Can physics measure the work a porter does? What do you need to know to measure a porter's work? What produces work, and how is power different from work?

The body of a porter carrying load on head on a level road uses more energy than is required when the porter is resting completely. In this way the porter is doing some work.

However, when we think of theoretical definition of work, there is no force resisting the horizontal movement of the load when carried on a level road, and therefore there is no work done when a porter carrying a load on his head walk along a level road.

When the porter is going down the stairs the gravitational energy acting on the load is assisting the downward movement of the load. In this way energy is released by downward movement of the load. Thus the porter is doing negative work on the load.

second mirror gathers light from a faraway object and reflects it to another mirror. - a.

this is poiselle's equation from fluid mechanic in physics. mech eng