Hi all, Well, I'll try to explain my doubt with an example. The idea of force formalizes what we intuitively think as a push or a pull, torque formalizes what we think as a turn, linear and angular momentum respectively capture the idea of "how much something is moving" and "how much something is turning". We have all of those intuitive concepts, and then we formalize then defining then mathematicaly or with experiments, like mass that are able to formalize the concept of how hard is to make something change it's state of motion. Work however is something I fail to see what it formalizes. People usually say: "well, work is change in energy", then you ask what is energy and the answer is "it's a measure of the ability of performing work". In this way it is getting circular. Some people then try to answer telling how to define units of energy. This tells ways to measure and quantize energy, but doesn't give a feel for what it is. The other possibility to explain work is simply give the formula: work is the line integral of the force along the trajectory of a particle. This is the formula, but what concept it captures? Momentum as p = mv as a first approximation is a good way to capture the "quantity of motion" concept: it's proportional to how much of the thing is moving (mass) and proportional to the velocity. Work is simply in the simples terms force multiplied by distance. What intuitive concept is this formalizing? Thanks very much in advance.