example again, and think about how it might be designed so that it fails to satisfy (2). Suppose that we were designing a treadmill to be used by Olympic sprinters in training. We might decide that we need fine-grained speed control only at very high speeds, and that it’s more important for the athletes to get up to sprint speed quickly than to have fine control over lower speeds. With that in mind, we might design the treadmill such that if the speed is less than (say) 10 MPH, each button press increments or decrements the speed by 2 MPH. Once the speed hits 10 MPH, though, we need more fine grained control, so each button press only changes the current speed by 1 MPH. At 15 MPH, things get even more fine grained, and each press once again changes things by .1 MPH. In this case, condition (2) is not satisfied: the relationship between the quantities of interest in the system (number of button presses and speed of the belt) doesn’t vary at a constant rate. Just knowing that you’ve pressed the “up arrow” button three times in the last minute is no longer enough for me to calculate how much the speed of the belt has changed: I need to know what the starting speed was, and I need to know how the relationship between button presses and speed changes varies with speed. Predicting the behavior of systems like this is thus a bit more complicated, as there is a higher-order relationship present between the changing quantities of the system.
5.1.2 Two Illustrations of Non-Linearity
The logistic function for population growth in ecology is an oft-cited example of a real-world non-linear system. The logistic function models the growth of a population of individuals as a function of time, given some basic information about the context in which the population exists (e.g. the carrying-capacity of the environment). One way of formulating the equation is: