Question 39: When is the work done by a force on a body : (a) positive, (b) negative, and (c) zero?

Answer: (a) Positive work – Work done is positive when the force aplied and the displacement caused are in same direction.
Example: When we kick a football, it moves in the direction of the force applied and hence, the work done in this case is positive.

(b) Negative work – Work done is negative when the force applied and the displacement caused are in opposite direction.
Example: Work done by the frictional force on a rolling football to decrease its motion is an example of negative work, as the direction of frictional force (acting on the ball) is opposite to the direction of the football’s motion

(c) Zero work – Work done can be zero for the following two reasons:
1. The displacement is zero.
2. The force and displacement are perpendicular to each other.
Work done by the earth on a satellite moving around the earth is zero as the direction of the motion of the satellite is always perpendicular to the direction of the gravitational force.