When we talk about energy we tend to focus on the change in kinetic energy and change in potential energy, where the change in potential energy is the negative of the work done by conservative forces....When we talk about energy we tend to focus on the change in kinetic energy and change in potential energy, where the change in potential energy is the negative of the work done by conservative forces. Since the forces change over the course of the motion of the block, the acceleration of the block is not constant throughout the motion of the block.
