Work

The Merriam-Webster online dictionary defines work as 'an activity in which one exerts strength or faculties to do or perform something'. For example, a standing gate man exerts strength and does his duty. Similarly, a person carrying a load on his head and walking on a horizontal platform is also doing some work. But, in physics, work is defined in a slightly different way: work is the product of the magnitude of the applied force, F and the magnitude of the displacement, s of the force in its direction. It is given by the formula: W = F s cos(theta), where theta is the angle between the direction of the force and its displacement. Therefore, the work done depends upon (i) the magnitude of a force, (ii) the displacement of the force, and (iii) the cosine of the angle between the direction of the force and that of the displacement. The work done by a force in making some displacement is maximum, if the applied force and its displacement are in the same direction and the work done by a force is 0 if the two quantities are directed at right angle. 

Therefore, a workman carrying a load on his head on a horizontal platform does zero work in carrying the load, but he still does work against friction between his feet and the ground. 

Test Yourself:

(1) Compute the work done by a force of 200N if it makes a displacement of 2 meters (i) in its own direction, (ii) at an angle of 30 degrees with its direction, and (iii) at right angle to its direction.