Analog computers are especially well-suited to representing situations described by differential equations. Occasionally, they were used when a system of differential equations proved very difficult to solve by traditional means. As a simple example, the dynamics of a spring-mass system can be described by the equation formula_1, with formula_2 as the vertical position of a mass formula_3, formula_4 the damping coefficient, formula_5 the spring constant and formula_6 the gravity of Earth. For analog computing, the equation is programmed as formula_7. The equivalent analog circuit consists of two integrators for the state variables formula_8 (speed) and formula_2 (position), one inverter, and three potentiometers. The circuit has to consider that both integration and addition units invert the signal polarity.