In the seventeenth century, Robert Hooke conducted many experiments with springs. In all the data he recorded, he found that the force needed to push down on a spring or stretch it out from its usual resting point was equal to the spring’s stiffness multiplied by the distance it is pushed or pulled from its resting point. He wrote the equation F = kX, in which F is the force, k is the spring’s stiffness, and X is the distance the end of the spring moves from its usual resting point when no push or pull is applied. The equation is still used today.