# Example 1: Simple Population Model

A simple system dynamics model can be used to show unrestricted exponential growth over time.

## Viewing this model in Nova

To run this model:

- Launch Nova
- Select Menu Item File | Browse Model Library
- Open folder 1-Population Models and double-click
**SimplePopModel**

The model displays in Nova as shown

### About this model

The two large central panels are called the *Modeling Canvas* and the *Dashboard*. The modeling canvas shows a graphic representation of the model as a set of interacting components. In fact, the model is constructed by placing such components on the modeling canvas, connecting them appropriately, and specifying their mathematical relationships using *component expressions* (not shown).

In the model, population is a stock called **pop** whose change is specified by a flow called **birth**. The birth rate is provided by the component expression `rate * pop`

. Here **rate** is a term which represents the constant or computed quantity contained in its component expression. In this model **rate** is provided by a slider called **birth_rate**, which is visible in the dashboard.

Also visible in the dashboard are a graph and table for displaying model output. These each have stubs in the model canvas that show how they get their data from **pop**.

The mathematical model represented here is given by the differential equation: `d pop/dt = rate * pop`

### Running the model

To run this model follow these steps:

Running this model:

- Click on the
**Capture**button in the tool bar. - Click
**Load**next to “Capture.” - Click
**Exec**to run the model through multiple iterations. Click**Stop**to stop the run before completion. - To run the model step by step, click
**Capture**,**Load**,**Init**, then**Step**. - To rerun the model you may omit the
**Capture**,**Load**steps. - To have the model run more slowly adjust the speed slider above the Dashboard.