Work is a scalar quantity. It’s value may be positive, negative or even zero.
(I) Positive Work
If a force acting on a body has a component in the direction of the displacement, then the work done by the force is positive.
Example:- When a horse pulls a card, the applied force and the displacement are in the same direction. So, work done by horse is positive.
(II) Negative Work
If a force acting on a body has a component in the opposite direction of displacement, then work done is negative.
Example:- When a breaks are applied to a moving vehicle, the work done by the breaking force is negative, this is because the breaking force and the displacement act in opposite directions.
(III) Zero Work
Work done by force is zero, if the body gets displaced in the direction perpendicular to the direction of the applied force.
Example:- A body moving in a circle with constant speed. Every point of the circular path the centripetal force and the displacement are mutually perpendicular. So, the work done by centripetal force is zero.
Work Done W = FSCos90 = 0