Using Stock Elements

Stock Elements are perhaps the most important elements in GoldSim. Stock elements are important because they have the property that they accumulate past events and provide systems with inertia and memory, and hence are responsible for internally generating the dynamic behavior of a system. Without such elements, your models could respond to outside (exogenous) drivers, but could not generate any dynamics of their own. While there are other elements that behave in this way that are not Stocks (i.e., Delay elements), most (if not all) real-world dynamic systems will involve at least one Stock.

Stock elements have the special property that their outputs are influenced by what has happened in the past. That is, their outputs are determined by the previous values of their inputs.

GoldSim provides three types of Stock elements, listed below in terms of increasing complexity:

• Integrators are intended to integrate information. For example, integrating a time series of the velocity of a projectile using an Integrator would yield the height of the projectile.

• Reservoirs are intended to integrate/accumulate material flows and represent things such as reservoirs of water, financial account balances, and inventories.

• Pools are more powerful versions of Reservoirs (i.e., they are also intended to integrate/accumulate material flows). They have additional features to more easily accommodate multiple inflows and outflows.