Question

Define work. Give its expression and examples of +ve, –ve, and zero work done.

Answer 1

Definition of work done

1. When a body moves through a certain distance under the effect of a force applied to it, work is said to be done.
2. It is given as, $W=FS\cos \theta$, where, $F$ is force, $S$ is distance and $\theta$ is the angle between force and displacement.

Positive work

1. When the direction of motion is the same as the direction of force applied, positive work is said to be done on a body.
2. For example, when we kick a football, it moves in the direction of the force applied. Therefore, positive work is done on football.

Negative work

1. When the direction of motion is completely opposite to the direction of the force applied, negative work is said to be done on a body.
2. For example, when a ball rolls on a horizontal surface, the force of friction acts opposite to the direction of its motion. Therefore, negative work is done by the force on the ball.
   

Zero work

1. When the direction of motion is perpendicular to the direction of the force applied, zero work is said to be done.
2. For example, when we walk holding a bag, the force of gravitation acts on the bag in a downward direction, and the direction of our motion is horizontal. The direction of motion is perpendicular to the direction of the force. Therefore, no work is done by the force on the bag. Work done is also zero when the displacement of a body is zero.
3. For example, if we push a wall, force is applied but no displacement occurs, therefore, no work is done.