 # Continuous Simulations

In continuous simulations, the states of simulation objects change continuously, as modeled by differential equations, using fixed-increment time progression. Continuous dynamic systems (like physical systems with material objects moving in space) can only be captured by a continuous simulation model, while discrete dynamic systems (like a predator-prey ecosystem) can be captured either in a more abstract manner by a continuous simulation model (like the Lotka-Volterra equations) or in a more realistic manner by a discrete event simulation model (since birth, death and predator-prey encounters are discrete events).

Notice that the concept of continuous simulation does not require using a continuous time model and a continuous space model, nor does it require using continuous state variables (attributes with a floating-point number range), though when describing a continuous dynamic system with a continuous simulation model it is natural to use a continuous time and space model and attributes with a floating-point number range for representing continuous state variables.

Summary: The term "continuous" in continuous simulation does not refer to the underlying time and space models, nor does it refer to the range of state variables. Rather it refers exclusively to the nature of state changes, and we could therefore also speak of continuous state change simulation.

The approach of modeling a discrete dynamic system with the help of differential equations described in the form of stock-flow models is called System Dynamics. A web-based simulation platform supporting System Dynamics is Insight Maker. Any System Dynamics modeling framework could also be used for modeling continuous dynamic systems such as physical systems. However, typical application domains of System Dynamics are biology and social sciences, where systems are essentially discrete.

An alternative is the declarative equation-based modeling approach of Modelica, which is a general language for continuous simulation supported by many commercial and also by open source tools, mainly used in science and engineering.

## Examples of System Dynamics Models

Notice that for all typical System Dynamics examples we could also have a Discrete Event Simulation model (often called "agent-based" or "individual-based" model since not based on modeling aggregates, but rather on modeling individuals).

Predator-Prey Model
Based on the Lotka-Volterra equations.
Susceptible-Infected-Recovered (SIR) Disease Model
An epidemiological model that computes the theoretical number of individuals infected with a contagious disease in a closed population over time. See also a discrete SIR model.
World3
A System Dynamics model of the interactions between population, industrial growth, food production and limits in the ecosystems of the Earth. Reconstructed with Insight Maker. Originally due to the Club of Rome, see The Limits to Growth.

## Examples of Physics Simulation Models

Solar System
A simulation that is programmed with CSS 3D animations.
Galaxy
A spiral galaxy with 5000 stars (by Jonas Wagner, 2010-08-18).
Cloth Simulation
Mass-Spring Physics
A 3D Double Pendulum
In this Modelica example, the pendulum consists of two cylinders.