Solutions:

I If the body’s displacement is in the direction of force, the work done is positive;

Therefore, W = F S.

When a coolie elevates a load against gravity, the work done by the coolie on the load is called coolie work. Coolie’s displacement as well as the force he exerts are both upward.

(ii) If the body’s displacement is in the opposite direction of the force, the work done is negative,

hence W = − F S.

When a body moves on a surface, the force of friction between the body and the surface is in the opposite direction of the body’s motion, and so the work done by the force of friction is negative.