Does "work done" actually have any real meaningful sense or value? I've got no problem with "work done" when moving an object in a conservative field. This makes sense - you've got to put in energy when, for example, lifting an object in a gravitational field, and this energy is the work done, and you get this energy back if the object then falls back to its original position. In this case, the work done is simply another name for the energy required to move 'up' in the field, and is equal to the (change of) the potential energy of the object. But what if you're not talking about moving in a conservative field? If you just push against a wall, for example, no work is done because the wall doesn't move. But there are all sorts of energy processes going on and energy is expended. If you push a car, there is some "work done" if the car moves. But, again, you have to expend energy to overcome friction, and so on. In both cases, the "work done" is a small fraction of the energy input, and it doesn't immediately seem to have any value. So, what actually is the value of "work done"? What's the point of it? Wouldn't it make more sense to dispense with it entirely and just say that the kinetic energy of the wall or the car that you're pushing is one part of the energy of the system? Why does "work done" have any value over and above kinetic energy? Wikipedia doesn't seem to help here, because its derivation of kinetic energy is in terms of work done when lifting an object in a gravitational field, which seems to me to be rather circular.