# Lesson 1 - Unit 6 Overview

"We frequently look into the future of mankind and see dangers. We see if we carry on doing what we are doing in 20 years' time there will be no rainforests left, just to use one example. Looking into the future may be one of the reasons that brains evolved in the first place."

- Richard Dawkins

In the previous Unit, we covered some of the fundamentals of working with elements in GoldSim, and we concluded by building a simple model.  It is important to understand, however, that the model that was built was a static calculation rather than a dynamic simulation.  That is, it carried out a calculation (multiplying three Data elements) exactly once.  Nothing was changing or evolving with time in the model.  In a dynamic simulation, the system changes with time, either due to external “forcing functions” (e.g., rainfall), and/or the internal structure of the system itself (e.g., storage/memory, delays and feedbacks). In a dynamic simulation, we don’t just compute outputs once (what is the value now?), but we do it multiple times, looking forward in time (what will the value be in the future?).

In this Unit we will discuss how to carry out a dynamic simulation in GoldSim.  In doing so, we will also introduce two of the most commonly used elements (the Reservoir and the Selector).

Specifically, we will discuss the following:

• Understanding dynamic behavior;
• Representing time in a dynamic simulation;
• Specifying the Simulation Settings for time;
• How to make a model vary with time;
• Introduction to the Reservoir element;
• Understanding simulation modes;
• Running a model and viewing results;
• Using Run Properties to explicitly reference time;
• Representing complex if-then logic;
• Formatting results and using Result elements; and
• Exporting results from a Time History Result element.

This Unit also includes two Exercises.

This Unit has a total of 14 Lessons (including this overview and a summary at the end). Starting with this Unit, the material will become a bit more difficult, and hence the time to complete the Units will likely be longer.