A boy carrying a box on his head is walking on a level road from one place to another on a straight road is doing no work. The statement is

boy carrying a box on horizontal surface is doing no work against gravity, as gravitational force acts vertical down ward, and displacement is horizontal, angle between force of gravitation and displacement are perpendicular to each other hence no work is done by gravitational force.work done by gravitational force =(Force of gravity).(displacement) force is perpendicular to displacement  w =F S cosθ=FS cos90=0