Zero work: ∵ W = Fdcosθ
If θ = 90°, then cosθ = 0, therefore W = 0
For example- work done by centripetal force in uniform circular motion is zero.
Positive work:
Again W = Fd cosθ
When θ is less than 90°, then cosθ is positive, and hence the work done will be positive.
e.g. when 0 = 0°, then cos0 = 1 then W = Fd.
For example: When a man lifts a weight upwards, then the work done by the man is positive.
Negative work: When θ = 180°, then cosθ = -1, then W is negative.
For example: When a man lifts a weight, the work done by the gravitational force is negative.