This is a collection of models which illustrate the idea of feedback loops, which form the basis of all living systems.
1. The first is an open-loop model - that is, it contains no feedback. The process of gravitational acceleration g changes the speed of a falling ball, but this speed has no feedback effect on gravity.
2. The second model contains a positive feedback loop. A greater number of Bacteria causes the number of cell divisions to increase, and the process of cell division itself also increases the number of Bacteria. The process 'division' and the state variable 'Bacteria' both have a positive effect on each other, generating unconstrained growth.
3. The third model contains a negative feedback loop. A greater number of TumourCells causes the number of cell deaths to increase, but the process of cell death itself decreases the number of TumourCells. The process 'death' negatively influences the state variable 'TumourCells', while 'TumourCells' positively influences 'death', causing the TumourCells to decay to zero.