Workdone is given by, w =Fscosθ where θ is the angle between the directions of force applied and displacement. In cases 1 and 3,θ=0 ∴ work done =0 In case 2.,w=fscos0=mgh,h being height covered. In case 4 , force is always perpendicular to motion i.e., θ=90°, hence w=0