The angular displacement is defined as the angle through which an object moves on a circular path. It is the angle, in radians, between the initial and final positions.
(θf - θi) = angular displacement
θ = s/r
θ = angular displacement through which movement has occurred
s = distance travelled
r = radius of the circle
In case 2, θ = pi/2 and r = R
Therefore, s = (pi/2)xR
Now Workdone, W = F x s (since force is tangential to surface i.e a force which acts on a moving body in the direction of a tangent to the path of the body)
W = F x (pi/2)R.