...The ODE systems where I'd be most helpful are the 1st- and 2nd-order linear systems that occur in nature. Examples include natural decay, temperature change, free-fall against air drag, capacitor (dis)charge, or a spring-mass oscillation "playing catch" between its two energy forms. Higher-order systems can often simplify into 1st- or 2nd-order frames when a slower element dominates. (read more)