$A$ porter moving vertically upstairs with a load on his head does work. Why?

(N/A) Work is defined as the product of force and displacement in the direction of the force. When a porter moves vertically upstairs,he applies an upward force to balance the weight of the load (acting downwards). Since the displacement of the porter is also in the upward direction,the angle between the force applied by the porter and the displacement is $0^{\circ}$. Therefore,the work done by the porter is positive and non-zero $(W = F \cdot s \cdot \cos(0^{\circ}) = F \cdot s)$.