When is the work done by a force said to be negative? Give one situation in which one of the forces acting on the object is doing positive work and the other is doing negative work.

(N/A) Work done is given by the formula $W = F S \cos \theta$,where $\theta$ is the angle between the force vector $F$ and the displacement vector $S$.
Work done is said to be negative when the angle $\theta$ between the force and the displacement is obtuse,i.e.,$90^{\circ} < \theta \leq 180^{\circ}$,because $\cos \theta$ is negative in this range.
$A$ common situation is a body falling vertically downwards through the air. In this case,the force of gravity acts in the direction of motion (downwards),so the work done by gravity is positive. Simultaneously,the air resistance (frictional force) acts in the opposite direction of motion (upwards),so the work done by air resistance is negative.