Work, Energy, and Power

6.1  (a) While the person lifts a bucket out of a well by means of a rope tied to the bucket, the direction of both the force and the displacement are same, hence the work done is positive.

(b) While lifting the bucket, he works against gravity, but the work done by the gravitational force is downward, hence the work done is negative.

(c) The direction of motion of the object is in the opposite direction of the frictional force; hence the work done is negative.

(d) While a body moves on a rough horizontal plane, the frictional forces try to oppose the motion. But since the applied force maintains uniform velocity of the object, the motion of the object and the applied force are in the same direction. Hence the work done is positive.

(e) Since the resistive force of air is trying to bring the vibrating pendulum to rest, the work done is negative.