As the brick in Example 9.8 moves downward, why is the magnitude of the force it exerts on the spring given by Eq. 1 in Example 9.8 and not by simply the magnitude \(m g\) of the gravitational force exerted by Earth on the brick? 

Data from Example 9.8 

A brick of inertia \(m\) compresses a spring of spring constant \(k\) so that the free end of the spring is displaced from its relaxed position. What is the work done by the brick on the spring during the compression?

Fbsx = +k(x x0). (1)