Work done when force is applied at an angle to motion is given by:

W= F×S cos θ

Where, F = force

S = displacement

θ = angle between direction of force and direction of motion.

Work done by a coolie in carrying load on his head is ZERO with respect to gravity. This is because the coolie applies force against force of gravity which acts in vertical downward direction and displacement of load takes place in horizontal direction hence angle between displacement and force becomes 90°.

cos 90°= 0 which means work done = 0

So, a coolie does not work, when he moves on a level road while carrying a box on his head.