Work done by a constant force is given as W=Fscosθ where, F= applied force s= displacement and θ= angle between the direction of force and displacement. When, θ=0∘, then W=Fs (positive) When, θ=90∘, then W=Fscos90∘=0( zero ) When, θ=180∘, then W=Fscos180∘=−Fs (negative) Hence, work done may be positive, negative or zero.