The Net Work is positive if the KE increases,
negative if it decreases.
An object moves vertically up in a vacuum. KE decreases,
PE increases at the same time by the same amount. Work
must thus be negative. Clearly it the work done by F=mg.
But work is not energy; we already used work to give
meaning to PE (and to KE). Why do have to talk about work
again? The usefulness of W is that it helps us to derive
formulas for various forms of energies. (And for power,
the rate at which an energy transformation takes place.)
Ludwik Kowalski