Work done by a constant force is given as
\(W=F s \cos \theta\)
where, \(F=\) applied force
\(s=\) displacement
and \(\theta=\) angle between the direction of force and displacement.
When, \(\theta=0^{\circ}\), then \(W=F s\) (positive)
When, \(\theta=90^{\circ}\), then \(W=F s \cos 90^{\circ}\)
\(=0 \text { (zero) }\)
When, \(\theta=180^{\circ}\), then \(W=F s \cos 180^{\circ}\)
\(=-F s \text { (negative) }\)
Hence, work done may be positive, negative or zero.