We note that oscillations about the inverted position can be formally described by the same differential equation, equation (5), with negative values of ω2 = g/l. In other words, we can consider ω2 as a control parameter whose variation is physically equivalent to changing the gravitational force exerted on the pendulum. When this control parameter is diminished through zero to negative values, the constant (gravitational) torque in equation (5) first turns to zero and then changes its sign to the opposite. Such a “gravity” tends to bring the pendulum into the inverted position ϕ = π, destabilizing the position ϕ = 0 of the unforced pendulum: The inverted position with ω2 < 0 in equation (5) is equivalent to the hanging down position with the positive value of ω2 of the same magnitude.