A block of mass \(m\) is suspended vertically from an ideal spring of force constant \(k\) in a uniform downward gravitational field \(g\). The block oscillates vertically with amplitude \(A\) about its static equilibrium position \(y_{eq}\) between an upper turning point at \(y_{eq} - A\) and a lower turning point at \(y_{eq} + A\). Although the gravitational force acts continuously downward throughout the oscillation, precise measurements confirm that the time required for the block to travel downward from the upper turning point to the lower turning point is strictly equal to the time required to travel upward from the lower turning point to the upper turning point. Which of the following explanations correctly accounts for why the unidirectional downward force of gravity does not cause the duration of the downward stroke to differ from the duration of the upward stroke?