right side for defining parameters.

Putting in the values as given below (left side) and pushing the sim-setup-button _{}
followed by the simulate-button
_{} (after having told Vensim where to save its
outputs), makes the model run and integrate the flow to calculate the
stock. Clicking on the rectangle Stock
followed by the graph-button
_{} makes the software generate the
graph as shown in the plot on the right side.

In mathematical terms the above Vensim-model stands for

\(Stock=\int\limits_{0}^{t}\!Flow\,dt\) respectively \( N=\int \limits_{0}^{t}\gamma N\, dt\)