Answer
See explanation.
Work Step by Step
a. When the block is pressed against the spring, compressing it, the displacement and the spring force are in opposite directions, so the spring force does negative work. The potential energy stored in the spring increases – it has more PE when it is more compressed.
b. Here the displacement is upward, and the spring force is also upward, so the spring force does positive work. The potential energy stored in the spring decreases – it has less PE when it is not compressed as far.
